cache.rs

use super::{
    value_initializer::{InitResult, ValueInitializer},
    CacheBuilder, ConcurrentCacheExt, Iter, OwnedKeyEntrySelector, PredicateId,
    RefKeyEntrySelector,
};
use crate::{
    common::{
        concurrent::{
            constants::{MAX_SYNC_REPEATS, WRITE_RETRY_INTERVAL_MICROS},
            housekeeper::{self, InnerSync},
            Weigher, WriteOp,
        },
        time::Instant,
    },
    notification::{self, EvictionListener},
    sync_base::base_cache::{BaseCache, HouseKeeperArc},
    Entry, Policy, PredicateError,
};

#[cfg(feature = "unstable-debug-counters")]
use crate::common::concurrent::debug_counters::CacheDebugStats;

use crossbeam_channel::{Sender, TrySendError};
use std::{
    borrow::Borrow,
    collections::hash_map::RandomState,
    fmt,
    future::Future,
    hash::{BuildHasher, Hash},
    sync::Arc,
    time::Duration,
};

/// A thread-safe, futures-aware concurrent in-memory cache.
///
/// `Cache` supports full concurrency of retrievals and a high expected concurrency
/// for updates. It can be accessed inside and outside of asynchronous contexts.
///
/// `Cache` utilizes a lock-free concurrent hash table as the central key-value
/// storage. `Cache` performs a best-effort bounding of the map using an entry
/// replacement algorithm to determine which entries to evict when the capacity is
/// exceeded.
///
/// To use this cache, enable a crate feature called "future".
///
/// # Table of Contents
///
/// - [Example: `insert`, `get` and `invalidate`](#example-insert-get-and-invalidate)
/// - [Avoiding to clone the value at `get`](#avoiding-to-clone-the-value-at-get)
/// - [Example: Size-based Eviction](#example-size-based-eviction)
/// - [Example: Time-based Expirations](#example-time-based-expirations)
/// - [Example: Eviction Listener](#example-eviction-listener)
///     - [You should avoid eviction listener to panic](#you-should-avoid-eviction-listener-to-panic)
///     - [Delivery Modes for Eviction Listener](#delivery-modes-for-eviction-listener)
/// - [Thread Safety](#thread-safety)
/// - [Sharing a cache across threads](#sharing-a-cache-across-threads)
/// - [Hashing Algorithm](#hashing-algorithm)
///
/// # Example: `insert`, `get` and `invalidate`
///
/// Cache entries are manually added using an insert method, and are stored in the
/// cache until either evicted or manually invalidated:
///
/// - Inside an async context (`async fn` or `async` block), use
///   [`insert`](#method.insert), [`get_with`](#method.get_with) or
///   [`invalidate`](#method.invalidate) methods for updating the cache and `await`
///   them.
/// - Outside any async context, use [`blocking`](#method.blocking) method to access
///   blocking version of [`insert`](./struct.BlockingOp.html#method.insert) or
///   [`invalidate`](struct.BlockingOp.html#method.invalidate) methods.
///
/// Here's an example of reading and updating a cache by using multiple asynchronous
/// tasks with [Tokio][tokio-crate] runtime:
///
/// [tokio-crate]: https://crates.io/crates/tokio
///
///```rust
/// // Cargo.toml
/// //
/// // [dependencies]
/// // moka = { version = "0.9", features = ["future"] }
/// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
/// // futures-util = "0.3"
///
/// use moka::future::Cache;
///
/// #[tokio::main]
/// async fn main() {
///     const NUM_TASKS: usize = 16;
///     const NUM_KEYS_PER_TASK: usize = 64;
///
///     fn value(n: usize) -> String {
///         format!("value {}", n)
///     }
///
///     // Create a cache that can store up to 10,000 entries.
///     let cache = Cache::new(10_000);
///
///     // Spawn async tasks and write to and read from the cache.
///     let tasks: Vec<_> = (0..NUM_TASKS)
///         .map(|i| {
///             // To share the same cache across the async tasks, clone it.
///             // This is a cheap operation.
///             let my_cache = cache.clone();
///             let start = i * NUM_KEYS_PER_TASK;
///             let end = (i + 1) * NUM_KEYS_PER_TASK;
///
///             tokio::spawn(async move {
///                 // Insert 64 entries. (NUM_KEYS_PER_TASK = 64)
///                 for key in start..end {
///                     // insert() is an async method, so await it.
///                     my_cache.insert(key, value(key)).await;
///                     // get() returns Option<String>, a clone of the stored value.
///                     assert_eq!(my_cache.get(&key), Some(value(key)));
///                 }
///
///                 // Invalidate every 4 element of the inserted entries.
///                 for key in (start..end).step_by(4) {
///                     // invalidate() is an async method, so await it.
///                     my_cache.invalidate(&key).await;
///                 }
///             })
///         })
///         .collect();
///
///     // Wait for all tasks to complete.
///     futures_util::future::join_all(tasks).await;
///
///     // Verify the result.
///     for key in 0..(NUM_TASKS * NUM_KEYS_PER_TASK) {
///         if key % 4 == 0 {
///             assert_eq!(cache.get(&key), None);
///         } else {
///             assert_eq!(cache.get(&key), Some(value(key)));
///         }
///     }
/// }
/// ```
///
/// If you want to atomically initialize and insert a value when the key is not
/// present, you might want to check other insertion methods
/// [`get_with`](#method.get_with) and [`try_get_with`](#method.try_get_with).
///
/// # Avoiding to clone the value at `get`
///
/// The return type of `get` method is `Option<V>` instead of `Option<&V>`. Every
/// time `get` is called for an existing key, it creates a clone of the stored value
/// `V` and returns it. This is because the `Cache` allows concurrent updates from
/// threads so a value stored in the cache can be dropped or replaced at any time by
/// any other thread. `get` cannot return a reference `&V` as it is impossible to
/// guarantee the value outlives the reference.
///
/// If you want to store values that will be expensive to clone, wrap them by
/// `std::sync::Arc` before storing in a cache. [`Arc`][rustdoc-std-arc] is a
/// thread-safe reference-counted pointer and its `clone()` method is cheap.
///
/// [rustdoc-std-arc]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html
///
/// # Example: Size-based Eviction
///
/// ```rust
/// // Cargo.toml
/// //
/// // [dependencies]
/// // moka = { version = "0.9", features = ["future"] }
/// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
/// // futures-util = "0.3"
///
/// use std::convert::TryInto;
/// use moka::future::Cache;
///
/// #[tokio::main]
/// async fn main() {
///     // Evict based on the number of entries in the cache.
///     let cache = Cache::builder()
///         // Up to 10,000 entries.
///         .max_capacity(10_000)
///         // Create the cache.
///         .build();
///     cache.insert(1, "one".to_string()).await;
///
///     // Evict based on the byte length of strings in the cache.
///     let cache = Cache::builder()
///         // A weigher closure takes &K and &V and returns a u32
///         // representing the relative size of the entry.
///         .weigher(|_key, value: &String| -> u32 {
///             value.len().try_into().unwrap_or(u32::MAX)
///         })
///         // This cache will hold up to 32MiB of values.
///         .max_capacity(32 * 1024 * 1024)
///         .build();
///     cache.insert(2, "two".to_string()).await;
/// }
/// ```
///
/// If your cache should not grow beyond a certain size, use the `max_capacity`
/// method of the [`CacheBuilder`][builder-struct] to set the upper bound. The cache
/// will try to evict entries that have not been used recently or very often.
///
/// At the cache creation time, a weigher closure can be set by the `weigher` method
/// of the `CacheBuilder`. A weigher closure takes `&K` and `&V` as the arguments and
/// returns a `u32` representing the relative size of the entry:
///
/// - If the `weigher` is _not_ set, the cache will treat each entry has the same
///   size of `1`. This means the cache will be bounded by the number of entries.
/// - If the `weigher` is set, the cache will call the weigher to calculate the
///   weighted size (relative size) on an entry. This means the cache will be bounded
///   by the total weighted size of entries.
///
/// Note that weighted sizes are not used when making eviction selections.
///
/// [builder-struct]: ./struct.CacheBuilder.html
///
/// # Example: Time-based Expirations
///
/// `Cache` supports the following expiration policies:
///
/// - **Time to live**: A cached entry will be expired after the specified duration
///   past from `insert`.
/// - **Time to idle**: A cached entry will be expired after the specified duration
///   past from `get` or `insert`.
///
/// ```rust
/// // Cargo.toml
/// //
/// // [dependencies]
/// // moka = { version = "0.9", features = ["future"] }
/// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
/// // futures-util = "0.3"
///
/// use moka::future::Cache;
/// use std::time::Duration;
///
/// #[tokio::main]
/// async fn main() {
///     let cache = Cache::builder()
///         // Time to live (TTL): 30 minutes
///         .time_to_live(Duration::from_secs(30 * 60))
///         // Time to idle (TTI):  5 minutes
///         .time_to_idle(Duration::from_secs( 5 * 60))
///         // Create the cache.
///         .build();
///
///     // This entry will expire after 5 minutes (TTI) if there is no get().
///     cache.insert(0, "zero").await;
///
///     // This get() will extend the entry life for another 5 minutes.
///     cache.get(&0);
///
///     // Even though we keep calling get(), the entry will expire
///     // after 30 minutes (TTL) from the insert().
/// }
/// ```
///
/// # Example: Eviction Listener
///
/// A `Cache` can be configured with an eviction listener, a closure that is called
/// every time there is a cache eviction. The listener takes three parameters: the
/// key and value of the evicted entry, and the
/// [`RemovalCause`](../notification/enum.RemovalCause.html) to indicate why the
/// entry was evicted.
///
/// An eviction listener can be used to keep other data structures in sync with the
/// cache, for example.
///
/// The following example demonstrates how to use an eviction listener with
/// time-to-live expiration to manage the lifecycle of temporary files on a
/// filesystem. The cache stores the paths of the files, and when one of them has
/// expired, the eviction lister will be called with the path, so it can remove the
/// file from the filesystem.
///
/// ```rust
/// // Cargo.toml
/// //
/// // [dependencies]
/// // anyhow = "1.0"
/// // uuid = { version = "1.1", features = ["v4"] }
/// // tokio = { version = "1.18", features = ["fs", "macros", "rt-multi-thread", "sync", "time"] }
///
/// use moka::future::Cache;
///
/// use anyhow::{anyhow, Context};
/// use std::{
///     io,
///     path::{Path, PathBuf},
///     sync::Arc,
///     time::Duration,
/// };
/// use tokio::{fs, sync::RwLock};
/// use uuid::Uuid;
///
/// /// The DataFileManager writes, reads and removes data files.
/// struct DataFileManager {
///     base_dir: PathBuf,
///     file_count: usize,
/// }
///
/// impl DataFileManager {
///     fn new(base_dir: PathBuf) -> Self {
///         Self {
///             base_dir,
///             file_count: 0,
///         }
///     }
///
///     async fn write_data_file(&mut self, contents: String) -> io::Result<PathBuf> {
///         loop {
///             // Generate a unique file path.
///             let mut path = self.base_dir.to_path_buf();
///             path.push(Uuid::new_v4().as_hyphenated().to_string());
///
///             if path.exists() {
///                 continue; // This path is already taken by others. Retry.
///             }
///
///             // We have got a unique file path, so create the file at
///             // the path and write the contents to the file.
///             fs::write(&path, contents).await?;
///             self.file_count += 1;
///             println!(
///                 "Created a data file at {:?} (file count: {})",
///                 path, self.file_count
///             );
///
///             // Return the path.
///             return Ok(path);
///         }
///     }
///
///     async fn read_data_file(&self, path: impl AsRef<Path>) -> io::Result<String> {
///         // Reads the contents of the file at the path, and return the contents.
///         fs::read_to_string(path).await
///     }
///
///     async fn remove_data_file(&mut self, path: impl AsRef<Path>) -> io::Result<()> {
///         // Remove the file at the path.
///         fs::remove_file(path.as_ref()).await?;
///         self.file_count -= 1;
///         println!(
///             "Removed a data file at {:?} (file count: {})",
///             path.as_ref(),
///             self.file_count
///         );
///
///         Ok(())
///     }
/// }
///
/// #[tokio::main]
/// async fn main() -> anyhow::Result<()> {
///     // Create an instance of the DataFileManager and wrap it with
///     // Arc<RwLock<_>> so it can be shared across threads.
///     let file_mgr = DataFileManager::new(std::env::temp_dir());
///     let file_mgr = Arc::new(RwLock::new(file_mgr));
///
///     let file_mgr1 = Arc::clone(&file_mgr);
///     let rt = tokio::runtime::Handle::current();
///
///     // Create an eviction lister closure.
///     let listener = move |k, v: PathBuf, cause| {
///         // Try to remove the data file at the path `v`.
///         println!(
///             "\n== An entry has been evicted. k: {:?}, v: {:?}, cause: {:?}",
///             k, v, cause
///         );
///         rt.block_on(async {
///             // Acquire the write lock of the DataFileManager.
///             let mut mgr = file_mgr1.write().await;
///             // Remove the data file. We must handle error cases here to
///             // prevent the listener from panicking.
///             if let Err(_e) = mgr.remove_data_file(v.as_path()).await {
///                 eprintln!("Failed to remove a data file at {:?}", v);
///             }
///         });
///     };
///
///     // Create the cache. Set time to live for two seconds and set the
///     // eviction listener.
///     let cache = Cache::builder()
///         .max_capacity(100)
///         .time_to_live(Duration::from_secs(2))
///         .eviction_listener_with_queued_delivery_mode(listener)
///         .build();
///
///     // Insert an entry to the cache.
///     // This will create and write a data file for the key "user1", store the
///     // path of the file to the cache, and return it.
///     println!("== try_get_with()");
///     let path = cache
///         .try_get_with("user1", async {
///             let mut mgr = file_mgr.write().await;
///             let path = mgr
///                 .write_data_file("user data".into())
///                 .await
///                 .with_context(|| format!("Failed to create a data file"))?;
///             Ok(path) as anyhow::Result<_>
///         })
///         .await
///         .map_err(|e| anyhow!("{}", e))?;
///
///     // Read the data file at the path and print the contents.
///     println!("\n== read_data_file()");
///     {
///         let mgr = file_mgr.read().await;
///         let contents = mgr
///             .read_data_file(path.as_path())
///             .await
///             .with_context(|| format!("Failed to read data from {:?}", path))?;
///         println!("contents: {}", contents);
///     }
///
///     // Sleep for five seconds. While sleeping, the cache entry for key "user1"
///     // will be expired and evicted, so the eviction lister will be called to
///     // remove the file.
///     tokio::time::sleep(Duration::from_secs(5)).await;
///
///     Ok(())
/// }
/// ```
///
/// ## You should avoid eviction listener to panic
///
/// It is very important to make an eviction listener closure not to panic.
/// Otherwise, the cache will stop calling the listener after a panic. This is an
/// intended behavior because the cache cannot know whether it is memory safe or not
/// to call the panicked lister again.
///
/// When a listener panics, the cache will swallow the panic and disable the
/// listener. If you want to know when a listener panics and the reason of the panic,
/// you can enable an optional `logging` feature of Moka and check error-level logs.
///
/// To enable the `logging`, do the followings:
///
/// 1. In `Cargo.toml`, add the crate feature `logging` for `moka`.
/// 2. Set the logging level for `moka` to `error` or any lower levels (`warn`,
///    `info`, ...):
///     - If you are using the `env_logger` crate, you can achieve this by setting
///       `RUST_LOG` environment variable to `moka=error`.
/// 3. If you have more than one caches, you may want to set a distinct name for each
///    cache by using cache builder's [`name`][builder-name-method] method. The name
///    will appear in the log.
///
/// [builder-name-method]: ./struct.CacheBuilder.html#method.name
///
/// ## Delivery Modes for Eviction Listener
///
/// The [`DeliveryMode`][delivery-mode] specifies how and when an eviction
/// notification should be delivered to an eviction listener. Currently, the
/// `future::Cache` supports only one delivery mode: `Queued` mode.
///
/// A future version of `future::Cache` will support `Immediate` mode, which will be
/// easier to use in many use cases than queued mode. Unlike the `future::Cache`,
/// the `sync::Cache` already supports it.
///
/// Once `future::Cache` supports the immediate mode, the `eviction_listener` and
/// `eviction_listener_with_conf` methods will be added to the
/// `future::CacheBuilder`. The former will use the immediate mode, and the latter
/// will take a custom configurations to specify the queued mode. The current method
/// `eviction_listener_with_queued_delivery_mode` will be deprecated.
///
/// For more details about the delivery modes, see [this section][sync-delivery-modes]
/// of `sync::Cache` documentation.
///
/// [delivery-mode]: ../notification/enum.DeliveryMode.html
/// [sync-delivery-modes]: ../sync/struct.Cache.html#delivery-modes-for-eviction-listener
///
/// # Thread Safety
///
/// All methods provided by the `Cache` are considered thread-safe, and can be safely
/// accessed by multiple concurrent threads.
///
/// - `Cache<K, V, S>` requires trait bounds `Send`, `Sync` and `'static` for `K`
///   (key), `V` (value) and `S` (hasher state).
/// - `Cache<K, V, S>` will implement `Send` and `Sync`.
///
/// # Sharing a cache across asynchronous tasks
///
/// To share a cache across async tasks (or OS threads), do one of the followings:
///
/// - Create a clone of the cache by calling its `clone` method and pass it to other
///   task.
/// - Wrap the cache by a `sync::OnceCell` or `sync::Lazy` from
///   [once_cell][once-cell-crate] create, and set it to a `static` variable.
///
/// Cloning is a cheap operation for `Cache` as it only creates thread-safe
/// reference-counted pointers to the internal data structures.
///
/// [once-cell-crate]: https://crates.io/crates/once_cell
///
/// # Hashing Algorithm
///
/// By default, `Cache` uses a hashing algorithm selected to provide resistance
/// against HashDoS attacks. It will be the same one used by
/// `std::collections::HashMap`, which is currently SipHash 1-3.
///
/// While SipHash's performance is very competitive for medium sized keys, other
/// hashing algorithms will outperform it for small keys such as integers as well as
/// large keys such as long strings. However those algorithms will typically not
/// protect against attacks such as HashDoS.
///
/// The hashing algorithm can be replaced on a per-`Cache` basis using the
/// [`build_with_hasher`][build-with-hasher-method] method of the `CacheBuilder`.
/// Many alternative algorithms are available on crates.io, such as the
/// [aHash][ahash-crate] crate.
///
/// [build-with-hasher-method]: ./struct.CacheBuilder.html#method.build_with_hasher
/// [ahash-crate]: https://crates.io/crates/ahash
///
pub struct Cache<K, V, S = RandomState> {
    base: BaseCache<K, V, S>,
    value_initializer: Arc<ValueInitializer<K, V, S>>,
}

