[issue11618] Locks broken wrt timeouts on Windows

[sbt]

sbt <shibturn at gmail.com> added the comment:

> Btw, the locktimeout.patch appears to have a race condition.  
> LeaveNonRecursiveMutex may SetEvent when there is no thread waiting 
> (because a timeout just occurred, but the thread on which it happened 
> is still somewhere around line #62 ).  This will cause the next 
> WaitForSingleObject() to succeed, when it shouldn't.

I believe the lock is still in a consistent state.  If this race happens and SetEvent() is called then we will must have mutex->owned > -1 because the timed out waiter is still counted by mutex->owned.  This prevents the tests involving interlocked functions from giving true.  Thus WaitForSingleObject() is the ONLY way for a waiter to get the lock.

In other words, as soon as a timeout happens the fast "interlocked path" gets blocked.  It is only unblocked again after a call to WaitForSingleObject() succeeds: then the thread which now owns the lock fixes mutex->owned using mutex->timeouts and the interlocked path is operational again (unless another timeout happens).

I can certainly understand the desire to follow the KISS principle.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11618>
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