UniversalEvent's race condition on cancellation #375
Description
UniversalEvent (and consequently UniversalResult) has a race condition in the _unblock_waiters() method. Since discarding a future object from the _waiting set is done without holding the lock[1][2][3], it may occur at iteration time in the _unblock_waiters() method. As a result, RuntimeError will be raised.
Since this is a multi-threaded issue, it cannot be reliably reproduced. Instead, below is synthetic code that demonstrates that behavior.
items = {1, 2, 3}
for item in items: # raises RuntimeError
items.discard(item + 1)The quickest solution is to copy the set before iterating.
Note that this is actually #352. The fixes were not sufficient - they reduce the reproducibility of the issue, but they do not fix the issue itself. Getting a reference to the set and calling its method are different operations, and context switching can happen in between.