in the _cache dictionary, even if we already got the result. We need to keep a
reference to the overlapped object, otherwise the memory may be reused and
GetQueuedCompletionStatus() may use random bytes and behaves badly.
There is still a hack for ConnectNamedPipe(): the overlapped object is not
register into _cache if the overlapped object completed directly.
Log also an error in debug mode in ProactorIocp._loop() if we get an unexpected
event.
Add a protection in ProactorIocp.close() to avoid blocking, even if it should
not happen. I still don't understand exactly why some the completion of some
overlapped objects are not notified.
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