// TODO: https://github.com/moka-rs/moka/issues/54
#[allow(clippy::non_send_fields_in_send_ty)]
unsafe impl<K, V, S> Send for Cache<K, V, S>
where
    K: Send + Sync,
    V: Send + Sync,
    S: Send,
{
}

unsafe impl<K, V, S> Sync for Cache<K, V, S>
where
    K: Send + Sync,
    V: Send + Sync,
    S: Sync,
{
}

// NOTE: We cannot do `#[derive(Clone)]` because it will add `Clone` bound to `K`.
impl<K, V, S> Clone for Cache<K, V, S> {
    /// Makes a clone of this shared cache.
    ///
    /// This operation is cheap as it only creates thread-safe reference counted
    /// pointers to the shared internal data structures.
    fn clone(&self) -> Self {
        Self {
            base: self.base.clone(),
            value_initializer: Arc::clone(&self.value_initializer),
        }
    }
}

impl<K, V, S> fmt::Debug for Cache<K, V, S>
where
    K: fmt::Debug + Eq + Hash + Send + Sync + 'static,
    V: fmt::Debug + Clone + Send + Sync + 'static,
    // TODO: Remove these bounds from S.
    S: BuildHasher + Clone + Send + Sync + 'static,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut d_map = f.debug_map();

        for (k, v) in self.iter() {
            d_map.entry(&k, &v);
        }

        d_map.finish()
    }
}

impl<K, V, S> Cache<K, V, S> {
    /// Returns cache’s name.
    pub fn name(&self) -> Option<&str> {
        self.base.name()
    }

    /// Returns a read-only cache policy of this cache.
    ///
    /// At this time, cache policy cannot be modified after cache creation.
    /// A future version may support to modify it.
    pub fn policy(&self) -> Policy {
        self.base.policy()
    }

    /// Returns an approximate number of entries in this cache.
    ///
    /// The value returned is _an estimate_; the actual count may differ if there are
    /// concurrent insertions or removals, or if some entries are pending removal due
    /// to expiration. This inaccuracy can be mitigated by performing a `sync()`
    /// first.
    ///
    /// # Example
    ///
    /// ```rust
    /// // Cargo.toml
    /// //
    /// // [dependencies]
    /// // moka = { version = "0.9", features = ["future"] }
    /// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
    /// use moka::future::Cache;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let cache = Cache::new(10);
    ///     cache.insert('n', "Netherland Dwarf").await;
    ///     cache.insert('l', "Lop Eared").await;
    ///     cache.insert('d', "Dutch").await;
    ///
    ///     // Ensure an entry exists.
    ///     assert!(cache.contains_key(&'n'));
    ///
    ///     // However, followings may print stale number zeros instead of threes.
    ///     println!("{}", cache.entry_count());   // -> 0
    ///     println!("{}", cache.weighted_size()); // -> 0
    ///
    ///     // To mitigate the inaccuracy, bring `ConcurrentCacheExt` trait to
    ///     // the scope so we can use `sync` method.
    ///     use moka::future::ConcurrentCacheExt;
    ///     // Call `sync` to run pending internal tasks.
    ///     cache.sync();
    ///
    ///     // Followings will print the actual numbers.
    ///     println!("{}", cache.entry_count());   // -> 3
    ///     println!("{}", cache.weighted_size()); // -> 3
    /// }
    /// ```
    ///
    pub fn entry_count(&self) -> u64 {
        self.base.entry_count()
    }

    /// Returns an approximate total weighted size of entries in this cache.
    ///
    /// The value returned is _an estimate_; the actual size may differ if there are
    /// concurrent insertions or removals, or if some entries are pending removal due
    /// to expiration. This inaccuracy can be mitigated by performing a `sync()`
    /// first. See [`entry_count`](#method.entry_count) for a sample code.
    pub fn weighted_size(&self) -> u64 {
        self.base.weighted_size()
    }

    #[cfg(feature = "unstable-debug-counters")]
    #[cfg_attr(docsrs, doc(cfg(feature = "unstable-debug-counters")))]
    pub fn debug_stats(&self) -> CacheDebugStats {
        self.base.debug_stats()
    }
}

impl<K, V> Cache<K, V, RandomState>
where
    K: Hash + Eq + Send + Sync + 'static,
    V: Clone + Send + Sync + 'static,
{
    /// Constructs a new `Cache<K, V>` that will store up to the `max_capacity`.
    ///
    /// To adjust various configuration knobs such as `initial_capacity` or
    /// `time_to_live`, use the [`CacheBuilder`][builder-struct].
    ///
    /// [builder-struct]: ./struct.CacheBuilder.html
    pub fn new(max_capacity: u64) -> Self {
        let build_hasher = RandomState::default();
        Self::with_everything(
            None,
            Some(max_capacity),
            None,
            build_hasher,
            None,
            None,
            None,
            None,
            None,
            false,
            housekeeper::Configuration::new_thread_pool(true),
        )
    }

    /// Returns a [`CacheBuilder`][builder-struct], which can builds a `Cache` with
    /// various configuration knobs.
    ///
    /// [builder-struct]: ./struct.CacheBuilder.html
    pub fn builder() -> CacheBuilder<K, V, Cache<K, V, RandomState>> {
        CacheBuilder::default()
    }
}

impl<K, V, S> Cache<K, V, S>
where
    K: Hash + Eq + Send + Sync + 'static,
    V: Clone + Send + Sync + 'static,
    S: BuildHasher + Clone + Send + Sync + 'static,
{
    // https://rust-lang.github.io/rust-clippy/master/index.html#too_many_arguments
    #[allow(clippy::too_many_arguments)]
    pub(crate) fn with_everything(
        name: Option<String>,
        max_capacity: Option<u64>,
        initial_capacity: Option<usize>,
        build_hasher: S,
        weigher: Option<Weigher<K, V>>,
        eviction_listener: Option<EvictionListener<K, V>>,
        eviction_listener_conf: Option<notification::Configuration>,
        time_to_live: Option<Duration>,
        time_to_idle: Option<Duration>,
        invalidator_enabled: bool,
        housekeeper_conf: housekeeper::Configuration,
    ) -> Self {
        Self {
            base: BaseCache::new(
                name,
                max_capacity,
                initial_capacity,
                build_hasher.clone(),
                weigher,
                eviction_listener,
                eviction_listener_conf,
                time_to_live,
                time_to_idle,
                invalidator_enabled,
                housekeeper_conf,
            ),
            value_initializer: Arc::new(ValueInitializer::with_hasher(build_hasher)),
        }
    }

    /// Returns `true` if the cache contains a value for the key.
    ///
    /// Unlike the `get` method, this method is not considered a cache read operation,
    /// so it does not update the historic popularity estimator or reset the idle
    /// timer for the key.
    ///
    /// The key may be any borrowed form of the cache's key type, but `Hash` and `Eq`
    /// on the borrowed form _must_ match those for the key type.
    pub fn contains_key<Q>(&self, key: &Q) -> bool
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.base.contains_key_with_hash(key, self.base.hash(key))
    }

    /// Returns a _clone_ of the value corresponding to the key.
    ///
    /// If you want to store values that will be expensive to clone, wrap them by
    /// `std::sync::Arc` before storing in a cache. [`Arc`][rustdoc-std-arc] is a
    /// thread-safe reference-counted pointer and its `clone()` method is cheap.
    ///
    /// The key may be any borrowed form of the cache's key type, but `Hash` and `Eq`
    /// on the borrowed form _must_ match those for the key type.
    ///
    /// [rustdoc-std-arc]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html
    pub fn get<Q>(&self, key: &Q) -> Option<V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.base
            .get_with_hash(key, self.base.hash(key), false)
            .map(Entry::into_value)
    }

    /// Takes a key `K` and returns an [`OwnedKeyEntrySelector`] that can be used to
    /// select or insert an entry.
    ///
    /// [`OwnedKeyEntrySelector`]: ./struct.OwnedKeyEntrySelector.html
    ///
    /// # Example
    ///
    /// ```rust
    /// // Cargo.toml
    /// //
    /// // [dependencies]
    /// // moka = { version = "0.10", features = ["future"] }
    /// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
    ///
    /// use moka::future::Cache;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let cache: Cache<String, u32> = Cache::new(100);
    ///     let key = "key1".to_string();
    ///
    ///     let entry = cache.entry(key.clone()).or_insert(3).await;
    ///     assert!(entry.is_fresh());
    ///     assert_eq!(entry.key(), &key);
    ///     assert_eq!(entry.into_value(), 3);
    ///
    ///     let entry = cache.entry(key).or_insert(6).await;
    ///     // Not fresh because the value was already in the cache.
    ///     assert!(!entry.is_fresh());
    ///     assert_eq!(entry.into_value(), 3);
    /// }
    /// ```
    pub fn entry(&self, key: K) -> OwnedKeyEntrySelector<'_, K, V, S>
    where
        K: Hash + Eq,
    {
        let hash = self.base.hash(&key);
        OwnedKeyEntrySelector::new(key, hash, self)
    }

    /// Takes a reference `&Q` of a key and returns an [`RefKeyEntrySelector`] that
    /// can be used to select or insert an entry.
    ///
    /// [`RefKeyEntrySelector`]: ./struct.RefKeyEntrySelector.html
    ///
    /// # Example
    ///
    /// ```rust
    /// // Cargo.toml
    /// //
    /// // [dependencies]
    /// // moka = { version = "0.10", features = ["future"] }
    /// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
    ///
    /// use moka::future::Cache;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let cache: Cache<String, u32> = Cache::new(100);
    ///     let key = "key1".to_string();
    ///
    ///     let entry = cache.entry_by_ref(&key).or_insert(3).await;
    ///     assert!(entry.is_fresh());
    ///     assert_eq!(entry.key(), &key);
    ///     assert_eq!(entry.into_value(), 3);
    ///
    ///     let entry = cache.entry_by_ref(&key).or_insert(6).await;
    ///     // Not fresh because the value was already in the cache.
    ///     assert!(!entry.is_fresh());
    ///     assert_eq!(entry.into_value(), 3);
    /// }
    /// ```
    pub fn entry_by_ref<'a, Q>(&'a self, key: &'a Q) -> RefKeyEntrySelector<'a, K, Q, V, S>
    where
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        let hash = self.base.hash(key);
        RefKeyEntrySelector::new(key, hash, self)
    }

