moka::future::Cache::get_or_insert_with() panics if previously inserting task aborted

[niklasf]

When writing a web server, it appears that hyper::Server can abort tasks, if the requester has gone away. moka::future::Cache does not like that and panics. Is this bug? Or is there a recommended way to deal with it?

Minimized example:

[package]
name = "moka-future-bug"
version = "0.1.0"
edition = "2021"

[dependencies]
moka = { version = "0.6.2", features = ["future"] }
tokio = { version = "1.15.0", features = ["full"] }

use moka::future::Cache;
use std::time::Duration;

#[tokio::main]
async fn main() {
    let cache_a: Cache<(), ()> = Cache::new(1);

    let cache_b = cache_a.clone();

    let handle = tokio::task::spawn(async move {
        cache_b
            .get_or_insert_with((), async {
                tokio::time::sleep(Duration::from_millis(1000)).await;
            })
            .await;
    });

    tokio::time::sleep(Duration::from_millis(500)).await;
    handle.abort();

    cache_a.get_or_insert_with((), async {}).await; // panics!
}

Backtrace:

thread 'main' panicked at 'Too many retries. Tried to read the return value from the `init` \
                                future but failed 200 times. Maybe the `init` kept panicking?', /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/moka-0.6.2/src/future/value_initializer.rs:138:33
stack backtrace:
   0: rust_begin_unwind
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/core/src/panicking.rs:107:14
   2: moka::future::value_initializer::ValueInitializer<K,V,S>::do_try_init::{{closure}}
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/moka-0.6.2/src/future/value_initializer.rs:138:33
   3: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/core/src/future/mod.rs:80:19
   4: moka::future::value_initializer::ValueInitializer<K,V,S>::init_or_read::{{closure}}
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/moka-0.6.2/src/future/value_initializer.rs:53:9
   5: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/core/src/future/mod.rs:80:19
   6: moka::future::cache::Cache<K,V,S>::get_or_insert_with_hash_and_fun::{{closure}}
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/moka-0.6.2/src/future/cache.rs:621:15
   7: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/core/src/future/mod.rs:80:19
   8: moka::future::cache::Cache<K,V,S>::get_or_insert_with::{{closure}}
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/moka-0.6.2/src/future/cache.rs:363:9
   9: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/core/src/future/mod.rs:80:19
  10: moka_future_bug::main::{{closure}}
             at ./src/main.rs:21:5
  11: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/core/src/future/mod.rs:80:19
  12: tokio::park::thread::CachedParkThread::block_on::{{closure}}
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/park/thread.rs:263:54
  13: tokio::coop::with_budget::{{closure}}
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/coop.rs:102:9
  14: std::thread::local::LocalKey<T>::try_with
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/std/src/thread/local.rs:413:16
  15: std::thread::local::LocalKey<T>::with
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/std/src/thread/local.rs:389:9
  16: tokio::coop::with_budget
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/coop.rs:95:5
  17: tokio::coop::budget
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/coop.rs:72:5
  18: tokio::park::thread::CachedParkThread::block_on
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/park/thread.rs:263:31
  19: tokio::runtime::enter::Enter::block_on
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/runtime/enter.rs:151:13
  20: tokio::runtime::thread_pool::ThreadPool::block_on
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/runtime/thread_pool/mod.rs:77:9
  21: tokio::runtime::Runtime::block_on
             at /home/niklas/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/runtime/mod.rs:463:43
  22: moka_future_bug::main
             at ./src/main.rs:21:5
  23: core::ops::function::FnOnce::call_once
             at /rustc/efec545293b9263be9edfb283a7aa66350b3acbf/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[tatsuya6502]

Thank you for reporting. I confirmed the issue and I think it is a bug. I will investigate and fix it.

[barkanido]

Hey @tatsuya6502 lmk if you think I could help here

[tatsuya6502]

Hi @barkanido — Thank you for the offer. I will do this by myself because this one is a bit tricky to solve. I will need some experiments.

[tatsuya6502]

This issue will be fixed by PR #60.

After merging the PR, I will start a release process for Moka v0.6.3. I will run a regular pre-release testing, which usually takes 2.5 hours, and If everything goes well, I will publish v0.6.3 to crates.io.

[tatsuya6502]

Published v0.6.3 with the fix to crates.io.

[niklasf]

Wow, thanks for the super fast tournaround on this!

Aside: It looks like in a web service it can still be a good idea to call get_or_*_insert_with from a new independent task, so clients cannot stall initialization by causing aborts - at least if other clients may be interested in the same key.