async-condvar-fair
Condition variables for async Rust. Features:
-
Fair. [
notify_one
] always wakes up the longest-waiting waiter. -
Novel [
Baton
] facility to help avoid dropped notifications which can otherwise arise through async task cancellation. -
Works with any async runtime.
-
Can work with any kind of mutex (sync or async).
-
100% safe code. (Uses
parking_lot
anddlv-list
internally.) -
Can handle multiple different mutexes interacting with multiple different condvars, if you like that kind of thing.
Main entrypoint
The main entrypoint to this library is [Condvar
].
See its documentation for details of the constructors and
methods available.
Example
// async-condvar-fair = { version = "0.2", features = "parking_lot_0_12" }
// parking_lot = { version = "0.12", features = ["send_guard"] }
# #[cfg(feature = "parking_lot_0_12")] mod demo {
use std::collections::VecDeque;
use async_condvar_fair::{Condvar, BatonExt};
use parking_lot::Mutex;
struct Shared {
cv: Condvar,
queue: Mutex<VecDeque<String>>,
}
impl Shared {
pub async fn procssor(&self) {
let mut guard = self.queue.lock();
let mut baton = None;
loop {
while let Some(entry) = guard.pop_front() {
println!("processing {:?}", &entry);
}
baton.dispose();
let got = self.cv.wait_baton(guard).await;
guard = got.0;
baton = got.1;
}
}
}
# }
Mutexes - choosing a mutex, sync vs async
Note that it can make perfect sense to have an async condvar, but protect the shared state with a sync mutex. If the operations which occur while holding the mutex are fast (and, in particular, do not perform IO), this is usually the best choice.
[parking_lot::Mutex
] is a good choice for such a sync mutex, and is
conveniently supported by async-condvar-fair
. You will probably
want to enable parking_lot
's send_guard
feature.
If you use a sync mutex, you probably don't intend to be awaiting with
the mutex held, so you can probably use plain [wait
] (rather than
[wait_baton
]).
If the operations you do while holding the mutex are slow, you should
use the async mutex supplied with your async runtime. In this case,
if you are using [notify_one
] you should consider using
[wait_baton
] to avoid task cancellation causing lost notifications:
see [Baton
].
Mutex guard, Send
ing between threads
In a multithreaded async runtime, tasks can move between threads.
This means the mutex guard needs to be Send
. Async mutex guards
supplied with multithreaded async runtimes are all Send
. Sync mutex
guards often aren't.
For example, [std::sync::MutexGuard
], which can only be used with
Condvar
with a non-multithreaded runtime, because futures which have
a local variable with such a guard will not be Send
. Use
[parking_lot
] with send_guard
enabled, instead.
Mutexes - how to pass the mutex to wait_baton
et al
[Condvar::wait_baton
] and [wait
] can in principle work with any mutex.
But they need to know how to relock the mutex.
For the most convenient mutexes, like [parking_lot::Mutex
], you can
just pass the guard to wait
. Condvar
will use the guard to unlock
the mutex, and then to relock it again during wakeup.
But for many mutexes, this is not possible, because the guard type
does not provide a way to get back to the unlocked mutex reference.
This is the case, for example, for [std::sync::Mutex
].
For these inconvenient mutexes, you can pass a tuple to [wait_baton
]
or [wait
], or use [wait_no_relock
] and relock the mutex yourself.
Builtin mutex support, and async-condvar-fair
crate features
feature | mutex type | pass to [wait ] / [wait_baton ] |
---|---|---|
always enabled | [std::sync::Mutex ] |
(MutexGuard, &Mutex) [2] |
parking_lot_N |
[parking_lot::Mutex ] |
MutexGuard [1] |
parking_lot_N |
[parking_lot::FairMutex ] |
FairMutexGuard [1] |
tokio |
[tokio::sync::Mutex ] |
(MutexGuard, &Mutex) |
tokio |
[Arc ]< [tokio::sync::Mutex ]> |
(OwnedMutexGuard, Arc<Mutex>) |
tokio |
[tokio::sync::RwLock ] |
(RwLockReadGuard, &Mutex) |
tokio |
[tokio::sync::RwLock ] |
(RwLockWriteGuard, &Mutex) |
tokio |
[Arc ]< [tokio::sync::RwLock ]> |
(OwnedRwLockReadGuard, Arc<RwLock>) |
tokio |
[Arc ]< [tokio::sync::RwLock ]> |
(OwnedRwLockWriteGuard, Arc<RwLock>) |
smol |
[smol::lock::Mutex ] |
MutexGuard |
smol |
[smol::lock::RwLock ] |
(RwLockReadGuard, &RwLock) |
smol |
[smol::lock::RwLock ] |
(RwLockWriteGuard, &RwLock) |
smol |
[Arc ]< [smol::lock::Mutex ]> |
MutexGuardArc |
Notes: 0. None of these are not enabled by default, since each one implies a dependency on the corresponding crate.
- Enable the
parking_lot_N
feature corresponding to theparking_lot
version you are using (as specified in yourCargo.toml
). Enableparking_lot
'ssend_guard
feature for use with a multithreaded runtime. - Single-threaded runtimes only.
Use with other mutexes
The support listed above is all achieved through provided
implementations of the [RelockMutexGuard
] trait.
If you want to use a mutex type without builtin support in
async-condvar-wait
, use the [RelockMutexGuard!
] macro to define a
suitable impl, define that impl by hand, or use [wait_no_relock
] and
relock the mutex yourself each time.
Example of passing a tuple of guard and mutex
# fn main() {
# #[cfg(feature = "parking_lot_0_12")] {
# use std::collections::VecDeque;
# use async_condvar_fair::{Condvar, BatonExt};
use std::sync::Mutex;
struct Shared {
cv: Condvar,
queue: Mutex<VecDeque<String>>,
}
#
# impl Shared {
# pub async fn procssor(&self) {
# let mut guard = self.queue.lock().unwrap();
# let mut baton = None;
loop {
while let Some(entry) = guard.pop_front() {
println!("processing {:?}", &entry);
if entry.is_empty() { break }
}
baton.dispose();
let got = self.cv.wait_baton((guard, &self.queue)).await;
guard = got.0;
baton = got.1;
}
# }
# } } }
Change log - breaking changes and significant bug fixes
-
1.0.1 2024-02-20:
- No API or functional changes.
- Relax
dlv-list
dependencies (both upwards and downwards). - Explicitly declare our MSRV (1.56).
- Minor fixes to docs.
- Testing improvements.
- Rename Cargo.lock.example to Cargo.lock.
-
1.0.0 2022-10-11:
- Update our own version to 1.x.
- Bump
dlv-list
dependency. No API or functional changes.
-
0.2.2 2022-03-27:
- Bump
dlv-list
dependency. No API or functional changes.
- Bump
-
0.2.1 2022-03-27:
- Breaking change:
New
parking_lot_N
features for providingRelockMutexGuard
impls for specificparking_lot
versions. When upgrading from 0.1.0, specify the new featureparking_lot_0_11
. - Support for
parking_lot
0.9, 0.10, 0.12.
- Breaking change:
New
-
0.2.0: version number not used.
-
0.1.0, 2021-07-05
- Initial release.
Copyright, licence, and NO WARRANTY
Copyright Ian Jackson and contributors
to the Rust package async-condvar-fair.
There is NO WARRANTY.
See the files GPL-3
and LICENCE
in the source tree.
SPDX-License-Identifier: GPL-3.0-or-later