    /// Returns a _clone_ of the value corresponding to the key. If the value does
    /// not exist, resolve the `init` future and inserts the output.
    ///
    /// # Concurrent calls on the same key
    ///
    /// This method guarantees that concurrent calls on the same not-existing key are
    /// coalesced into one evaluation of the `init` future. Only one of the calls
    /// evaluates its future, and other calls wait for that future to resolve.
    ///
    /// The following code snippet demonstrates this behavior:
    ///
    /// ```rust
    /// // Cargo.toml
    /// //
    /// // [dependencies]
    /// // moka = { version = "0.9", features = ["future"] }
    /// // futures-util = "0.3"
    /// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
    /// use moka::future::Cache;
    /// use std::sync::Arc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     const TEN_MIB: usize = 10 * 1024 * 1024; // 10MiB
    ///     let cache = Cache::new(100);
    ///
    ///     // Spawn four async tasks.
    ///     let tasks: Vec<_> = (0..4_u8)
    ///         .map(|task_id| {
    ///             let my_cache = cache.clone();
    ///             tokio::spawn(async move {
    ///                 println!("Task {} started.", task_id);
    ///
    ///                 // Insert and get the value for key1. Although all four async tasks
    ///                 // will call `get_with` at the same time, the `init` async
    ///                 // block must be resolved only once.
    ///                 let value = my_cache
    ///                     .get_with("key1", async move {
    ///                         println!("Task {} inserting a value.", task_id);
    ///                         Arc::new(vec![0u8; TEN_MIB])
    ///                     })
    ///                     .await;
    ///
    ///                 // Ensure the value exists now.
    ///                 assert_eq!(value.len(), TEN_MIB);
    ///                 assert!(my_cache.get(&"key1").is_some());
    ///
    ///                 println!("Task {} got the value. (len: {})", task_id, value.len());
    ///             })
    ///         })
    ///         .collect();
    ///
    ///     // Run all tasks concurrently and wait for them to complete.
    ///     futures_util::future::join_all(tasks).await;
    /// }
    /// ```
    ///
    /// **A Sample Result**
    ///
    /// - The `init` future (async black) was resolved exactly once by task 3.
    /// - Other tasks were blocked until task 3 inserted the value.
    ///
    /// ```console
    /// Task 0 started.
    /// Task 3 started.
    /// Task 1 started.
    /// Task 2 started.
    /// Task 3 inserting a value.
    /// Task 3 got the value. (len: 10485760)
    /// Task 0 got the value. (len: 10485760)
    /// Task 1 got the value. (len: 10485760)
    /// Task 2 got the value. (len: 10485760)
    /// ```
    ///
    /// # Panics
    ///
    /// This method panics when the `init` future has panicked. When it happens, only
    /// the caller whose `init` future panicked will get the panic (e.g. only task 3
    /// in the above sample). If there are other calls in progress (e.g. task 0, 1
    /// and 2 above), this method will restart and resolve one of the remaining
    /// `init` futures.
    ///
    pub async fn get_with(&self, key: K, init: impl Future<Output = V>) -> V {
        let hash = self.base.hash(&key);
        let key = Arc::new(key);
        let replace_if = None as Option<fn(&V) -> bool>;
        self.get_or_insert_with_hash_and_fun(key, hash, init, replace_if, false)
            .await
            .into_value()
    }

    /// Similar to [`get_with`](#method.get_with), but instead of passing an owned
    /// key, you can pass a reference to the key. If the key does not exist in the
    /// cache, the key will be cloned to create new entry in the cache.
    pub async fn get_with_by_ref<Q>(&self, key: &Q, init: impl Future<Output = V>) -> V
    where
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        let hash = self.base.hash(key);
        let replace_if = None as Option<fn(&V) -> bool>;

        self.get_or_insert_with_hash_by_ref_and_fun(key, hash, init, replace_if, false)
            .await
            .into_value()
    }

    /// Deprecated, replaced with
    /// [`entry()::or_insert_with_if()`](./struct.OwnedKeyEntrySelector.html#method.or_insert_with_if)
    #[deprecated(since = "0.10.0", note = "Replaced with `entry().or_insert_with_if()`")]
    pub async fn get_with_if(
        &self,
        key: K,
        init: impl Future<Output = V>,
        replace_if: impl FnMut(&V) -> bool,
    ) -> V {
        let hash = self.base.hash(&key);
        let key = Arc::new(key);
        self.get_or_insert_with_hash_and_fun(key, hash, init, Some(replace_if), false)
            .await
            .into_value()
    }

    /// Returns a _clone_ of the value corresponding to the key. If the value does
    /// not exist, resolves the `init` future, and inserts the value if `Some(value)`
    /// was returned. If `None` was returned from the future, this method does not
    /// insert a value and returns `None`.
    ///
    /// # Concurrent calls on the same key
    ///
    /// This method guarantees that concurrent calls on the same not-existing key are
    /// coalesced into one evaluation of the `init` future. Only one of the calls
    /// evaluates its future, and other calls wait for that future to resolve.
    ///
    /// The following code snippet demonstrates this behavior:
    ///
    /// ```rust
    /// // Cargo.toml
    /// //
    /// // [dependencies]
    /// // moka = { version = "0.9", features = ["future"] }
    /// // futures-util = "0.3"
    /// // reqwest = "0.11"
    /// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
    /// use moka::future::Cache;
    ///
    /// // This async function tries to get HTML from the given URI.
    /// async fn get_html(task_id: u8, uri: &str) -> Option<String> {
    ///     println!("get_html() called by task {}.", task_id);
    ///     reqwest::get(uri).await.ok()?.text().await.ok()
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let cache = Cache::new(100);
    ///
    ///     // Spawn four async tasks.
    ///     let tasks: Vec<_> = (0..4_u8)
    ///         .map(|task_id| {
    ///             let my_cache = cache.clone();
    ///             tokio::spawn(async move {
    ///                 println!("Task {} started.", task_id);
    ///
    ///                 // Try to insert and get the value for key1. Although
    ///                 // all four async tasks will call `try_get_with`
    ///                 // at the same time, get_html() must be called only once.
    ///                 let value = my_cache
    ///                     .optionally_get_with(
    ///                         "key1",
    ///                         get_html(task_id, "https://www.rust-lang.org"),
    ///                     ).await;
    ///
    ///                 // Ensure the value exists now.
    ///                 assert!(value.is_some());
    ///                 assert!(my_cache.get(&"key1").is_some());
    ///
    ///                 println!(
    ///                     "Task {} got the value. (len: {})",
    ///                     task_id,
    ///                     value.unwrap().len()
    ///                 );
    ///             })
    ///         })
    ///         .collect();
    ///
    ///     // Run all tasks concurrently and wait for them to complete.
    ///     futures_util::future::join_all(tasks).await;
    /// }
    /// ```
    ///
    /// **A Sample Result**
    ///
    /// - `get_html()` was called exactly once by task 2.
    /// - Other tasks were blocked until task 2 inserted the value.
    ///
    /// ```console
    /// Task 1 started.
    /// Task 0 started.
    /// Task 2 started.
    /// Task 3 started.
    /// get_html() called by task 2.
    /// Task 2 got the value. (len: 19419)
    /// Task 1 got the value. (len: 19419)
    /// Task 0 got the value. (len: 19419)
    /// Task 3 got the value. (len: 19419)
    /// ```
    ///
    /// # Panics
    ///
    /// This method panics when the `init` future has panicked. When it happens, only
    /// the caller whose `init` future panicked will get the panic (e.g. only task 2
    /// in the above sample). If there are other calls in progress (e.g. task 0, 1
    /// and 3 above), this method will restart and resolve one of the remaining
    /// `init` futures.
    ///
    pub async fn optionally_get_with<F>(&self, key: K, init: F) -> Option<V>
    where
        F: Future<Output = Option<V>>,
    {
        let hash = self.base.hash(&key);
        let key = Arc::new(key);
        self.get_or_optionally_insert_with_hash_and_fun(key, hash, init, false)
            .await
            .map(Entry::into_value)
    }

    /// Similar to [`optionally_get_with`](#method.optionally_get_with), but instead
    /// of passing an owned key, you can pass a reference to the key. If the key does
    /// not exist in the cache, the key will be cloned to create new entry in the
    /// cache.
    pub async fn optionally_get_with_by_ref<F, Q>(&self, key: &Q, init: F) -> Option<V>
    where
        F: Future<Output = Option<V>>,
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        let hash = self.base.hash(key);
        self.get_or_optionally_insert_with_hash_by_ref_and_fun(key, hash, init, false)
            .await
            .map(Entry::into_value)
    }

    /// Returns a _clone_ of the value corresponding to the key. If the value does
    /// not exist, resolves the `init` future, and inserts the value if `Ok(value)`
    /// was returned. If `Err(_)` was returned from the future, this method does not
    /// insert a value and returns the `Err` wrapped by [`std::sync::Arc`][std-arc].
    ///
    /// [std-arc]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html
    ///
    /// # Concurrent calls on the same key
    ///
    /// This method guarantees that concurrent calls on the same not-existing key are
    /// coalesced into one evaluation of the `init` future (as long as these
    /// futures return the same error type). Only one of the calls evaluates its
    /// future, and other calls wait for that future to resolve.
    ///
    /// The following code snippet demonstrates this behavior:
    ///
    /// ```rust
    /// // Cargo.toml
    /// //
    /// // [dependencies]
    /// // moka = { version = "0.9", features = ["future"] }
    /// // futures-util = "0.3"
    /// // reqwest = "0.11"
    /// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
    /// use moka::future::Cache;
    ///
    /// // This async function tries to get HTML from the given URI.
    /// async fn get_html(task_id: u8, uri: &str) -> Result<String, reqwest::Error> {
    ///     println!("get_html() called by task {}.", task_id);
    ///     reqwest::get(uri).await?.text().await
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let cache = Cache::new(100);
    ///
    ///     // Spawn four async tasks.
    ///     let tasks: Vec<_> = (0..4_u8)
    ///         .map(|task_id| {
    ///             let my_cache = cache.clone();
    ///             tokio::spawn(async move {
    ///                 println!("Task {} started.", task_id);
    ///
    ///                 // Try to insert and get the value for key1. Although
    ///                 // all four async tasks will call `try_get_with`
    ///                 // at the same time, get_html() must be called only once.
    ///                 let value = my_cache
    ///                     .try_get_with(
    ///                         "key1",
    ///                         get_html(task_id, "https://www.rust-lang.org"),
    ///                     ).await;
    ///
    ///                 // Ensure the value exists now.
    ///                 assert!(value.is_ok());
    ///                 assert!(my_cache.get(&"key1").is_some());
    ///
    ///                 println!(
    ///                     "Task {} got the value. (len: {})",
    ///                     task_id,
    ///                     value.unwrap().len()
    ///                 );
    ///             })
    ///         })
    ///         .collect();
    ///
    ///     // Run all tasks concurrently and wait for them to complete.
    ///     futures_util::future::join_all(tasks).await;
    /// }
    /// ```
    ///
    /// **A Sample Result**
    ///
    /// - `get_html()` was called exactly once by task 2.
    /// - Other tasks were blocked until task 2 inserted the value.
    ///
    /// ```console
    /// Task 1 started.
    /// Task 0 started.
    /// Task 2 started.
    /// Task 3 started.
    /// get_html() called by task 2.
    /// Task 2 got the value. (len: 19419)
    /// Task 1 got the value. (len: 19419)
    /// Task 0 got the value. (len: 19419)
    /// Task 3 got the value. (len: 19419)
    /// ```
    ///
    /// # Panics
    ///
    /// This method panics when the `init` future has panicked. When it happens, only
    /// the caller whose `init` future panicked will get the panic (e.g. only task 2
    /// in the above sample). If there are other calls in progress (e.g. task 0, 1
    /// and 3 above), this method will restart and resolve one of the remaining
    /// `init` futures.
    ///
    pub async fn try_get_with<F, E>(&self, key: K, init: F) -> Result<V, Arc<E>>
    where
        F: Future<Output = Result<V, E>>,
        E: Send + Sync + 'static,
    {
        let hash = self.base.hash(&key);
        let key = Arc::new(key);
        self.get_or_try_insert_with_hash_and_fun(key, hash, init, false)
            .await
            .map(Entry::into_value)
    }

    /// Similar to [`try_get_with`](#method.try_get_with), but instead of passing an
    /// owned key, you can pass a reference to the key. If the key does not exist in
    /// the cache, the key will be cloned to create new entry in the cache.
    pub async fn try_get_with_by_ref<F, E, Q>(&self, key: &Q, init: F) -> Result<V, Arc<E>>
    where
        F: Future<Output = Result<V, E>>,
        E: Send + Sync + 'static,
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        let hash = self.base.hash(key);
        self.get_or_try_insert_with_hash_by_ref_and_fun(key, hash, init, false)
            .await
            .map(Entry::into_value)
    }

    /// Inserts a key-value pair into the cache.
    ///
    /// If the cache has this key present, the value is updated.
    pub async fn insert(&self, key: K, value: V) {
        let hash = self.base.hash(&key);
        let key = Arc::new(key);
        self.insert_with_hash(key, hash, value).await
    }

    fn do_blocking_insert(&self, key: K, value: V) {
        let hash = self.base.hash(&key);
        let key = Arc::new(key);
        let (op, now) = self.base.do_insert_with_hash(key, hash, value);
        let hk = self.base.housekeeper.as_ref();
        Self::blocking_schedule_write_op(
            self.base.inner.as_ref(),
            &self.base.write_op_ch,
            op,
            now,
            hk,
        )
        .expect("Failed to insert");
    }

    /// Discards any cached value for the key.
    ///
    /// The key may be any borrowed form of the cache's key type, but `Hash` and `Eq`
    /// on the borrowed form _must_ match those for the key type.
    pub async fn invalidate<Q>(&self, key: &Q)
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        let hash = self.base.hash(key);
        if let Some(kv) = self.base.remove_entry(key, hash) {
            if self.base.is_removal_notifier_enabled() {
                self.base.notify_invalidate(&kv.key, &kv.entry)
            }
            let op = WriteOp::Remove(kv);
            let now = self.base.current_time_from_expiration_clock();
            let hk = self.base.housekeeper.as_ref();
            Self::schedule_write_op(
                self.base.inner.as_ref(),
                &self.base.write_op_ch,
                op,
                now,
                hk,
            )
            .await
            .expect("Failed to remove");
            crossbeam_epoch::pin().flush();
        }
    }

    fn do_blocking_invalidate<Q>(&self, key: &Q)
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        let hash = self.base.hash(key);
        if let Some(kv) = self.base.remove_entry(key, hash) {
            let op = WriteOp::Remove(kv);
            let now = self.base.current_time_from_expiration_clock();
            let hk = self.base.housekeeper.as_ref();
            Self::blocking_schedule_write_op(
                self.base.inner.as_ref(),
                &self.base.write_op_ch,
                op,
                now,
                hk,
            )
            .expect("Failed to remove");
        }
    }

    /// Discards all cached values.
    ///
    /// This method returns immediately and a background thread will evict all the
    /// cached values inserted before the time when this method was called. It is
    /// guaranteed that the `get` method must not return these invalidated values
    /// even if they have not been evicted.
    ///
    /// Like the `invalidate` method, this method does not clear the historic
    /// popularity estimator of keys so that it retains the client activities of
    /// trying to retrieve an item.
    pub fn invalidate_all(&self) {
        self.base.invalidate_all();
    }

    /// Discards cached values that satisfy a predicate.
    ///
    /// `invalidate_entries_if` takes a closure that returns `true` or `false`. This
    /// method returns immediately and a background thread will apply the closure to
    /// each cached value inserted before the time when `invalidate_entries_if` was
    /// called. If the closure returns `true` on a value, that value will be evicted
    /// from the cache.
    ///
    /// Also the `get` method will apply the closure to a value to determine if it
    /// should have been invalidated. Therefore, it is guaranteed that the `get`
    /// method must not return invalidated values.
    ///
    /// Note that you must call
    /// [`CacheBuilder::support_invalidation_closures`][support-invalidation-closures]
    /// at the cache creation time as the cache needs to maintain additional internal
    /// data structures to support this method. Otherwise, calling this method will
    /// fail with a
    /// [`PredicateError::InvalidationClosuresDisabled`][invalidation-disabled-error].
    ///
    /// Like the `invalidate` method, this method does not clear the historic
    /// popularity estimator of keys so that it retains the client activities of
    /// trying to retrieve an item.
    ///
    /// [support-invalidation-closures]: ./struct.CacheBuilder.html#method.support_invalidation_closures
    /// [invalidation-disabled-error]: ../enum.PredicateError.html#variant.InvalidationClosuresDisabled
    pub fn invalidate_entries_if<F>(&self, predicate: F) -> Result<PredicateId, PredicateError>
    where
        F: Fn(&K, &V) -> bool + Send + Sync + 'static,
    {
        self.base.invalidate_entries_if(Arc::new(predicate))
    }

