/
lazy.rs
838 lines (745 loc) · 24 KB
/
lazy.rs
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//! Lazy values and one-time initialization of static data.
use crate::{
cell::{Cell, UnsafeCell},
fmt,
mem::{self, MaybeUninit},
ops::{Deref, Drop},
panic::{RefUnwindSafe, UnwindSafe},
sync::Once,
};
#[doc(inline)]
#[unstable(feature = "once_cell", issue = "74465")]
pub use core::lazy::*;
/// A synchronization primitive which can be written to only once.
///
/// This type is a thread-safe `OnceCell`.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncOnceCell;
///
/// static CELL: SyncOnceCell<String> = SyncOnceCell::new();
/// assert!(CELL.get().is_none());
///
/// std::thread::spawn(|| {
/// let value: &String = CELL.get_or_init(|| {
/// "Hello, World!".to_string()
/// });
/// assert_eq!(value, "Hello, World!");
/// }).join().unwrap();
///
/// let value: Option<&String> = CELL.get();
/// assert!(value.is_some());
/// assert_eq!(value.unwrap().as_str(), "Hello, World!");
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct SyncOnceCell<T> {
once: Once,
// Whether or not the value is initialized is tracked by `state_and_queue`.
value: UnsafeCell<MaybeUninit<T>>,
}
// Why do we need `T: Send`?
// Thread A creates a `SyncOnceCell` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T: Sync + Send> Sync for SyncOnceCell<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T: Send> Send for SyncOnceCell<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for SyncOnceCell<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T> Default for SyncOnceCell<T> {
fn default() -> SyncOnceCell<T> {
SyncOnceCell::new()
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: fmt::Debug> fmt::Debug for SyncOnceCell<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.get() {
Some(v) => f.debug_tuple("Once").field(v).finish(),
None => f.write_str("Once(Uninit)"),
}
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Clone> Clone for SyncOnceCell<T> {
fn clone(&self) -> SyncOnceCell<T> {
let cell = Self::new();
if let Some(value) = self.get() {
match cell.set(value.clone()) {
Ok(()) => (),
Err(_) => unreachable!(),
}
}
cell
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T> From<T> for SyncOnceCell<T> {
fn from(value: T) -> Self {
let cell = Self::new();
match cell.set(value) {
Ok(()) => cell,
Err(_) => unreachable!(),
}
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: PartialEq> PartialEq for SyncOnceCell<T> {
fn eq(&self, other: &SyncOnceCell<T>) -> bool {
self.get() == other.get()
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Eq> Eq for SyncOnceCell<T> {}
impl<T> SyncOnceCell<T> {
/// Creates a new empty cell.
#[unstable(feature = "once_cell", issue = "74465")]
pub const fn new() -> SyncOnceCell<T> {
SyncOnceCell { once: Once::new(), value: UnsafeCell::new(MaybeUninit::uninit()) }
}
/// Gets the reference to the underlying value.
///
/// Returns `None` if the cell is empty, or being initialized. This
/// method never blocks.
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get(&self) -> Option<&T> {
if self.is_initialized() {
// Safe b/c checked is_initialized
Some(unsafe { self.get_unchecked() })
} else {
None
}
}
/// Gets the mutable reference to the underlying value.
///
/// Returns `None` if the cell is empty. This method never blocks.
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get_mut(&mut self) -> Option<&mut T> {
if self.is_initialized() {
// Safe b/c checked is_initialized and we have a unique access
Some(unsafe { self.get_unchecked_mut() })
} else {
None
}
}
/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell's value was updated.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncOnceCell;
///
/// static CELL: SyncOnceCell<i32> = SyncOnceCell::new();
///
/// fn main() {
/// assert!(CELL.get().is_none());
///
/// std::thread::spawn(|| {
/// assert_eq!(CELL.set(92), Ok(()));
/// }).join().unwrap();
///
/// assert_eq!(CELL.set(62), Err(62));
/// assert_eq!(CELL.get(), Some(&92));
/// }
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn set(&self, value: T) -> Result<(), T> {
let mut value = Some(value);
self.get_or_init(|| value.take().unwrap());
match value {
None => Ok(()),
Some(value) => Err(value),
}
}
/// Gets the contents of the cell, initializing it with `f` if the cell
/// was empty.
