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There are some problems with the new Windows overlapped implementation
of PipeConnection in the default branch.
1) poll(0) can return False when an empty string is in the pipe: if
 the next message in the pipe is b"" then PeekNamedPipe() returns
 (0, 0) which is indistiguishable from the case where the pipe is
 empty.
 This affects versions 2.6-3.2 of Python on Windows as well. For
 old versions I would just document the fact that poll() will fail
 to see that there is data in the pipe if the next message is an
 empty string. In practice this is not a big deal since pipes are
 primarily used for pickled objects which will never be the empty
 string.
2) _poll() forgets to check self._buffered, so it can return False
 even if there is buffered data.
3) Even if (2) is fixed, _poll() with a non-zero timeout can mess up
 message boundaries if the pipe contains messages of length zero or
 one. To fix this overlapped_GetOverlappedResult() needs to be
 changed to not swallow and forget ERROR_MORE_DATA errors.
4) In _poll() there is a race condition: if the read operation
 completes after WaitForMultipleObjects() timesout, but before the
 operation is cancelled, then the data read will be discarded.
5) The code assumes that if CancelIo()/CancelIoEx() returns
 successfully then the OVERLAPPED structure and associated buffer
 may be deallocated immediately. But the documentation says that if
 CancelIo()/CancelIoEx() succeeds then cancellation has only been
 *requested*:
 http://msdn.microsoft.com/en-us/library/aa363792%28v=vs.85%29.aspx
 If [CancelIoEx()] succeeds, the return value is nonzero. The
 cancel operation for all pending I/O operations issued by the
 calling thread for the specified file handle was successfully
 *requested*. The application must not free or reuse the
 OVERLAPPED structure associated with the canceled I/O operations
 until they have *completed*. The thread can use the
 GetOverlappedResult function to determine when the I/O operations
 themselves have been completed.
 We should wait for cancellation to *complete* by using
 GetOverlappedResult(..., TRUE), otherwise we risk a crash. If the
 operation was cancelled then FALSE is returned with GetLastError()
 == ERROR_OPERATION_ABORTED.
 I would suggest making it a programming error for the overlapped
 object to be deallocated while the operation is still pending, and
 to print a RuntimeError if that happens. (This does not prevent us
 from still trying to prevent a crash using CancelIoEx().)
6) Not a bug exactly, but poll(timeout) is no longer interruptible
 with Ctrl-C. This also means that Queue.get() is no longer
 interruptible.
--
The unit tests attached pass on Unix. On Windows versions up to 3.2,
only test_empty_string fails. On Windows version 3.3 all three fail.