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Add RwLockWriteGuard::{downgrade_map, try_downgrade_map}. #5527

Merged
merged 5 commits into from Mar 8, 2023
Merged

Add RwLockWriteGuard::{downgrade_map, try_downgrade_map}. #5527

merged 5 commits into from Mar 8, 2023

Commits on Mar 7, 2023

  1. Add RwLockWriteGuard::{downgrade_map, try_downgrade_map}. 

    `downgrade_map` can already be made using a combination of `RwLockWriteGuard::downgrade` and `RwLockReadGuard::map`, but this version runs `f` *before* downgrading, in case it's useful for anyone and just for completion's sake.

The real use case is with `try_downgrade_map`. In this case, we run `f` *before* we downgrade, so we can return the write lock in case it returns `None`. This isn't implementable in safe code, at least not without one of the following tradeoffs:
* Call `f` twice: Not always doable, when it's expensive or not idempotent.
* Downgrade, call `f`, then re-lock if it returned `None`: If you're doing double-checked locking, this goes against the whole point of it, because you unlock and so state might have changed in the meantime.
    @Zenithsiz
    Zenithsiz committed Mar 7, 2023
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  2. Fixed RwLockWriteGuard::{downgrade_map, try_downgrade_map} not prop… 

    …erly downgrading.

`RwLockWriteGuard::{downgrade_map, try_downgrade_map}` doctests now test whether a read lock can be obtained afterwards.
    @Zenithsiz
    Zenithsiz committed Mar 7, 2023
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  3. Added OwnedRwLockWriteGuard::{downgrade_map, try_downgrade_map}. 

    These are mirrors of the existing methods on `RwLockLockWriteGuard`.
    @Zenithsiz
    Zenithsiz committed Mar 7, 2023
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  4. Improved some doctests readability.

    @Zenithsiz
    Zenithsiz committed Mar 7, 2023
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  5. Added tests for `{Mapped}RwLockWriteGuard::{downgrade_map, try_downgr… 

    …ade_map}`.
    @Zenithsiz
    Zenithsiz committed Mar 7, 2023
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