Rollup merge of #76932 - fusion-engineering-forks:condvar-promise, r=sfackler
Relax promises about condition variable. For quite a while now, there have been plans to at some point use parking_lot or some other more efficient implementation of mutexes and condition variables. Right now, Mutex and CondVar both Box the 'real' mutex/condvar inside, to give it a stable address. This was done because implementations like pthread and Windows critical sections may not be moved. More efficient implementations based on futexes, WaitOnAddress, Windows SRW locks, parking_lot, etc. may be moved (while not borrowed), so wouldn't need boxing. However, not boxing them (which would be great goal to achieve), breaks a promise std currently makes about CondVar. CondVar promises to panic when used with different mutexes, to ensure consistent behaviour on all platforms. To this check, a mutex is considered 'the same' if the address of the 'real mutex' in the Box is the same. This address doesn't change when moving a `std::mutex::Mutex` object, effectively giving it an identity that survives moves of the Mutex object. If we ever switch to a non-boxed version, they no longer carry such an identity, and this check can no longer be made. Four options: 1. Always box mutexes. 2. Add a `MutexId` similar to `ThreadId`. Making mutexes bigger, and making it hard to ever have a `const fn new` for them. 3. Making the requirement of CondVar stricter: panic if the Mutex object itself moved. 4. Making the promise of CondVar weaker: don't promise to panic. 1, 2, and 3 seem like bad options. This PR updates the documentation for 4.
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@ -78,13 +78,9 @@ impl WaitTimeoutResult {
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/// and a mutex. The predicate is always verified inside of the mutex before
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/// determining that a thread must block.
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///
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/// Functions in this module will block the current **thread** of execution and
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/// are bindings to system-provided condition variables where possible. Note
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/// that this module places one additional restriction over the system condition
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/// variables: each condvar can be used with precisely one mutex at runtime. Any
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/// attempt to use multiple mutexes on the same condition variable will result
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/// in a runtime panic. If this is not desired, then the unsafe primitives in
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/// `sys` do not have this restriction but may result in undefined behavior.
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/// Functions in this module will block the current **thread** of execution.
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/// Note that any attempt to use multiple mutexes on the same condition
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/// variable may result in a runtime panic.
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///
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/// # Examples
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///
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@ -159,10 +155,8 @@ impl Condvar {
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///
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/// # Panics
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///
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/// This function will [`panic!`] if it is used with more than one mutex
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/// over time. Each condition variable is dynamically bound to exactly one
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/// mutex to ensure defined behavior across platforms. If this functionality
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/// is not desired, then unsafe primitives in `sys` are provided.
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/// This function may [`panic!`] if it is used with more than one mutex
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/// over time.
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///
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/// [`notify_one`]: Self::notify_one
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/// [`notify_all`]: Self::notify_all
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