    /// Creates an iterator visiting all key-value pairs in arbitrary order. The
    /// iterator element type is `(Arc<K>, V)`, where `V` is a clone of a stored
    /// value.
    ///
    /// Iterators do not block concurrent reads and writes on the cache. An entry can
    /// be inserted to, invalidated or evicted from a cache while iterators are alive
    /// on the same cache.
    ///
    /// Unlike the `get` method, visiting entries via an iterator do not update the
    /// historic popularity estimator or reset idle timers for keys.
    ///
    /// # Guarantees
    ///
    /// In order to allow concurrent access to the cache, iterator's `next` method
    /// does _not_ guarantee the following:
    ///
    /// - It does not guarantee to return a key-value pair (an entry) if its key has
    ///   been inserted to the cache _after_ the iterator was created.
    ///   - Such an entry may or may not be returned depending on key's hash and
    ///     timing.
    ///
    /// and the `next` method guarantees the followings:
    ///
    /// - It guarantees not to return the same entry more than once.
    /// - It guarantees not to return an entry if it has been removed from the cache
    ///   after the iterator was created.
    ///     - Note: An entry can be removed by following reasons:
    ///         - Manually invalidated.
    ///         - Expired (e.g. time-to-live).
    ///         - Evicted as the cache capacity exceeded.
    ///
    /// # Examples
    ///
    /// ```rust
    /// // Cargo.toml
    /// //
    /// // [dependencies]
    /// // moka = { version = "0.9", features = ["future"] }
    /// // tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
    /// use moka::future::Cache;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let cache = Cache::new(100);
    ///     cache.insert("Julia", 14).await;
    ///
    ///     let mut iter = cache.iter();
    ///     let (k, v) = iter.next().unwrap(); // (Arc<K>, V)
    ///     assert_eq!(*k, "Julia");
    ///     assert_eq!(v, 14);
    ///
    ///     assert!(iter.next().is_none());
    /// }
    /// ```
    ///
    pub fn iter(&self) -> Iter<'_, K, V> {
        use crate::sync_base::iter::{Iter as InnerIter, ScanningGet};

        let inner = InnerIter::with_single_cache_segment(&self.base, self.base.num_cht_segments());
        Iter::new(inner)
    }

    /// Returns a `BlockingOp` for this cache. It provides blocking
    /// [`insert`](./struct.BlockingOp.html#method.insert) and
    /// [`invalidate`](struct.BlockingOp.html#method.invalidate) methods, which
    /// can be called outside of asynchronous contexts.
    pub fn blocking(&self) -> BlockingOp<'_, K, V, S> {
        BlockingOp(self)
    }
}

impl<'a, K, V, S> IntoIterator for &'a Cache<K, V, S>
where
    K: Hash + Eq + Send + Sync + 'static,
    V: Clone + Send + Sync + 'static,
    S: BuildHasher + Clone + Send + Sync + 'static,
{
    type Item = (Arc<K>, V);

    type IntoIter = Iter<'a, K, V>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

impl<K, V, S> ConcurrentCacheExt<K, V> for Cache<K, V, S>
where
    K: Hash + Eq + Send + Sync + 'static,
    V: Clone + Send + Sync + 'static,
    S: BuildHasher + Clone + Send + Sync + 'static,
{
    fn sync(&self) {
        self.base.inner.sync(MAX_SYNC_REPEATS);
    }
}

//
// private methods
//
impl<K, V, S> Cache<K, V, S>
where
    K: Hash + Eq + Send + Sync + 'static,
    V: Clone + Send + Sync + 'static,
    S: BuildHasher + Clone + Send + Sync + 'static,
{
    pub(crate) async fn get_or_insert_with_hash_and_fun(
        &self,
        key: Arc<K>,
        hash: u64,
        init: impl Future<Output = V>,
        mut replace_if: Option<impl FnMut(&V) -> bool>,
        need_key: bool,
    ) -> Entry<K, V> {
        let maybe_entry =
            self.base
                .get_with_hash_but_ignore_if(&key, hash, replace_if.as_mut(), need_key);
        if let Some(entry) = maybe_entry {
            entry
        } else {
            self.insert_with_hash_and_fun(key, hash, init, replace_if, need_key)
                .await
        }
    }

    pub(crate) async fn get_or_insert_with_hash_by_ref_and_fun<Q>(
        &self,
        key: &Q,
        hash: u64,
        init: impl Future<Output = V>,
        mut replace_if: Option<impl FnMut(&V) -> bool>,
        need_key: bool,
    ) -> Entry<K, V>
    where
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        let maybe_entry =
            self.base
                .get_with_hash_but_ignore_if(key, hash, replace_if.as_mut(), need_key);
        if let Some(entry) = maybe_entry {
            entry
        } else {
            let key = Arc::new(key.to_owned());
            self.insert_with_hash_and_fun(key, hash, init, replace_if, need_key)
                .await
        }
    }

    async fn insert_with_hash_and_fun(
        &self,
        key: Arc<K>,
        hash: u64,
        init: impl Future<Output = V>,
        mut replace_if: Option<impl FnMut(&V) -> bool>,
        need_key: bool,
    ) -> Entry<K, V> {
        use futures_util::FutureExt;

        let get = || {
            self.base
                .get_with_hash_but_no_recording(&key, hash, replace_if.as_mut())
        };
        let insert = |v| self.insert_with_hash(key.clone(), hash, v).boxed();

        let k = if need_key {
            Some(Arc::clone(&key))
        } else {
            None
        };

        match self
            .value_initializer
            .init_or_read(Arc::clone(&key), get, init, insert)
            .await
        {
            InitResult::Initialized(v) => {
                crossbeam_epoch::pin().flush();
                Entry::new(k, v, true)
            }
            InitResult::ReadExisting(v) => Entry::new(k, v, false),
            InitResult::InitErr(_) => unreachable!(),
        }
    }

    pub(crate) async fn get_or_insert_with_hash(
        &self,
        key: Arc<K>,
        hash: u64,
        init: impl FnOnce() -> V,
    ) -> Entry<K, V> {
        match self.base.get_with_hash(&key, hash, true) {
            Some(entry) => entry,
            None => {
                let value = init();
                self.insert_with_hash(Arc::clone(&key), hash, value.clone())
                    .await;
                Entry::new(Some(key), value, true)
            }
        }
    }

    pub(crate) async fn get_or_insert_with_hash_by_ref<Q>(
        &self,
        key: &Q,
        hash: u64,
        init: impl FnOnce() -> V,
    ) -> Entry<K, V>
    where
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        match self.base.get_with_hash(key, hash, true) {
            Some(entry) => entry,
            None => {
                let key = Arc::new(key.to_owned());
                let value = init();
                self.insert_with_hash(Arc::clone(&key), hash, value.clone())
                    .await;
                Entry::new(Some(key), value, true)
            }
        }
    }

    pub(crate) async fn get_or_optionally_insert_with_hash_and_fun<F>(
        &self,
        key: Arc<K>,
        hash: u64,
        init: F,
        need_key: bool,
    ) -> Option<Entry<K, V>>
    where
        F: Future<Output = Option<V>>,
    {
        let entry = self.base.get_with_hash(&key, hash, need_key);
        if entry.is_some() {
            return entry;
        }

        self.optionally_insert_with_hash_and_fun(key, hash, init, need_key)
            .await
    }

    pub(crate) async fn get_or_optionally_insert_with_hash_by_ref_and_fun<F, Q>(
        &self,
        key: &Q,
        hash: u64,
        init: F,
        need_key: bool,
    ) -> Option<Entry<K, V>>
    where
        F: Future<Output = Option<V>>,
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        let entry = self.base.get_with_hash(key, hash, need_key);
        if entry.is_some() {
            return entry;
        }

        let key = Arc::new(key.to_owned());
        self.optionally_insert_with_hash_and_fun(key, hash, init, need_key)
            .await
    }

    async fn optionally_insert_with_hash_and_fun<F>(
        &self,
        key: Arc<K>,
        hash: u64,
        init: F,
        need_key: bool,
    ) -> Option<Entry<K, V>>
    where
        F: Future<Output = Option<V>>,
    {
        use futures_util::FutureExt;

        let get = || {
            let ignore_if = None as Option<&mut fn(&V) -> bool>;
            self.base
                .get_with_hash_but_no_recording(&key, hash, ignore_if)
        };
        let insert = |v| self.insert_with_hash(key.clone(), hash, v).boxed();

        let k = if need_key {
            Some(Arc::clone(&key))
        } else {
            None
        };

        match self
            .value_initializer
            .optionally_init_or_read(Arc::clone(&key), get, init, insert)
            .await
        {
            InitResult::Initialized(v) => {
                crossbeam_epoch::pin().flush();
                Some(Entry::new(k, v, true))
            }
            InitResult::ReadExisting(v) => Some(Entry::new(k, v, false)),
            InitResult::InitErr(_) => None,
        }
    }

    pub(super) async fn get_or_try_insert_with_hash_and_fun<F, E>(
        &self,
        key: Arc<K>,
        hash: u64,
        init: F,
        need_key: bool,
    ) -> Result<Entry<K, V>, Arc<E>>
    where
        F: Future<Output = Result<V, E>>,
        E: Send + Sync + 'static,
    {
        if let Some(entry) = self.base.get_with_hash(&key, hash, need_key) {
            return Ok(entry);
        }

        self.try_insert_with_hash_and_fun(key, hash, init, need_key)
            .await
    }

    pub(super) async fn get_or_try_insert_with_hash_by_ref_and_fun<F, E, Q>(
        &self,
        key: &Q,
        hash: u64,
        init: F,
        need_key: bool,
    ) -> Result<Entry<K, V>, Arc<E>>
    where
        F: Future<Output = Result<V, E>>,
        E: Send + Sync + 'static,
        K: Borrow<Q>,
        Q: ToOwned<Owned = K> + Hash + Eq + ?Sized,
    {
        if let Some(entry) = self.base.get_with_hash(key, hash, need_key) {
            return Ok(entry);
        }
        let key = Arc::new(key.to_owned());
        self.try_insert_with_hash_and_fun(key, hash, init, need_key)
            .await
    }

    async fn try_insert_with_hash_and_fun<F, E>(
        &self,
        key: Arc<K>,
        hash: u64,
        init: F,
        need_key: bool,
    ) -> Result<Entry<K, V>, Arc<E>>
    where
        F: Future<Output = Result<V, E>>,
        E: Send + Sync + 'static,
    {
        use futures_util::FutureExt;

        let get = || {
            let ignore_if = None as Option<&mut fn(&V) -> bool>;
            self.base
                .get_with_hash_but_no_recording(&key, hash, ignore_if)
        };
        let insert = |v| self.insert_with_hash(key.clone(), hash, v).boxed();

        let k = if need_key {
            Some(Arc::clone(&key))
        } else {
            None
        };

        match self
            .value_initializer
            .try_init_or_read(Arc::clone(&key), get, init, insert)
            .await
        {
            InitResult::Initialized(v) => {
                crossbeam_epoch::pin().flush();
                Ok(Entry::new(k, v, true))
            }
            InitResult::ReadExisting(v) => Ok(Entry::new(k, v, false)),
            InitResult::InitErr(e) => {
                crossbeam_epoch::pin().flush();
                Err(e)
            }
        }
    }

    async fn insert_with_hash(&self, key: Arc<K>, hash: u64, value: V) {
        let (op, now) = self.base.do_insert_with_hash(key, hash, value);
        let hk = self.base.housekeeper.as_ref();
        Self::schedule_write_op(
            self.base.inner.as_ref(),
            &self.base.write_op_ch,
            op,
            now,
            hk,
        )
        .await
        .expect("Failed to insert");
    }

    #[inline]
    async fn schedule_write_op(
        inner: &impl InnerSync,
        ch: &Sender<WriteOp<K, V>>,
        op: WriteOp<K, V>,
        now: Instant,
        housekeeper: Option<&HouseKeeperArc<K, V, S>>,
    ) -> Result<(), TrySendError<WriteOp<K, V>>> {
        let mut op = op;

        // TODO: Try to replace the timer with an async event listener to see if it
        // can provide better performance.
        loop {
            BaseCache::apply_reads_writes_if_needed(inner, ch, now, housekeeper);
            match ch.try_send(op) {
                Ok(()) => break,
                Err(TrySendError::Full(op1)) => {
                    op = op1;
                    async_io::Timer::after(Duration::from_micros(WRITE_RETRY_INTERVAL_MICROS))
                        .await;
                }
                Err(e @ TrySendError::Disconnected(_)) => return Err(e),
            }
        }
        Ok(())
    }

    #[inline]
    fn blocking_schedule_write_op(
        inner: &impl InnerSync,
        ch: &Sender<WriteOp<K, V>>,
        op: WriteOp<K, V>,
        now: Instant,
        housekeeper: Option<&HouseKeeperArc<K, V, S>>,
    ) -> Result<(), TrySendError<WriteOp<K, V>>> {
        let mut op = op;

        loop {
            BaseCache::apply_reads_writes_if_needed(inner, ch, now, housekeeper);
            match ch.try_send(op) {
                Ok(()) => break,
                Err(TrySendError::Full(op1)) => {
                    op = op1;
                    std::thread::sleep(Duration::from_micros(WRITE_RETRY_INTERVAL_MICROS));
                }
                Err(e @ TrySendError::Disconnected(_)) => return Err(e),
            }
        }
        Ok(())
    }
}

// For unit tests.
#[cfg(test)]
impl<K, V, S> Cache<K, V, S>
where
    K: Hash + Eq + Send + Sync + 'static,
    V: Clone + Send + Sync + 'static,
    S: BuildHasher + Clone + Send + Sync + 'static,
{
    fn is_table_empty(&self) -> bool {
        self.entry_count() == 0
    }

    fn invalidation_predicate_count(&self) -> usize {
        self.base.invalidation_predicate_count()
    }

    fn reconfigure_for_testing(&mut self) {
        self.base.reconfigure_for_testing();
    }

    fn set_expiration_clock(&self, clock: Option<crate::common::time::Clock>) {
        self.base.set_expiration_clock(clock);
    }
}

pub struct BlockingOp<'a, K, V, S>(&'a Cache<K, V, S>);

impl<'a, K, V, S> BlockingOp<'a, K, V, S>
where
    K: Hash + Eq + Send + Sync + 'static,
    V: Clone + Send + Sync + 'static,
    S: BuildHasher + Clone + Send + Sync + 'static,
{
    /// Inserts a key-value pair into the cache. If the cache has this key present,
    /// the value is updated.
    ///
    /// This method is intended for use cases where you are inserting from
    /// synchronous code.
    pub fn insert(&self, key: K, value: V) {
        self.0.do_blocking_insert(key, value)
    }

    /// Discards any cached value for the key.
    ///
    /// This method is intended for use cases where you are invalidating from
    /// synchronous code.
    ///
    /// The key may be any borrowed form of the cache's key type, but `Hash` and `Eq`
    /// on the borrowed form _must_ match those for the key type.
    pub fn invalidate<Q>(&self, key: &Q)
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.0.do_blocking_invalidate(key)
    }
}

