feat: Implement write lock downgrading - #65
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LGTM. I made some code style changes and this is ready for merging.
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Thanks for your help. |
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Description
This PR introduces a
downgrade()method toRwLockWriteGuardand its variants, addressing Issue #62.Currently, a task holding a write lock must drop it and re-acquire a read lock to allow concurrent reads. This is inefficient and creates a small window for race conditions. The
downgrade()method provides a way to atomically and efficiently convert an exclusive write lock into a shared read lock, improving performance and ensuring safety.Implementation
The
downgrade()method has been added to all fourRwLockWriteGuardvariants:RwLockWriteGuardMappedRwLockWriteGuardOwnedRwLockWriteGuardOwnedMappedRwLockWriteGuardThe implementation atomically converts the lock by releasing
max_readers - 1permits from the underlying semaphore, leaving one permit for the new read guard.ManuallyDropis used to prevent the original write guard'sDropimplementation from running, ensuring panic safety and correct ownership transfer of the lock.To implement
RwLockWriteGuard::downgrade, I'll need to create a newRwLockReadGuard. However, thelockfield ofRwLockReadGuardis private. To avoid changing the access permissions of thelockfield directly (given that changing it topub(super)would grant excessive access, and Rust doesn't offer more fine-grained access control, which is a trade-off point tison and I discussed in PR #61), So, my current plan is to add a dedicated constructor forRwLockReadGuard. I will name itfrom_write_downgrade()instead of genericnew()and constrain its API by requiring theWriteGuardto be passed in. This makes its purpose clear and prevents misuse.