///
/// Many threads may call `get_or_init` concurrently with different
/// initializing functions, but it is guaranteed that only one function
/// will be executed.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and the cell
/// remains uninitialized.
///
/// It is an error to reentrantly initialize the cell from `f`. The
/// exact outcome is unspecified. Current implementation deadlocks, but
/// this may be changed to a panic in the future.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncOnceCell;
///
/// let cell = SyncOnceCell::new();
/// let value = cell.get_or_init(|| 92);
/// assert_eq!(value, &92);
/// let value = cell.get_or_init(|| unreachable!());
/// assert_eq!(value, &92);
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get_or_init<F>(&self, f: F) -> &T
where
F: FnOnce() -> T,
{
match self.get_or_try_init(|| Ok::<T, !>(f())) {
Ok(val) => val,
}
}
/// Gets the contents of the cell, initializing it with `f` if
/// the cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and
/// the cell remains uninitialized.
///
/// It is an error to reentrantly initialize the cell from `f`.
/// The exact outcome is unspecified. Current implementation
/// deadlocks, but this may be changed to a panic in the future.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncOnceCell;
///
/// let cell = SyncOnceCell::new();
/// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
/// assert!(cell.get().is_none());
/// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
/// Ok(92)
/// });
/// assert_eq!(value, Ok(&92));
/// assert_eq!(cell.get(), Some(&92))
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<T, E>,
{
// Fast path check
// NOTE: We need to perform an acquire on the state in this method
// in order to correctly synchronize `SyncLazy::force`. This is
// currently done by calling `self.get()`, which in turn calls
// `self.is_initialized()`, which in turn performs the acquire.
if let Some(value) = self.get() {
return Ok(value);
}
self.initialize(f)?;
debug_assert!(self.is_initialized());
// Safety: The inner value has been initialized
Ok(unsafe { self.get_unchecked() })
}
/// Consumes the `SyncOnceCell`, returning the wrapped value. Returns
/// `None` if the cell was empty.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncOnceCell;
///
/// let cell: SyncOnceCell<String> = SyncOnceCell::new();
/// assert_eq!(cell.into_inner(), None);
///
/// let cell = SyncOnceCell::new();
/// cell.set("hello".to_string()).unwrap();
/// assert_eq!(cell.into_inner(), Some("hello".to_string()));
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn into_inner(mut self) -> Option<T> {
// Safety: Safe because we immediately free `self` without dropping
let inner = unsafe { self.take_inner() };
// Don't drop this `SyncOnceCell`. We just moved out one of the fields, but didn't set
// the state to uninitialized.
mem::forget(self);
inner
}
/// Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state.
///
/// Has no effect and returns `None` if the `SyncOnceCell` hasn't been initialized.
///
/// Safety is guaranteed by requiring a mutable reference.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncOnceCell;
///
/// let mut cell: SyncOnceCell<String> = SyncOnceCell::new();
/// assert_eq!(cell.take(), None);
///
/// let mut cell = SyncOnceCell::new();
/// cell.set("hello".to_string()).unwrap();
/// assert_eq!(cell.take(), Some("hello".to_string()));
/// assert_eq!(cell.get(), None);
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn take(&mut self) -> Option<T> {
mem::take(self).into_inner()
}
/// Takes the wrapped value out of a `SyncOnceCell`.
/// Afterwards the cell is no longer initialized.
///
/// Safety: The cell must now be free'd WITHOUT dropping. No other usages of the cell
/// are valid. Only used by `into_inner` and `drop`.
unsafe fn take_inner(&mut self) -> Option<T> {
// The mutable reference guarantees there are no other threads that can observe us
// taking out the wrapped value.