// To see the debug prints, run test as `cargo test -- --nocapture`
#[cfg(test)]
mod tests {
    use super::{Cache, ConcurrentCacheExt};
    use crate::{common::time::Clock, notification::RemovalCause};

    use async_io::Timer;
    use parking_lot::Mutex;
    use std::{convert::Infallible, sync::Arc, time::Duration};

    #[tokio::test]
    async fn basic_single_async_task() {
        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        // We use non-async mutex in the eviction listener (because the listener
        // is a regular closure).
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .max_capacity(3)
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert("a", "alice").await;
        cache.insert("b", "bob").await;
        assert_eq!(cache.get(&"a"), Some("alice"));
        assert!(cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));
        assert_eq!(cache.get(&"b"), Some("bob"));
        cache.sync();
        // counts: a -> 1, b -> 1

        cache.insert("c", "cindy").await;
        assert_eq!(cache.get(&"c"), Some("cindy"));
        assert!(cache.contains_key(&"c"));
        // counts: a -> 1, b -> 1, c -> 1
        cache.sync();

        assert!(cache.contains_key(&"a"));
        assert_eq!(cache.get(&"a"), Some("alice"));
        assert_eq!(cache.get(&"b"), Some("bob"));
        assert!(cache.contains_key(&"b"));
        cache.sync();
        // counts: a -> 2, b -> 2, c -> 1

        // "d" should not be admitted because its frequency is too low.
        cache.insert("d", "david").await; //   count: d -> 0
        expected.push((Arc::new("d"), "david", RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.get(&"d"), None); //   d -> 1
        assert!(!cache.contains_key(&"d"));

        cache.insert("d", "david").await;
        expected.push((Arc::new("d"), "david", RemovalCause::Size));
        cache.sync();
        assert!(!cache.contains_key(&"d"));
        assert_eq!(cache.get(&"d"), None); //   d -> 2

        // "d" should be admitted and "c" should be evicted
        // because d's frequency is higher than c's.
        cache.insert("d", "dennis").await;
        expected.push((Arc::new("c"), "cindy", RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.get(&"a"), Some("alice"));
        assert_eq!(cache.get(&"b"), Some("bob"));
        assert_eq!(cache.get(&"c"), None);
        assert_eq!(cache.get(&"d"), Some("dennis"));
        assert!(cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));
        assert!(!cache.contains_key(&"c"));
        assert!(cache.contains_key(&"d"));

        cache.invalidate(&"b").await;
        expected.push((Arc::new("b"), "bob", RemovalCause::Explicit));
        cache.sync();
        assert_eq!(cache.get(&"b"), None);
        assert!(!cache.contains_key(&"b"));

        verify_notification_vec(&cache, actual, &expected);
    }

    #[test]
    fn basic_single_blocking_api() {
        let mut cache = Cache::new(3);
        cache.reconfigure_for_testing();

        // Make the cache exterior immutable.
        let cache = cache;

        cache.blocking().insert("a", "alice");
        cache.blocking().insert("b", "bob");
        assert_eq!(cache.get(&"a"), Some("alice"));
        assert_eq!(cache.get(&"b"), Some("bob"));
        cache.sync();
        // counts: a -> 1, b -> 1

        cache.blocking().insert("c", "cindy");
        assert_eq!(cache.get(&"c"), Some("cindy"));
        // counts: a -> 1, b -> 1, c -> 1
        cache.sync();

        assert_eq!(cache.get(&"a"), Some("alice"));
        assert_eq!(cache.get(&"b"), Some("bob"));
        cache.sync();
        // counts: a -> 2, b -> 2, c -> 1

        // "d" should not be admitted because its frequency is too low.
        cache.blocking().insert("d", "david"); //   count: d -> 0
        cache.sync();
        assert_eq!(cache.get(&"d"), None); //   d -> 1

        cache.blocking().insert("d", "david");
        cache.sync();
        assert_eq!(cache.get(&"d"), None); //   d -> 2

        // "d" should be admitted and "c" should be evicted
        // because d's frequency is higher than c's.
        cache.blocking().insert("d", "dennis");
        cache.sync();
        assert_eq!(cache.get(&"a"), Some("alice"));
        assert_eq!(cache.get(&"b"), Some("bob"));
        assert_eq!(cache.get(&"c"), None);
        assert_eq!(cache.get(&"d"), Some("dennis"));

        cache.blocking().invalidate(&"b");
        assert_eq!(cache.get(&"b"), None);
    }

    #[tokio::test]
    async fn size_aware_eviction() {
        let weigher = |_k: &&str, v: &(&str, u32)| v.1;

        let alice = ("alice", 10);
        let bob = ("bob", 15);
        let bill = ("bill", 20);
        let cindy = ("cindy", 5);
        let david = ("david", 15);
        let dennis = ("dennis", 15);

        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .max_capacity(31)
            .weigher(weigher)
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert("a", alice).await;
        cache.insert("b", bob).await;
        assert_eq!(cache.get(&"a"), Some(alice));
        assert!(cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));
        assert_eq!(cache.get(&"b"), Some(bob));
        cache.sync();
        // order (LRU -> MRU) and counts: a -> 1, b -> 1

        cache.insert("c", cindy).await;
        assert_eq!(cache.get(&"c"), Some(cindy));
        assert!(cache.contains_key(&"c"));
        // order and counts: a -> 1, b -> 1, c -> 1
        cache.sync();

        assert!(cache.contains_key(&"a"));
        assert_eq!(cache.get(&"a"), Some(alice));
        assert_eq!(cache.get(&"b"), Some(bob));
        assert!(cache.contains_key(&"b"));
        cache.sync();
        // order and counts: c -> 1, a -> 2, b -> 2

        // To enter "d" (weight: 15), it needs to evict "c" (w: 5) and "a" (w: 10).
        // "d" must have higher count than 3, which is the aggregated count
        // of "a" and "c".
        cache.insert("d", david).await; //   count: d -> 0
        expected.push((Arc::new("d"), david, RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.get(&"d"), None); //   d -> 1
        assert!(!cache.contains_key(&"d"));

        cache.insert("d", david).await;
        expected.push((Arc::new("d"), david, RemovalCause::Size));
        cache.sync();
        assert!(!cache.contains_key(&"d"));
        assert_eq!(cache.get(&"d"), None); //   d -> 2

        cache.insert("d", david).await;
        expected.push((Arc::new("d"), david, RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.get(&"d"), None); //   d -> 3
        assert!(!cache.contains_key(&"d"));

        cache.insert("d", david).await;
        expected.push((Arc::new("d"), david, RemovalCause::Size));
        cache.sync();
        assert!(!cache.contains_key(&"d"));
        assert_eq!(cache.get(&"d"), None); //   d -> 4

        // Finally "d" should be admitted by evicting "c" and "a".
        cache.insert("d", dennis).await;
        expected.push((Arc::new("c"), cindy, RemovalCause::Size));
        expected.push((Arc::new("a"), alice, RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.get(&"a"), None);
        assert_eq!(cache.get(&"b"), Some(bob));
        assert_eq!(cache.get(&"c"), None);
        assert_eq!(cache.get(&"d"), Some(dennis));
        assert!(!cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));
        assert!(!cache.contains_key(&"c"));
        assert!(cache.contains_key(&"d"));

        // Update "b" with "bill" (w: 15 -> 20). This should evict "d" (w: 15).
        cache.insert("b", bill).await;
        expected.push((Arc::new("b"), bob, RemovalCause::Replaced));
        expected.push((Arc::new("d"), dennis, RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.get(&"b"), Some(bill));
        assert_eq!(cache.get(&"d"), None);
        assert!(cache.contains_key(&"b"));
        assert!(!cache.contains_key(&"d"));

        // Re-add "a" (w: 10) and update "b" with "bob" (w: 20 -> 15).
        cache.insert("a", alice).await;
        cache.insert("b", bob).await;
        expected.push((Arc::new("b"), bill, RemovalCause::Replaced));
        cache.sync();
        assert_eq!(cache.get(&"a"), Some(alice));
        assert_eq!(cache.get(&"b"), Some(bob));
        assert_eq!(cache.get(&"d"), None);
        assert!(cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));
        assert!(!cache.contains_key(&"d"));

        // Verify the sizes.
        assert_eq!(cache.entry_count(), 2);
        assert_eq!(cache.weighted_size(), 25);

        verify_notification_vec(&cache, actual, &expected);
    }

    #[tokio::test]
    async fn basic_multi_async_tasks() {
        let num_tasks = 4;
        let cache = Cache::new(100);

        let tasks = (0..num_tasks)
            .map(|id| {
                let cache = cache.clone();
                if id == 0 {
                    tokio::spawn(async move {
                        cache.blocking().insert(10, format!("{}-100", id));
                        cache.get(&10);
                        cache.blocking().insert(20, format!("{}-200", id));
                        cache.blocking().invalidate(&10);
                    })
                } else {
                    tokio::spawn(async move {
                        cache.insert(10, format!("{}-100", id)).await;
                        cache.get(&10);
                        cache.insert(20, format!("{}-200", id)).await;
                        cache.invalidate(&10).await;
                    })
                }
            })
            .collect::<Vec<_>>();

        let _ = futures_util::future::join_all(tasks).await;

        assert!(cache.get(&10).is_none());
        assert!(cache.get(&20).is_some());
        assert!(!cache.contains_key(&10));
        assert!(cache.contains_key(&20));
    }

    #[tokio::test]
    async fn invalidate_all() {
        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .max_capacity(100)
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert("a", "alice").await;
        cache.insert("b", "bob").await;
        cache.insert("c", "cindy").await;
        assert_eq!(cache.get(&"a"), Some("alice"));
        assert_eq!(cache.get(&"b"), Some("bob"));
        assert_eq!(cache.get(&"c"), Some("cindy"));
        assert!(cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));
        assert!(cache.contains_key(&"c"));

        // `cache.sync()` is no longer needed here before invalidating. The last
        // modified timestamp of the entries were updated when they were inserted.
        // https://github.com/moka-rs/moka/issues/155

        cache.invalidate_all();
        expected.push((Arc::new("a"), "alice", RemovalCause::Explicit));
        expected.push((Arc::new("b"), "bob", RemovalCause::Explicit));
        expected.push((Arc::new("c"), "cindy", RemovalCause::Explicit));
        cache.sync();

        cache.insert("d", "david").await;
        cache.sync();

        assert!(cache.get(&"a").is_none());
        assert!(cache.get(&"b").is_none());
        assert!(cache.get(&"c").is_none());
        assert_eq!(cache.get(&"d"), Some("david"));
        assert!(!cache.contains_key(&"a"));
        assert!(!cache.contains_key(&"b"));
        assert!(!cache.contains_key(&"c"));
        assert!(cache.contains_key(&"d"));

        verify_notification_vec(&cache, actual, &expected);
    }

    // This test is for https://github.com/moka-rs/moka/issues/155
    #[tokio::test]
    async fn invalidate_all_without_sync() {
        let cache = Cache::new(1024);

        assert_eq!(cache.get(&0), None);
        cache.insert(0, 1).await;
        assert_eq!(cache.get(&0), Some(1));

        cache.invalidate_all();
        assert_eq!(cache.get(&0), None);
    }

    #[tokio::test]
    async fn invalidate_entries_if() -> Result<(), Box<dyn std::error::Error>> {
        use std::collections::HashSet;

        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .max_capacity(100)
            .support_invalidation_closures()
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        let (clock, mock) = Clock::mock();
        cache.set_expiration_clock(Some(clock));

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert(0, "alice").await;
        cache.insert(1, "bob").await;
        cache.insert(2, "alex").await;
        cache.sync();

        mock.increment(Duration::from_secs(5)); // 5 secs from the start.
        cache.sync();

        assert_eq!(cache.get(&0), Some("alice"));
        assert_eq!(cache.get(&1), Some("bob"));
        assert_eq!(cache.get(&2), Some("alex"));
        assert!(cache.contains_key(&0));
        assert!(cache.contains_key(&1));
        assert!(cache.contains_key(&2));

        let names = ["alice", "alex"].iter().cloned().collect::<HashSet<_>>();
        cache.invalidate_entries_if(move |_k, &v| names.contains(v))?;
        assert_eq!(cache.invalidation_predicate_count(), 1);
        expected.push((Arc::new(0), "alice", RemovalCause::Explicit));
        expected.push((Arc::new(2), "alex", RemovalCause::Explicit));

        mock.increment(Duration::from_secs(5)); // 10 secs from the start.

        cache.insert(3, "alice").await;

        // Run the invalidation task and wait for it to finish. (TODO: Need a better way than sleeping)
        cache.sync(); // To submit the invalidation task.
        std::thread::sleep(Duration::from_millis(200));
        cache.sync(); // To process the task result.
        std::thread::sleep(Duration::from_millis(200));

        assert!(cache.get(&0).is_none());
        assert!(cache.get(&2).is_none());
        assert_eq!(cache.get(&1), Some("bob"));
        // This should survive as it was inserted after calling invalidate_entries_if.
        assert_eq!(cache.get(&3), Some("alice"));

        assert!(!cache.contains_key(&0));
        assert!(cache.contains_key(&1));
        assert!(!cache.contains_key(&2));
        assert!(cache.contains_key(&3));

        assert_eq!(cache.entry_count(), 2);
        assert_eq!(cache.invalidation_predicate_count(), 0);

        mock.increment(Duration::from_secs(5)); // 15 secs from the start.

        cache.invalidate_entries_if(|_k, &v| v == "alice")?;
        cache.invalidate_entries_if(|_k, &v| v == "bob")?;
        assert_eq!(cache.invalidation_predicate_count(), 2);
        // key 1 was inserted before key 3.
        expected.push((Arc::new(1), "bob", RemovalCause::Explicit));
        expected.push((Arc::new(3), "alice", RemovalCause::Explicit));

        // Run the invalidation task and wait for it to finish. (TODO: Need a better way than sleeping)
        cache.sync(); // To submit the invalidation task.
        std::thread::sleep(Duration::from_millis(200));
        cache.sync(); // To process the task result.
        std::thread::sleep(Duration::from_millis(200));

        assert!(cache.get(&1).is_none());
        assert!(cache.get(&3).is_none());

        assert!(!cache.contains_key(&1));
        assert!(!cache.contains_key(&3));

        assert_eq!(cache.entry_count(), 0);
        assert_eq!(cache.invalidation_predicate_count(), 0);

        verify_notification_vec(&cache, actual, &expected);

        Ok(())
    }

    #[tokio::test]
    async fn time_to_live() {
        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .max_capacity(100)
            .time_to_live(Duration::from_secs(10))
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        let (clock, mock) = Clock::mock();
        cache.set_expiration_clock(Some(clock));

