[issue8800] add threading.RWLock

[Richard Oudkerk]

Richard Oudkerk added the comment:

> Multiprocessing:  Because there is no way I know to share a list of 
> owning thread ids, this version is more limited

Why do you need a *shared* list?  I think it should be fine to use a per-process list of owning thread ids.   So the current thread owns the lock if and only if it is in the current process's list of owners.

(On Unix you should probably clear the list when you fork by using multiprocessing.util.register_after_fork() in the initializer.)

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<http://bugs.python.org/issue8800>
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