[issue8800] add threading.RWLock

[Richard Oudkerk]

Richard Oudkerk added the comment:

I think Sebastian's algorithm does not correctly deal with the non-blocking case.  Consider the following situation:

* Thread-1 successfully acquires exclusive lock.
  Now num_got_lock == 1.

* Thread-2 blocks waiting for shared lock.
  Will block until (num_got_lock == 1 and excl_count == 0).
  Now num_got_lock == 1.

* Thread-3 does non-blocking acquire of shared lock but fails.
  Now num_got_lock == 2.

Now, since num_got_lock == 2, the predicate that Thread-2 is waiting for will not happen until num_got_lock overflows.

This is probably fixable if we just prevent a failed non-blocking acquire from modifying num_acq_lock and num_got_lock.  (But I don't see how to extend the algorithm to allow timeouts.)

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