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert("a", "alice").await;
        cache.sync();

        mock.increment(Duration::from_secs(5)); // 5 secs from the start.
        cache.sync();

        assert_eq!(cache.get(&"a"), Some("alice"));
        assert!(cache.contains_key(&"a"));

        mock.increment(Duration::from_secs(5)); // 10 secs.
        expected.push((Arc::new("a"), "alice", RemovalCause::Expired));
        assert_eq!(cache.get(&"a"), None);
        assert!(!cache.contains_key(&"a"));

        assert_eq!(cache.iter().count(), 0);

        cache.sync();
        assert!(cache.is_table_empty());

        cache.insert("b", "bob").await;
        cache.sync();

        assert_eq!(cache.entry_count(), 1);

        mock.increment(Duration::from_secs(5)); // 15 secs.
        cache.sync();

        assert_eq!(cache.get(&"b"), Some("bob"));
        assert!(cache.contains_key(&"b"));
        assert_eq!(cache.entry_count(), 1);

        cache.insert("b", "bill").await;
        expected.push((Arc::new("b"), "bob", RemovalCause::Replaced));
        cache.sync();

        mock.increment(Duration::from_secs(5)); // 20 secs
        cache.sync();

        assert_eq!(cache.get(&"b"), Some("bill"));
        assert!(cache.contains_key(&"b"));
        assert_eq!(cache.entry_count(), 1);

        mock.increment(Duration::from_secs(5)); // 25 secs
        expected.push((Arc::new("b"), "bill", RemovalCause::Expired));

        assert_eq!(cache.get(&"a"), None);
        assert_eq!(cache.get(&"b"), None);
        assert!(!cache.contains_key(&"a"));
        assert!(!cache.contains_key(&"b"));

        assert_eq!(cache.iter().count(), 0);

        cache.sync();
        assert!(cache.is_table_empty());

        verify_notification_vec(&cache, actual, &expected);
    }

    #[tokio::test]
    async fn time_to_idle() {
        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .max_capacity(100)
            .time_to_idle(Duration::from_secs(10))
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        let (clock, mock) = Clock::mock();
        cache.set_expiration_clock(Some(clock));

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert("a", "alice").await;
        cache.sync();

        mock.increment(Duration::from_secs(5)); // 5 secs from the start.
        cache.sync();

        assert_eq!(cache.get(&"a"), Some("alice"));

        mock.increment(Duration::from_secs(5)); // 10 secs.
        cache.sync();

        cache.insert("b", "bob").await;
        cache.sync();

        assert_eq!(cache.entry_count(), 2);

        mock.increment(Duration::from_secs(2)); // 12 secs.
        cache.sync();

        // contains_key does not reset the idle timer for the key.
        assert!(cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));
        cache.sync();

        assert_eq!(cache.entry_count(), 2);

        mock.increment(Duration::from_secs(3)); // 15 secs.
        expected.push((Arc::new("a"), "alice", RemovalCause::Expired));

        assert_eq!(cache.get(&"a"), None);
        assert_eq!(cache.get(&"b"), Some("bob"));
        assert!(!cache.contains_key(&"a"));
        assert!(cache.contains_key(&"b"));

        assert_eq!(cache.iter().count(), 1);

        cache.sync();
        assert_eq!(cache.entry_count(), 1);

        mock.increment(Duration::from_secs(10)); // 25 secs
        expected.push((Arc::new("b"), "bob", RemovalCause::Expired));

        assert_eq!(cache.get(&"a"), None);
        assert_eq!(cache.get(&"b"), None);
        assert!(!cache.contains_key(&"a"));
        assert!(!cache.contains_key(&"b"));

        assert_eq!(cache.iter().count(), 0);

        cache.sync();
        assert!(cache.is_table_empty());

        verify_notification_vec(&cache, actual, &expected);
    }

    #[tokio::test]
    async fn test_iter() {
        const NUM_KEYS: usize = 50;

        fn make_value(key: usize) -> String {
            format!("val: {}", key)
        }

        let cache = Cache::builder()
            .max_capacity(100)
            .time_to_idle(Duration::from_secs(10))
            .build();

        for key in 0..NUM_KEYS {
            cache.insert(key, make_value(key)).await;
        }

        let mut key_set = std::collections::HashSet::new();

        for (key, value) in &cache {
            assert_eq!(value, make_value(*key));

            key_set.insert(*key);
        }

        // Ensure there are no missing or duplicate keys in the iteration.
        assert_eq!(key_set.len(), NUM_KEYS);
    }

    /// Runs 16 async tasks at the same time and ensures no deadlock occurs.
    ///
    /// - Eight of the task will update key-values in the cache.
    /// - Eight others will iterate the cache.
    ///
    #[tokio::test]
    async fn test_iter_multi_async_tasks() {
        use std::collections::HashSet;

        const NUM_KEYS: usize = 1024;
        const NUM_TASKS: usize = 16;

        fn make_value(key: usize) -> String {
            format!("val: {}", key)
        }

        let cache = Cache::builder()
            .max_capacity(2048)
            .time_to_idle(Duration::from_secs(10))
            .build();

        // Initialize the cache.
        for key in 0..NUM_KEYS {
            cache.insert(key, make_value(key)).await;
        }

        let rw_lock = Arc::new(tokio::sync::RwLock::<()>::default());
        let write_lock = rw_lock.write().await;

        let tasks = (0..NUM_TASKS)
            .map(|n| {
                let cache = cache.clone();
                let rw_lock = Arc::clone(&rw_lock);

                if n % 2 == 0 {
                    // This thread will update the cache.
                    tokio::spawn(async move {
                        let read_lock = rw_lock.read().await;
                        for key in 0..NUM_KEYS {
                            // TODO: Update keys in a random order?
                            cache.insert(key, make_value(key)).await;
                        }
                        std::mem::drop(read_lock);
                    })
                } else {
                    // This thread will iterate the cache.
                    tokio::spawn(async move {
                        let read_lock = rw_lock.read().await;
                        let mut key_set = HashSet::new();
                        // let mut key_count = 0usize;
                        for (key, value) in &cache {
                            assert_eq!(value, make_value(*key));
                            key_set.insert(*key);
                            // key_count += 1;
                        }
                        // Ensure there are no missing or duplicate keys in the iteration.
                        assert_eq!(key_set.len(), NUM_KEYS);
                        std::mem::drop(read_lock);
                    })
                }
            })
            .collect::<Vec<_>>();

        // Let these threads to run by releasing the write lock.
        std::mem::drop(write_lock);

        let _ = futures_util::future::join_all(tasks).await;

        // Ensure there are no missing or duplicate keys in the iteration.
        let key_set = cache.iter().map(|(k, _v)| *k).collect::<HashSet<_>>();
        assert_eq!(key_set.len(), NUM_KEYS);
    }

    #[tokio::test]
    async fn get_with() {
        let cache = Cache::new(100);
        const KEY: u32 = 0;

        // This test will run five async tasks:
        //
        // Task1 will be the first task to call `get_with` for a key, so its async
        // block will be evaluated and then a &str value "task1" will be inserted to
        // the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `get_with` immediately.
                let v = cache1
                    .get_with(KEY, async {
                        // Wait for 300 ms and return a &str value.
                        Timer::after(Duration::from_millis(300)).await;
                        "task1"
                    })
                    .await;
                assert_eq!(v, "task1");
            }
        };

        // Task2 will be the second task to call `get_with` for the same key, so its
        // async block will not be evaluated. Once task1's async block finishes, it
        // will get the value inserted by task1's async block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `get_with`.
                Timer::after(Duration::from_millis(100)).await;
                let v = cache2.get_with(KEY, async { unreachable!() }).await;
                assert_eq!(v, "task1");
            }
        };

        // Task3 will be the third task to call `get_with` for the same key. By the
        // time it calls, task1's async block should have finished already and the
        // value should be already inserted to the cache. So its async block will not
        // be evaluated and will get the value inserted by task1's async block
        // immediately.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get_with`.
                Timer::after(Duration::from_millis(400)).await;
                let v = cache3.get_with(KEY, async { unreachable!() }).await;
                assert_eq!(v, "task1");
            }
        };

        // Task4 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache4.get(&KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task5 will call `get` for the same key. It will call after task1's async
        // block finished, so it will get the value insert by task1's async block.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get`.
                Timer::after(Duration::from_millis(400)).await;
                let maybe_v = cache5.get(&KEY);
                assert_eq!(maybe_v, Some("task1"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5);
    }

    #[tokio::test]
    async fn get_with_by_ref() {
        let cache = Cache::new(100);
        const KEY: &u32 = &0;

        // This test will run five async tasks:
        //
        // Task1 will be the first task to call `get_with_by_ref` for a key, so its async
        // block will be evaluated and then a &str value "task1" will be inserted to
        // the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `get_with_by_ref` immediately.
                let v = cache1
                    .get_with_by_ref(KEY, async {
                        // Wait for 300 ms and return a &str value.
                        Timer::after(Duration::from_millis(300)).await;
                        "task1"
                    })
                    .await;
                assert_eq!(v, "task1");
            }
        };

        // Task2 will be the second task to call `get_with_by_ref` for the same key, so its
        // async block will not be evaluated. Once task1's async block finishes, it
        // will get the value inserted by task1's async block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `get_with_by_ref`.
                Timer::after(Duration::from_millis(100)).await;
                let v = cache2.get_with_by_ref(KEY, async { unreachable!() }).await;
                assert_eq!(v, "task1");
            }
        };

        // Task3 will be the third task to call `get_with_by_ref` for the same key. By the
        // time it calls, task1's async block should have finished already and the
        // value should be already inserted to the cache. So its async block will not
        // be evaluated and will get the value inserted by task1's async block
        // immediately.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get_with_by_ref`.
                Timer::after(Duration::from_millis(400)).await;
                let v = cache3.get_with_by_ref(KEY, async { unreachable!() }).await;
                assert_eq!(v, "task1");
            }
        };

        // Task4 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache4.get(KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task5 will call `get` for the same key. It will call after task1's async
        // block finished, so it will get the value insert by task1's async block.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get`.
                Timer::after(Duration::from_millis(400)).await;
                let maybe_v = cache5.get(KEY);
                assert_eq!(maybe_v, Some("task1"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5);
    }

    #[tokio::test]
    async fn entry_or_insert_with_if() {
        let cache = Cache::new(100);
        const KEY: u32 = 0;

        // This test will run seven async tasks:
        //
        // Task1 will be the first task to call `or_insert_with_if` for a key, so its
        // async block will be evaluated and then a &str value "task1" will be
        // inserted to the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `or_insert_with_if` immediately.
                let v = cache1
                    .entry(KEY)
                    .or_insert_with_if(
                        async {
                            // Wait for 300 ms and return a &str value.
                            Timer::after(Duration::from_millis(300)).await;
                            "task1"
                        },
                        |_v| unreachable!(),
                    )
                    .await
                    .into_value();
                assert_eq!(v, "task1");
            }
        };

        // Task2 will be the second task to call `or_insert_with_if` for the same
        // key, so its async block will not be evaluated. Once task1's async block
        // finishes, it will get the value inserted by task1's async block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `or_insert_with_if`.
                Timer::after(Duration::from_millis(100)).await;
                let v = cache2
                    .entry(KEY)
                    .or_insert_with_if(async { unreachable!() }, |_v| unreachable!())
                    .await
                    .into_value();
                assert_eq!(v, "task1");
            }
        };

        // Task3 will be the third task to call `or_insert_with_if` for the same key.
        // By the time it calls, task1's async block should have finished already and
        // the value should be already inserted to the cache. Also task3's
        // `replace_if` closure returns `false`. So its async block will not be
        // evaluated and will get the value inserted by task1's async block
        // immediately.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 350 ms before calling `or_insert_with_if`.
                Timer::after(Duration::from_millis(350)).await;
                let v = cache3
                    .entry(KEY)
                    .or_insert_with_if(async { unreachable!() }, |v| {
                        assert_eq!(v, &"task1");
                        false
                    })
                    .await
                    .into_value();
                assert_eq!(v, "task1");
            }
        };

        // Task4 will be the fourth task to call `or_insert_with_if` for the same
        // key. The value should have been already inserted to the cache by task1.
        // However task4's `replace_if` closure returns `true`. So its async block
        // will be evaluated to replace the current value.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 400 ms before calling `or_insert_with_if`.
                Timer::after(Duration::from_millis(400)).await;
                let v = cache4
                    .entry(KEY)
                    .or_insert_with_if(async { "task4" }, |v| {
                        assert_eq!(v, &"task1");
                        true
                    })
                    .await
                    .into_value();
                assert_eq!(v, "task4");
            }
        };

        // Task5 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache5.get(&KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task6 will call `get` for the same key. It will call after task1's async
        // block finished, so it will get the value insert by task1's async block.
        let task6 = {
            let cache6 = cache.clone();
            async move {
                // Wait for 350 ms before calling `get`.
                Timer::after(Duration::from_millis(350)).await;
                let maybe_v = cache6.get(&KEY);
                assert_eq!(maybe_v, Some("task1"));
            }
        };

        // Task7 will call `get` for the same key. It will call after task4's async
        // block finished, so it will get the value insert by task4's async block.
        let task7 = {
            let cache7 = cache.clone();
            async move {
                // Wait for 450 ms before calling `get`.
                Timer::after(Duration::from_millis(450)).await;
                let maybe_v = cache7.get(&KEY);
                assert_eq!(maybe_v, Some("task4"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5, task6, task7);
    }

    #[tokio::test]
    async fn entry_by_ref_or_insert_with_if() {
        let cache = Cache::new(100);
        const KEY: &u32 = &0;

        // This test will run seven async tasks:
        //
        // Task1 will be the first task to call `or_insert_with_if` for a key, so its
        // async block will be evaluated and then a &str value "task1" will be
        // inserted to the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `or_insert_with_if` immediately.
                let v = cache1
                    .entry_by_ref(KEY)
                    .or_insert_with_if(
                        async {
                            // Wait for 300 ms and return a &str value.
                            Timer::after(Duration::from_millis(300)).await;
                            "task1"
                        },
                        |_v| unreachable!(),
                    )
                    .await
                    .into_value();
                assert_eq!(v, "task1");
            }
        };

        // Task2 will be the second task to call `or_insert_with_if` for the same
        // key, so its async block will not be evaluated. Once task1's async block
        // finishes, it will get the value inserted by task1's async block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `or_insert_with_if`.
                Timer::after(Duration::from_millis(100)).await;
                let v = cache2
                    .entry_by_ref(KEY)
                    .or_insert_with_if(async { unreachable!() }, |_v| unreachable!())
                    .await
                    .into_value();
                assert_eq!(v, "task1");
            }
        };

        // Task3 will be the third task to call `or_insert_with_if` for the same key.
        // By the time it calls, task1's async block should have finished already and
        // the value should be already inserted to the cache. Also task3's
        // `replace_if` closure returns `false`. So its async block will not be
        // evaluated and will get the value inserted by task1's async block
        // immediately.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 350 ms before calling `or_insert_with_if`.
                Timer::after(Duration::from_millis(350)).await;
                let v = cache3
                    .entry_by_ref(KEY)
                    .or_insert_with_if(async { unreachable!() }, |v| {
                        assert_eq!(v, &"task1");
                        false
                    })
                    .await
                    .into_value();
                assert_eq!(v, "task1");
            }
        };

        // Task4 will be the fourth task to call `or_insert_with_if` for the same
        // key. The value should have been already inserted to the cache by task1.
        // However task4's `replace_if` closure returns `true`. So its async block
        // will be evaluated to replace the current value.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 400 ms before calling `or_insert_with_if`.
                Timer::after(Duration::from_millis(400)).await;
                let v = cache4
                    .entry_by_ref(KEY)
                    .or_insert_with_if(async { "task4" }, |v| {
                        assert_eq!(v, &"task1");
                        true
                    })
                    .await
                    .into_value();
                assert_eq!(v, "task4");
            }
        };