// Right after this function `self` is supposed to be freed, so it makes little sense
// to atomically set the state to uninitialized.
if self.is_initialized() {
let value = mem::replace(&mut self.value, UnsafeCell::new(MaybeUninit::uninit()));
Some(value.into_inner().assume_init())
} else {
None
}
}
#[inline]
fn is_initialized(&self) -> bool {
self.once.is_completed()
}
#[cold]
fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<T, E>,
{
let mut res: Result<(), E> = Ok(());
let slot = &self.value;
// Ignore poisoning from other threads
// If another thread panics, then we'll be able to run our closure
self.once.call_once_force(|p| {
match f() {
Ok(value) => {
unsafe { (&mut *slot.get()).write(value) };
}
Err(e) => {
res = Err(e);
// Treat the underlying `Once` as poisoned since we
// failed to initialize our value. Calls
p.poison();
}
}
});
res
}
/// Safety: The value must be initialized
unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
(&*self.value.get()).get_ref()
}
/// Safety: The value must be initialized
unsafe fn get_unchecked_mut(&mut self) -> &mut T {
debug_assert!(self.is_initialized());
(&mut *self.value.get()).get_mut()
}
}
unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
fn drop(&mut self) {
// Safety: The cell is being dropped, so it can't be accessed again.
// We also don't touch the `T`, which validates our usage of #[may_dangle].
unsafe { self.take_inner() };
}
}
/// A value which is initialized on the first access.
///
/// This type is a thread-safe `Lazy`, and can be used in statics.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::collections::HashMap;
///
/// use std::lazy::SyncLazy;
///
/// static HASHMAP: SyncLazy<HashMap<i32, String>> = SyncLazy::new(|| {
/// println!("initializing");
/// let mut m = HashMap::new();
/// m.insert(13, "Spica".to_string());
/// m.insert(74, "Hoyten".to_string());
/// m
/// });
///
/// fn main() {
/// println!("ready");
/// std::thread::spawn(|| {
/// println!("{:?}", HASHMAP.get(&13));
/// }).join().unwrap();
/// println!("{:?}", HASHMAP.get(&74));
///
/// // Prints:
/// // ready
/// // initializing
/// // Some("Spica")
/// // Some("Hoyten")
/// }
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct SyncLazy<T, F = fn() -> T> {
cell: SyncOnceCell<T>,
init: Cell<Option<F>>,
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: fmt::Debug, F> fmt::Debug for SyncLazy<T, F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
}
}
// We never create a `&F` from a `&SyncLazy<T, F>` so it is fine
// to not impl `Sync` for `F`
// we do create a `&mut Option<F>` in `force`, but this is
// properly synchronized, so it only happens once
// so it also does not contribute to this impl.
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T, F: Send> Sync for SyncLazy<T, F> where SyncOnceCell<T>: Sync {}
// auto-derived `Send` impl is OK.
#[unstable(feature = "once_cell", issue = "74465")]
impl<T, F: UnwindSafe> RefUnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: RefUnwindSafe {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T, F: UnwindSafe> UnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: UnwindSafe {}
impl<T, F> SyncLazy<T, F> {
/// Creates a new lazy value with the given initializing
/// function.
#[unstable(feature = "once_cell", issue = "74465")]
pub const fn new(f: F) -> SyncLazy<T, F> {
SyncLazy { cell: SyncOnceCell::new(), init: Cell::new(Some(f)) }
}
}
impl<T, F: FnOnce() -> T> SyncLazy<T, F> {
/// Forces the evaluation of this lazy value and
/// returns a reference to result. This is equivalent
/// to the `Deref` impl, but is explicit.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncLazy;
///
/// let lazy = SyncLazy::new(|| 92);
///
/// assert_eq!(SyncLazy::force(&lazy), &92);
/// assert_eq!(&*lazy, &92);
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn force(this: &SyncLazy<T, F>) -> &T {
this.cell.get_or_init(|| match this.init.take() {
Some(f) => f(),
None => panic!("Lazy instance has previously been poisoned"),
})
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T, F: FnOnce() -> T> Deref for SyncLazy<T, F> {
type Target = T;
fn deref(&self) -> &T {
SyncLazy::force(self)
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Default> Default for SyncLazy<T> {
/// Creates a new lazy value using `Default` as the initializing function.
