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feat(rwlock): add an upgradable readers-writer lock #366

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1 change: 1 addition & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@ All notable changes to this project will be documented in this file.

### New features

* Add `rwlock::UpgradableRwLock` with a single upgradable reader, atomic upgrade and downgrade, and cancellation that releases a pending upgrade's guard; writers and upgradable readers share a FIFO reservation while ordinary readers use the data-permit queue.
* Add bounded MPMC `reserve` and `try_reserve` methods returning a borrowed `Permit`, so callers can wait for capacity before constructing a value; sends and reservations receive capacity in wait-queue order, and unused permits release it.

### Bug fixes
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11 changes: 9 additions & 2 deletions asyncband/src/rwlock/mod.rs
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Expand Up @@ -29,12 +29,16 @@
//! A write guard allows mutation and excludes every other guard. Both release their access on
//! drop, including during unwinding; a panic does not poison the lock.
//!
//! Waiting requests are served in queue order. A queued writer blocks readers behind it, even
//! while earlier readers still hold the lock. Consequently, keeping a read guard while waiting
//! [`RwLock`] serves waiting requests in queue order. A queued writer blocks readers behind it,
//! even while earlier readers still hold the lock. Consequently, keeping a read guard while waiting
//! for a write guard, or for another read behind a queued writer, can deadlock. The `try_` methods
//! never wait or reserve a queue position. Dropping a pending acquisition cancels its request;
//! a subsequent acquisition starts again at the back of the queue.
//!
//! [`UpgradableRwLock`] is a separate lock for inspecting a value before deciding to mutate it.
//! It reserves one reader's ability to upgrade and has its own [ordering
//! policy](UpgradableRwLock#ordering).
//!
//! # Updating and inspecting
//!
//! ```
Expand Down Expand Up @@ -114,6 +118,7 @@ mod owned_mapped_write_guard;
mod owned_read_guard;
mod owned_write_guard;
mod read_guard;
mod upgradable;
mod write_guard;

pub use self::mapped_read_guard::MappedRwLockReadGuard;
Expand All @@ -123,6 +128,8 @@ pub use self::owned_mapped_write_guard::OwnedMappedRwLockWriteGuard;
pub use self::owned_read_guard::OwnedRwLockReadGuard;
pub use self::owned_write_guard::OwnedRwLockWriteGuard;
pub use self::read_guard::RwLockReadGuard;
pub use self::upgradable::RwLockUpgradableReadGuard;
pub use self::upgradable::UpgradableRwLock;
pub use self::write_guard::RwLockWriteGuard;

/// A value with fair, asynchronous shared or exclusive access.
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