        // Task5 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache5.get(KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task6 will call `get` for the same key. It will call after task1's async
        // block finished, so it will get the value insert by task1's async block.
        let task6 = {
            let cache6 = cache.clone();
            async move {
                // Wait for 350 ms before calling `get`.
                Timer::after(Duration::from_millis(350)).await;
                let maybe_v = cache6.get(KEY);
                assert_eq!(maybe_v, Some("task1"));
            }
        };

        // Task7 will call `get` for the same key. It will call after task4's async
        // block finished, so it will get the value insert by task4's async block.
        let task7 = {
            let cache7 = cache.clone();
            async move {
                // Wait for 450 ms before calling `get`.
                Timer::after(Duration::from_millis(450)).await;
                let maybe_v = cache7.get(KEY);
                assert_eq!(maybe_v, Some("task4"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5, task6, task7);
    }

    #[tokio::test]
    async fn try_get_with() {
        use std::sync::Arc;

        // Note that MyError does not implement std::error::Error trait
        // like anyhow::Error.
        #[derive(Debug)]
        pub struct MyError(String);

        type MyResult<T> = Result<T, Arc<MyError>>;

        let cache = Cache::new(100);
        const KEY: u32 = 0;

        // This test will run eight async tasks:
        //
        // Task1 will be the first task to call `get_with` for a key, so its async
        // block will be evaluated and then an error will be returned. Nothing will
        // be inserted to the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `try_get_with` immediately.
                let v = cache1
                    .try_get_with(KEY, async {
                        // Wait for 300 ms and return an error.
                        Timer::after(Duration::from_millis(300)).await;
                        Err(MyError("task1 error".into()))
                    })
                    .await;
                assert!(v.is_err());
            }
        };

        // Task2 will be the second task to call `get_with` for the same key, so its
        // async block will not be evaluated. Once task1's async block finishes, it
        // will get the same error value returned by task1's async block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `try_get_with`.
                Timer::after(Duration::from_millis(100)).await;
                let v: MyResult<_> = cache2.try_get_with(KEY, async { unreachable!() }).await;
                assert!(v.is_err());
            }
        };

        // Task3 will be the third task to call `get_with` for the same key. By the
        // time it calls, task1's async block should have finished already, but the
        // key still does not exist in the cache. So its async block will be
        // evaluated and then an okay &str value will be returned. That value will be
        // inserted to the cache.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 400 ms before calling `try_get_with`.
                Timer::after(Duration::from_millis(400)).await;
                let v: MyResult<_> = cache3
                    .try_get_with(KEY, async {
                        // Wait for 300 ms and return an Ok(&str) value.
                        Timer::after(Duration::from_millis(300)).await;
                        Ok("task3")
                    })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task4 will be the fourth task to call `get_with` for the same key. So its
        // async block will not be evaluated. Once task3's async block finishes, it
        // will get the same okay &str value.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 500 ms before calling `try_get_with`.
                Timer::after(Duration::from_millis(500)).await;
                let v: MyResult<_> = cache4.try_get_with(KEY, async { unreachable!() }).await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task5 will be the fifth task to call `get_with` for the same key. So its
        // async block will not be evaluated. By the time it calls, task3's async
        // block should have finished already, so its async block will not be
        // evaluated and will get the value insert by task3's async block
        // immediately.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 800 ms before calling `try_get_with`.
                Timer::after(Duration::from_millis(800)).await;
                let v: MyResult<_> = cache5.try_get_with(KEY, async { unreachable!() }).await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task6 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task6 = {
            let cache6 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache6.get(&KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task7 will call `get` for the same key. It will call after task1's async
        // block finished with an error. So it will get none for the key.
        let task7 = {
            let cache7 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get`.
                Timer::after(Duration::from_millis(400)).await;
                let maybe_v = cache7.get(&KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task8 will call `get` for the same key. It will call after task3's async
        // block finished, so it will get the value insert by task3's async block.
        let task8 = {
            let cache8 = cache.clone();
            async move {
                // Wait for 800 ms before calling `get`.
                Timer::after(Duration::from_millis(800)).await;
                let maybe_v = cache8.get(&KEY);
                assert_eq!(maybe_v, Some("task3"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5, task6, task7, task8);
    }

    #[tokio::test]
    async fn try_get_with_by_ref() {
        use std::sync::Arc;

        // Note that MyError does not implement std::error::Error trait
        // like anyhow::Error.
        #[derive(Debug)]
        pub struct MyError(String);

        type MyResult<T> = Result<T, Arc<MyError>>;

        let cache = Cache::new(100);
        const KEY: &u32 = &0;

        // This test will run eight async tasks:
        //
        // Task1 will be the first task to call `try_get_with_by_ref` for a key, so
        // its async block will be evaluated and then an error will be returned.
        // Nothing will be inserted to the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `try_get_with_by_ref` immediately.
                let v = cache1
                    .try_get_with_by_ref(KEY, async {
                        // Wait for 300 ms and return an error.
                        Timer::after(Duration::from_millis(300)).await;
                        Err(MyError("task1 error".into()))
                    })
                    .await;
                assert!(v.is_err());
            }
        };

        // Task2 will be the second task to call `get_with` for the same key, so its
        // async block will not be evaluated. Once task1's async block finishes, it
        // will get the same error value returned by task1's async block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `try_get_with_by_ref`.
                Timer::after(Duration::from_millis(100)).await;
                let v: MyResult<_> = cache2
                    .try_get_with_by_ref(KEY, async { unreachable!() })
                    .await;
                assert!(v.is_err());
            }
        };

        // Task3 will be the third task to call `get_with` for the same key. By the
        // time it calls, task1's async block should have finished already, but the
        // key still does not exist in the cache. So its async block will be
        // evaluated and then an okay &str value will be returned. That value will be
        // inserted to the cache.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 400 ms before calling `try_get_with_by_ref`.
                Timer::after(Duration::from_millis(400)).await;
                let v: MyResult<_> = cache3
                    .try_get_with_by_ref(KEY, async {
                        // Wait for 300 ms and return an Ok(&str) value.
                        Timer::after(Duration::from_millis(300)).await;
                        Ok("task3")
                    })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task4 will be the fourth task to call `get_with` for the same key. So its
        // async block will not be evaluated. Once task3's async block finishes, it
        // will get the same okay &str value.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 500 ms before calling `try_get_with_by_ref`.
                Timer::after(Duration::from_millis(500)).await;
                let v: MyResult<_> = cache4
                    .try_get_with_by_ref(KEY, async { unreachable!() })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task5 will be the fifth task to call `get_with` for the same key. So its
        // async block will not be evaluated. By the time it calls, task3's async
        // block should have finished already, so its async block will not be
        // evaluated and will get the value insert by task3's async block
        // immediately.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 800 ms before calling `try_get_with_by_ref`.
                Timer::after(Duration::from_millis(800)).await;
                let v: MyResult<_> = cache5
                    .try_get_with_by_ref(KEY, async { unreachable!() })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task6 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task6 = {
            let cache6 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache6.get(KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task7 will call `get` for the same key. It will call after task1's async
        // block finished with an error. So it will get none for the key.
        let task7 = {
            let cache7 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get`.
                Timer::after(Duration::from_millis(400)).await;
                let maybe_v = cache7.get(KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task8 will call `get` for the same key. It will call after task3's async
        // block finished, so it will get the value insert by task3's async block.
        let task8 = {
            let cache8 = cache.clone();
            async move {
                // Wait for 800 ms before calling `get`.
                Timer::after(Duration::from_millis(800)).await;
                let maybe_v = cache8.get(KEY);
                assert_eq!(maybe_v, Some("task3"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5, task6, task7, task8);
    }

    #[tokio::test]
    async fn optionally_get_with() {
        let cache = Cache::new(100);
        const KEY: u32 = 0;

        // This test will run eight async tasks:
        //
        // Task1 will be the first task to call `optionally_get_with` for a key,
        // so its async block will be evaluated and then an None will be
        // returned. Nothing will be inserted to the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `try_get_with` immediately.
                let v = cache1
                    .optionally_get_with(KEY, async {
                        // Wait for 300 ms and return an None.
                        Timer::after(Duration::from_millis(300)).await;
                        None
                    })
                    .await;
                assert!(v.is_none());
            }
        };

        // Task2 will be the second task to call `optionally_get_with` for the same
        // key, so its async block will not be evaluated. Once task1's async block
        // finishes, it will get the same error value returned by task1's async
        // block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `optionally_get_with`.
                Timer::after(Duration::from_millis(100)).await;
                let v = cache2
                    .optionally_get_with(KEY, async { unreachable!() })
                    .await;
                assert!(v.is_none());
            }
        };

        // Task3 will be the third task to call `optionally_get_with` for the
        // same key. By the time it calls, task1's async block should have
        // finished already, but the key still does not exist in the cache. So
        // its async block will be evaluated and then an okay &str value will be
        // returned. That value will be inserted to the cache.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 400 ms before calling `optionally_get_with`.
                Timer::after(Duration::from_millis(400)).await;
                let v = cache3
                    .optionally_get_with(KEY, async {
                        // Wait for 300 ms and return an Some(&str) value.
                        Timer::after(Duration::from_millis(300)).await;
                        Some("task3")
                    })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task4 will be the fourth task to call `optionally_get_with` for the
        // same key. So its async block will not be evaluated. Once task3's
        // async block finishes, it will get the same okay &str value.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 500 ms before calling `try_get_with`.
                Timer::after(Duration::from_millis(500)).await;
                let v = cache4
                    .optionally_get_with(KEY, async { unreachable!() })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task5 will be the fifth task to call `optionally_get_with` for the
        // same key. So its async block will not be evaluated. By the time it
        // calls, task3's async block should have finished already, so its async
        // block will not be evaluated and will get the value insert by task3's
        // async block immediately.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 800 ms before calling `optionally_get_with`.
                Timer::after(Duration::from_millis(800)).await;
                let v = cache5
                    .optionally_get_with(KEY, async { unreachable!() })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task6 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task6 = {
            let cache6 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache6.get(&KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task7 will call `get` for the same key. It will call after task1's async
        // block finished with an error. So it will get none for the key.
        let task7 = {
            let cache7 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get`.
                Timer::after(Duration::from_millis(400)).await;
                let maybe_v = cache7.get(&KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task8 will call `get` for the same key. It will call after task3's async
        // block finished, so it will get the value insert by task3's async block.
        let task8 = {
            let cache8 = cache.clone();
            async move {
                // Wait for 800 ms before calling `get`.
                Timer::after(Duration::from_millis(800)).await;
                let maybe_v = cache8.get(&KEY);
                assert_eq!(maybe_v, Some("task3"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5, task6, task7, task8);
    }

    #[tokio::test]
    async fn optionally_get_with_by_ref() {
        let cache = Cache::new(100);
        const KEY: &u32 = &0;

        // This test will run eight async tasks:
        //
        // Task1 will be the first task to call `optionally_get_with_by_ref` for a
        // key, so its async block will be evaluated and then an None will be
        // returned. Nothing will be inserted to the cache.
        let task1 = {
            let cache1 = cache.clone();
            async move {
                // Call `try_get_with` immediately.
                let v = cache1
                    .optionally_get_with_by_ref(KEY, async {
                        // Wait for 300 ms and return an None.
                        Timer::after(Duration::from_millis(300)).await;
                        None
                    })
                    .await;
                assert!(v.is_none());
            }
        };

        // Task2 will be the second task to call `optionally_get_with_by_ref` for the
        // same key, so its async block will not be evaluated. Once task1's async
        // block finishes, it will get the same error value returned by task1's async
        // block.
        let task2 = {
            let cache2 = cache.clone();
            async move {
                // Wait for 100 ms before calling `optionally_get_with_by_ref`.
                Timer::after(Duration::from_millis(100)).await;
                let v = cache2
                    .optionally_get_with_by_ref(KEY, async { unreachable!() })
                    .await;
                assert!(v.is_none());
            }
        };

        // Task3 will be the third task to call `optionally_get_with_by_ref` for the
        // same key. By the time it calls, task1's async block should have
        // finished already, but the key still does not exist in the cache. So
        // its async block will be evaluated and then an okay &str value will be
        // returned. That value will be inserted to the cache.
        let task3 = {
            let cache3 = cache.clone();
            async move {
                // Wait for 400 ms before calling `optionally_get_with_by_ref`.
                Timer::after(Duration::from_millis(400)).await;
                let v = cache3
                    .optionally_get_with_by_ref(KEY, async {
                        // Wait for 300 ms and return an Some(&str) value.
                        Timer::after(Duration::from_millis(300)).await;
                        Some("task3")
                    })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task4 will be the fourth task to call `optionally_get_with_by_ref` for the
        // same key. So its async block will not be evaluated. Once task3's
        // async block finishes, it will get the same okay &str value.
        let task4 = {
            let cache4 = cache.clone();
            async move {
                // Wait for 500 ms before calling `try_get_with`.
                Timer::after(Duration::from_millis(500)).await;
                let v = cache4
                    .optionally_get_with_by_ref(KEY, async { unreachable!() })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task5 will be the fifth task to call `optionally_get_with_by_ref` for the
        // same key. So its async block will not be evaluated. By the time it
        // calls, task3's async block should have finished already, so its async
        // block will not be evaluated and will get the value insert by task3's
        // async block immediately.
        let task5 = {
            let cache5 = cache.clone();
            async move {
                // Wait for 800 ms before calling `optionally_get_with_by_ref`.
                Timer::after(Duration::from_millis(800)).await;
                let v = cache5
                    .optionally_get_with_by_ref(KEY, async { unreachable!() })
                    .await;
                assert_eq!(v.unwrap(), "task3");
            }
        };

        // Task6 will call `get` for the same key. It will call when task1's async
        // block is still running, so it will get none for the key.
        let task6 = {
            let cache6 = cache.clone();
            async move {
                // Wait for 200 ms before calling `get`.
                Timer::after(Duration::from_millis(200)).await;
                let maybe_v = cache6.get(KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task7 will call `get` for the same key. It will call after task1's async
        // block finished with an error. So it will get none for the key.
        let task7 = {
            let cache7 = cache.clone();
            async move {
                // Wait for 400 ms before calling `get`.
                Timer::after(Duration::from_millis(400)).await;
                let maybe_v = cache7.get(KEY);
                assert!(maybe_v.is_none());
            }
        };

        // Task8 will call `get` for the same key. It will call after task3's async
        // block finished, so it will get the value insert by task3's async block.
        let task8 = {
            let cache8 = cache.clone();
            async move {
                // Wait for 800 ms before calling `get`.
                Timer::after(Duration::from_millis(800)).await;
                let maybe_v = cache8.get(KEY);
                assert_eq!(maybe_v, Some("task3"));
            }
        };

        futures_util::join!(task1, task2, task3, task4, task5, task6, task7, task8);
    }