fn default() -> SyncLazy<T> {
SyncLazy::new(T::default)
}
}
#[cfg(test)]
mod tests {
use crate::{
lazy::{Lazy, SyncLazy, SyncOnceCell},
panic,
sync::{
atomic::{AtomicUsize, Ordering::SeqCst},
mpsc::channel,
Mutex,
},
thread,
};
#[test]
fn lazy_default() {
static CALLED: AtomicUsize = AtomicUsize::new(0);
struct Foo(u8);
impl Default for Foo {
fn default() -> Self {
CALLED.fetch_add(1, SeqCst);
Foo(42)
}
}
let lazy: Lazy<Mutex<Foo>> = <_>::default();
assert_eq!(CALLED.load(SeqCst), 0);
assert_eq!(lazy.lock().unwrap().0, 42);
assert_eq!(CALLED.load(SeqCst), 1);
lazy.lock().unwrap().0 = 21;
assert_eq!(lazy.lock().unwrap().0, 21);
assert_eq!(CALLED.load(SeqCst), 1);
}
#[test]
fn lazy_poisoning() {
let x: Lazy<String> = Lazy::new(|| panic!("kaboom"));
for _ in 0..2 {
let res = panic::catch_unwind(panic::AssertUnwindSafe(|| x.len()));
assert!(res.is_err());
}
}
fn spawn_and_wait<R: Send + 'static>(f: impl FnOnce() -> R + Send + 'static) -> R {
thread::spawn(f).join().unwrap()
}
#[test]
fn sync_once_cell() {
static ONCE_CELL: SyncOnceCell<i32> = SyncOnceCell::new();
assert!(ONCE_CELL.get().is_none());
spawn_and_wait(|| {
ONCE_CELL.get_or_init(|| 92);
assert_eq!(ONCE_CELL.get(), Some(&92));
});
ONCE_CELL.get_or_init(|| panic!("Kabom!"));
assert_eq!(ONCE_CELL.get(), Some(&92));
}
#[test]
fn sync_once_cell_get_mut() {
let mut c = SyncOnceCell::new();
assert!(c.get_mut().is_none());
c.set(90).unwrap();
*c.get_mut().unwrap() += 2;
assert_eq!(c.get_mut(), Some(&mut 92));
}
#[test]
fn sync_once_cell_get_unchecked() {
let c = SyncOnceCell::new();
c.set(92).unwrap();
unsafe {
assert_eq!(c.get_unchecked(), &92);
}
}
#[test]
fn sync_once_cell_drop() {
static DROP_CNT: AtomicUsize = AtomicUsize::new(0);
struct Dropper;
impl Drop for Dropper {
fn drop(&mut self) {
DROP_CNT.fetch_add(1, SeqCst);
}
}
let x = SyncOnceCell::new();
spawn_and_wait(move || {
x.get_or_init(|| Dropper);
assert_eq!(DROP_CNT.load(SeqCst), 0);
drop(x);
});
assert_eq!(DROP_CNT.load(SeqCst), 1);
}
#[test]
fn sync_once_cell_drop_empty() {
let x = SyncOnceCell::<String>::new();
drop(x);
}
#[test]
fn clone() {
let s = SyncOnceCell::new();
let c = s.clone();
assert!(c.get().is_none());
s.set("hello".to_string()).unwrap();
let c = s.clone();
assert_eq!(c.get().map(String::as_str), Some("hello"));
}
#[test]
fn get_or_try_init() {
let cell: SyncOnceCell<String> = SyncOnceCell::new();
assert!(cell.get().is_none());
let res = panic::catch_unwind(|| cell.get_or_try_init(|| -> Result<_, ()> { panic!() }));
assert!(res.is_err());
assert!(!cell.is_initialized());
assert!(cell.get().is_none());
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert_eq!(
cell.get_or_try_init(|| Ok::<_, ()>("hello".to_string())),
Ok(&"hello".to_string())
);
assert_eq!(cell.get(), Some(&"hello".to_string()));
}
#[test]
fn from_impl() {
assert_eq!(SyncOnceCell::from("value").get(), Some(&"value"));
assert_ne!(SyncOnceCell::from("foo").get(), Some(&"bar"));
}
#[test]
fn partialeq_impl() {
assert!(SyncOnceCell::from("value") == SyncOnceCell::from("value"));
assert!(SyncOnceCell::from("foo") != SyncOnceCell::from("bar"));