    #[tokio::test]
    // https://github.com/moka-rs/moka/issues/43
    async fn handle_panic_in_get_with() {
        use tokio::time::{sleep, Duration};

        let cache = Cache::new(16);
        let semaphore = Arc::new(tokio::sync::Semaphore::new(0));
        {
            let cache_ref = cache.clone();
            let semaphore_ref = semaphore.clone();
            tokio::task::spawn(async move {
                let _ = cache_ref
                    .get_with(1, async move {
                        semaphore_ref.add_permits(1);
                        sleep(Duration::from_millis(50)).await;
                        panic!("Panic during try_get_with");
                    })
                    .await;
            });
        }
        let _ = semaphore.acquire().await.expect("semaphore acquire failed");
        assert_eq!(cache.get_with(1, async { 5 }).await, 5);
    }

    #[tokio::test]
    // https://github.com/moka-rs/moka/issues/43
    async fn handle_panic_in_try_get_with() {
        use tokio::time::{sleep, Duration};

        let cache = Cache::new(16);
        let semaphore = Arc::new(tokio::sync::Semaphore::new(0));
        {
            let cache_ref = cache.clone();
            let semaphore_ref = semaphore.clone();
            tokio::task::spawn(async move {
                let _ = cache_ref
                    .try_get_with(1, async move {
                        semaphore_ref.add_permits(1);
                        sleep(Duration::from_millis(50)).await;
                        panic!("Panic during try_get_with");
                    })
                    .await as Result<_, Arc<Infallible>>;
            });
        }
        let _ = semaphore.acquire().await.expect("semaphore acquire failed");
        assert_eq!(
            cache.try_get_with(1, async { Ok(5) }).await as Result<_, Arc<Infallible>>,
            Ok(5)
        );
    }

    #[tokio::test]
    // https://github.com/moka-rs/moka/issues/59
    async fn abort_get_with() {
        use tokio::time::{sleep, Duration};

        let cache = Cache::new(16);
        let semaphore = Arc::new(tokio::sync::Semaphore::new(0));

        let handle;
        {
            let cache_ref = cache.clone();
            let semaphore_ref = semaphore.clone();

            handle = tokio::task::spawn(async move {
                let _ = cache_ref
                    .get_with(1, async move {
                        semaphore_ref.add_permits(1);
                        sleep(Duration::from_millis(50)).await;
                        unreachable!();
                    })
                    .await;
            });
        }

        let _ = semaphore.acquire().await.expect("semaphore acquire failed");
        handle.abort();

        assert_eq!(cache.get_with(1, async { 5 }).await, 5);
    }

    #[tokio::test]
    // https://github.com/moka-rs/moka/issues/59
    async fn abort_try_get_with() {
        use tokio::time::{sleep, Duration};

        let cache = Cache::new(16);
        let semaphore = Arc::new(tokio::sync::Semaphore::new(0));

        let handle;
        {
            let cache_ref = cache.clone();
            let semaphore_ref = semaphore.clone();

            handle = tokio::task::spawn(async move {
                let _ = cache_ref
                    .try_get_with(1, async move {
                        semaphore_ref.add_permits(1);
                        sleep(Duration::from_millis(50)).await;
                        unreachable!();
                    })
                    .await as Result<_, Arc<Infallible>>;
            });
        }

        let _ = semaphore.acquire().await.expect("semaphore acquire failed");
        handle.abort();

        assert_eq!(
            cache.try_get_with(1, async { Ok(5) }).await as Result<_, Arc<Infallible>>,
            Ok(5)
        );
    }

    #[tokio::test]
    async fn test_removal_notifications() {
        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .max_capacity(3)
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert('a', "alice").await;
        cache.invalidate(&'a').await;
        expected.push((Arc::new('a'), "alice", RemovalCause::Explicit));

        cache.sync();
        assert_eq!(cache.entry_count(), 0);

        cache.insert('b', "bob").await;
        cache.insert('c', "cathy").await;
        cache.insert('d', "david").await;
        cache.sync();
        assert_eq!(cache.entry_count(), 3);

        // This will be rejected due to the size constraint.
        cache.insert('e', "emily").await;
        expected.push((Arc::new('e'), "emily", RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.entry_count(), 3);

        // Raise the popularity of 'e' so it will be accepted next time.
        cache.get(&'e');
        cache.sync();

        // Retry.
        cache.insert('e', "eliza").await;
        // and the LRU entry will be evicted.
        expected.push((Arc::new('b'), "bob", RemovalCause::Size));
        cache.sync();
        assert_eq!(cache.entry_count(), 3);

        // Replace an existing entry.
        cache.insert('d', "dennis").await;
        expected.push((Arc::new('d'), "david", RemovalCause::Replaced));
        cache.sync();
        assert_eq!(cache.entry_count(), 3);

        verify_notification_vec(&cache, actual, &expected);
    }

    #[tokio::test]
    async fn test_removal_notifications_with_updates() {
        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener.
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| a1.lock().push((k, v, cause));

        // Create a cache with the eviction listener and also TTL and TTI.
        let mut cache = Cache::builder()
            .eviction_listener_with_queued_delivery_mode(listener)
            .time_to_live(Duration::from_secs(7))
            .time_to_idle(Duration::from_secs(5))
            .build();
        cache.reconfigure_for_testing();

        let (clock, mock) = Clock::mock();
        cache.set_expiration_clock(Some(clock));

        // Make the cache exterior immutable.
        let cache = cache;

        cache.insert("alice", "a0").await;
        cache.sync();

        // Now alice (a0) has been expired by the idle timeout (TTI).
        mock.increment(Duration::from_secs(6));
        expected.push((Arc::new("alice"), "a0", RemovalCause::Expired));
        assert_eq!(cache.get(&"alice"), None);

        // We have not ran sync after the expiration of alice (a0), so it is
        // still in the cache.
        assert_eq!(cache.entry_count(), 1);

        // Re-insert alice with a different value. Since alice (a0) is still
        // in the cache, this is actually a replace operation rather than an
        // insert operation. We want to verify that the RemovalCause of a0 is
        // Expired, not Replaced.
        cache.insert("alice", "a1").await;
        cache.sync();

        mock.increment(Duration::from_secs(4));
        assert_eq!(cache.get(&"alice"), Some("a1"));
        cache.sync();

        // Now alice has been expired by time-to-live (TTL).
        mock.increment(Duration::from_secs(4));
        expected.push((Arc::new("alice"), "a1", RemovalCause::Expired));
        assert_eq!(cache.get(&"alice"), None);

        // But, again, it is still in the cache.
        assert_eq!(cache.entry_count(), 1);

        // Re-insert alice with a different value and verify that the
        // RemovalCause of a1 is Expired (not Replaced).
        cache.insert("alice", "a2").await;
        cache.sync();

        assert_eq!(cache.entry_count(), 1);

        // Now alice (a2) has been expired by the idle timeout.
        mock.increment(Duration::from_secs(6));
        expected.push((Arc::new("alice"), "a2", RemovalCause::Expired));
        assert_eq!(cache.get(&"alice"), None);
        assert_eq!(cache.entry_count(), 1);

        // This invalidate will internally remove alice (a2).
        cache.invalidate(&"alice").await;
        cache.sync();
        assert_eq!(cache.entry_count(), 0);

        // Re-insert, and this time, make it expired by the TTL.
        cache.insert("alice", "a3").await;
        cache.sync();
        mock.increment(Duration::from_secs(4));
        assert_eq!(cache.get(&"alice"), Some("a3"));
        cache.sync();
        mock.increment(Duration::from_secs(4));
        expected.push((Arc::new("alice"), "a3", RemovalCause::Expired));
        assert_eq!(cache.get(&"alice"), None);
        assert_eq!(cache.entry_count(), 1);

        // This invalidate will internally remove alice (a2).
        cache.invalidate(&"alice").await;
        cache.sync();
        assert_eq!(cache.entry_count(), 0);

        verify_notification_vec(&cache, actual, &expected);
    }

    // NOTE: To enable the panic logging, run the following command:
    //
    // RUST_LOG=moka=info cargo test --features 'future, logging' -- \
    //   future::cache::tests::recover_from_panicking_eviction_listener --exact --nocapture
    //
    #[tokio::test]
    async fn recover_from_panicking_eviction_listener() {
        #[cfg(feature = "logging")]
        let _ = env_logger::builder().is_test(true).try_init();

        // The following `Vec`s will hold actual and expected notifications.
        let actual = Arc::new(Mutex::new(Vec::new()));
        let mut expected = Vec::new();

        // Create an eviction listener that panics when it see
        // a value "panic now!".
        let a1 = Arc::clone(&actual);
        let listener = move |k, v, cause| {
            if v == "panic now!" {
                panic!("Panic now!");
            }
            a1.lock().push((k, v, cause))
        };

        // Create a cache with the eviction listener.
        let mut cache = Cache::builder()
            .name("My Future Cache")
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        // Make the cache exterior immutable.
        let cache = cache;

        // Insert an okay value.
        cache.insert("alice", "a0").await;
        cache.sync();

        // Insert a value that will cause the eviction listener to panic.
        cache.insert("alice", "panic now!").await;
        expected.push((Arc::new("alice"), "a0", RemovalCause::Replaced));
        cache.sync();

        // Insert an okay value. This will replace the previous
        // value "panic now!" so the eviction listener will panic.
        cache.insert("alice", "a2").await;
        cache.sync();
        // No more removal notification should be sent.

        // Invalidate the okay value.
        cache.invalidate(&"alice").await;
        cache.sync();

        verify_notification_vec(&cache, actual, &expected);
    }

    // This test ensures that the `contains_key`, `get` and `invalidate` can use
    // borrowed form `&[u8]` for key with type `Vec<u8>`.
    // https://github.com/moka-rs/moka/issues/166
    #[tokio::test]
    async fn borrowed_forms_of_key() {
        let cache: Cache<Vec<u8>, ()> = Cache::new(1);

        let key = vec![1_u8];
        cache.insert(key.clone(), ()).await;

        // key as &Vec<u8>
        let key_v: &Vec<u8> = &key;
        assert!(cache.contains_key(key_v));
        assert_eq!(cache.get(key_v), Some(()));
        cache.invalidate(key_v).await;

        cache.insert(key, ()).await;

        // key as &[u8]
        let key_s: &[u8] = &[1_u8];
        assert!(cache.contains_key(key_s));
        assert_eq!(cache.get(key_s), Some(()));
        cache.invalidate(key_s).await;
    }

    #[tokio::test]
    async fn drop_value_immediately_after_eviction() {
        use crate::common::test_utils::{Counters, Value};

        const MAX_CAPACITY: u32 = 500;
        const KEYS: u32 = ((MAX_CAPACITY as f64) * 1.2) as u32;

        let counters = Arc::new(Counters::default());
        let counters1 = Arc::clone(&counters);

        let listener = move |_k, _v, cause| match cause {
            RemovalCause::Size => counters1.incl_evicted(),
            RemovalCause::Explicit => counters1.incl_invalidated(),
            _ => (),
        };

        let mut cache = Cache::builder()
            .max_capacity(MAX_CAPACITY as u64)
            .eviction_listener_with_queued_delivery_mode(listener)
            .build();
        cache.reconfigure_for_testing();

        // Make the cache exterior immutable.
        let cache = cache;

        for key in 0..KEYS {
            let value = Arc::new(Value::new(vec![0u8; 1024], &counters));
            cache.insert(key, value).await;
            counters.incl_inserted();
            cache.sync();
        }

        let eviction_count = KEYS - MAX_CAPACITY;

        // Retries will be needed when testing in a QEMU VM.
        const MAX_RETRIES: usize = 5;
        let mut retries = 0;
        loop {
            // Ensure all scheduled notifications have been processed.
            std::thread::sleep(Duration::from_millis(500));

            if counters.evicted() != eviction_count || counters.value_dropped() != eviction_count {
                if retries <= MAX_RETRIES {
                    retries += 1;
                    cache.sync();
                    continue;
                } else {
                    assert_eq!(counters.evicted(), eviction_count, "Retries exhausted");
                    assert_eq!(
                        counters.value_dropped(),
                        eviction_count,
                        "Retries exhausted"
                    );
                }
            }

            assert_eq!(counters.inserted(), KEYS, "inserted");
            assert_eq!(counters.value_created(), KEYS, "value_created");
            assert_eq!(counters.evicted(), eviction_count, "evicted");
            assert_eq!(counters.invalidated(), 0, "invalidated");
            assert_eq!(counters.value_dropped(), eviction_count, "value_dropped");

            break;
        }

        for key in 0..KEYS {
            cache.invalidate(&key).await;
            cache.sync();
        }

        let mut retries = 0;
        loop {
            // Ensure all scheduled notifications have been processed.
            std::thread::sleep(Duration::from_millis(500));

            if counters.invalidated() != MAX_CAPACITY || counters.value_dropped() != KEYS {
                if retries <= MAX_RETRIES {
                    retries += 1;
                    cache.sync();
                    continue;
                } else {
                    assert_eq!(counters.invalidated(), MAX_CAPACITY, "Retries exhausted");
                    assert_eq!(counters.value_dropped(), KEYS, "Retries exhausted");
                }
            }

            assert_eq!(counters.inserted(), KEYS, "inserted");
            assert_eq!(counters.value_created(), KEYS, "value_created");
            assert_eq!(counters.evicted(), eviction_count, "evicted");
            assert_eq!(counters.invalidated(), MAX_CAPACITY, "invalidated");
            assert_eq!(counters.value_dropped(), KEYS, "value_dropped");

            break;
        }

        std::mem::drop(cache);
        assert_eq!(counters.value_dropped(), KEYS, "value_dropped");
    }

    #[tokio::test]
    async fn test_debug_format() {
        let cache = Cache::new(10);
        cache.insert('a', "alice").await;
        cache.insert('b', "bob").await;
        cache.insert('c', "cindy").await;

        let debug_str = format!("{:?}", cache);
        assert!(debug_str.starts_with('{'));
        assert!(debug_str.contains(r#"'a': "alice""#));
        assert!(debug_str.contains(r#"'b': "bob""#));
        assert!(debug_str.contains(r#"'c': "cindy""#));
        assert!(debug_str.ends_with('}'));
    }

    type NotificationTuple<K, V> = (Arc<K>, V, RemovalCause);

    fn verify_notification_vec<K, V, S>(
        cache: &Cache<K, V, S>,
        actual: Arc<Mutex<Vec<NotificationTuple<K, V>>>>,
        expected: &[NotificationTuple<K, V>],
    ) where
        K: std::hash::Hash + Eq + std::fmt::Debug + Send + Sync + 'static,
        V: Eq + std::fmt::Debug + Clone + Send + Sync + 'static,
        S: std::hash::BuildHasher + Clone + Send + Sync + 'static,
    {
        // Retries will be needed when testing in a QEMU VM.
        const MAX_RETRIES: usize = 5;
        let mut retries = 0;
        loop {
            // Ensure all scheduled notifications have been processed.
            std::thread::sleep(Duration::from_millis(500));

            let actual = &*actual.lock();
            if actual.len() != expected.len() {
                if retries <= MAX_RETRIES {
                    retries += 1;
                    cache.sync();
                    continue;
                } else {
                    assert_eq!(actual.len(), expected.len(), "Retries exhausted");
                }
            }

            for (i, (actual, expected)) in actual.iter().zip(expected).enumerate() {
                assert_eq!(actual, expected, "expected[{}]", i);
            }

            break;
        }
    }
}