assert!(SyncOnceCell::<String>::new() == SyncOnceCell::new());
assert!(SyncOnceCell::<String>::new() != SyncOnceCell::from("value".to_owned()));
}
#[test]
fn into_inner() {
let cell: SyncOnceCell<String> = SyncOnceCell::new();
assert_eq!(cell.into_inner(), None);
let cell = SyncOnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));
}
#[test]
fn sync_lazy_new() {
static CALLED: AtomicUsize = AtomicUsize::new(0);
static SYNC_LAZY: SyncLazy<i32> = SyncLazy::new(|| {
CALLED.fetch_add(1, SeqCst);
92
});
assert_eq!(CALLED.load(SeqCst), 0);
spawn_and_wait(|| {
let y = *SYNC_LAZY - 30;
assert_eq!(y, 62);
assert_eq!(CALLED.load(SeqCst), 1);
});
let y = *SYNC_LAZY - 30;
assert_eq!(y, 62);
assert_eq!(CALLED.load(SeqCst), 1);
}
#[test]
fn sync_lazy_default() {
static CALLED: AtomicUsize = AtomicUsize::new(0);
struct Foo(u8);
impl Default for Foo {
fn default() -> Self {
CALLED.fetch_add(1, SeqCst);
Foo(42)
}
}
let lazy: SyncLazy<Mutex<Foo>> = <_>::default();
assert_eq!(CALLED.load(SeqCst), 0);
assert_eq!(lazy.lock().unwrap().0, 42);
assert_eq!(CALLED.load(SeqCst), 1);
lazy.lock().unwrap().0 = 21;
assert_eq!(lazy.lock().unwrap().0, 21);
assert_eq!(CALLED.load(SeqCst), 1);
}
#[test]
fn static_sync_lazy() {
static XS: SyncLazy<Vec<i32>> = SyncLazy::new(|| {
let mut xs = Vec::new();
xs.push(1);
xs.push(2);
xs.push(3);
xs
});
spawn_and_wait(|| {
assert_eq!(&*XS, &vec![1, 2, 3]);
});
assert_eq!(&*XS, &vec![1, 2, 3]);
}
#[test]
fn static_sync_lazy_via_fn() {
fn xs() -> &'static Vec<i32> {
static XS: SyncOnceCell<Vec<i32>> = SyncOnceCell::new();
XS.get_or_init(|| {
let mut xs = Vec::new();
xs.push(1);
xs.push(2);
xs.push(3);
xs
})
}
assert_eq!(xs(), &vec![1, 2, 3]);
}
#[test]
fn sync_lazy_poisoning() {
let x: SyncLazy<String> = SyncLazy::new(|| panic!("kaboom"));
for _ in 0..2 {
let res = panic::catch_unwind(|| x.len());
assert!(res.is_err());
}
}
#[test]
fn is_sync_send() {
fn assert_traits<T: Send + Sync>() {}
assert_traits::<SyncOnceCell<String>>();
assert_traits::<SyncLazy<String>>();
}
#[test]
fn eval_once_macro() {
macro_rules! eval_once {
(|| -> $ty:ty {
$($body:tt)*
}) => {{
static ONCE_CELL: SyncOnceCell<$ty> = SyncOnceCell::new();
fn init() -> $ty {
$($body)*
}
ONCE_CELL.get_or_init(init)
}};
}
let fib: &'static Vec<i32> = eval_once! {
|| -> Vec<i32> {
let mut res = vec![1, 1];
for i in 0..10 {
let next = res[i] + res[i + 1];
res.push(next);
}
res
}
};
assert_eq!(fib[5], 8)
}
#[test]
fn sync_once_cell_does_not_leak_partially_constructed_boxes() {
static ONCE_CELL: SyncOnceCell<String> = SyncOnceCell::new();
let n_readers = 10;
let n_writers = 3;
const MSG: &str = "Hello, World";
let (tx, rx) = channel();
for _ in 0..n_readers {
let tx = tx.clone();
thread::spawn(move || {
loop {
if let Some(msg) = ONCE_CELL.get() {
tx.send(msg).unwrap();
break;
}
#[cfg(target_env = "sgx")]
crate::thread::yield_now();
}
});
}
for _ in 0..n_writers {
thread::spawn(move || {
let _ = ONCE_CELL.set(MSG.to_owned());
});
}
for _ in 0..n_readers {
let msg = rx.recv().unwrap();
assert_eq!(msg, MSG);
}
}
#[test]
fn dropck() {
let cell = SyncOnceCell::new();
{
let s = String::new();
cell.set(&s).unwrap();
}
}
}