Between December 2014 and January 2015, once I succeeded to fix the root issue of the random asyncio crashes on Windows (Proactor Cancellation From Hell), I fixed more race conditions and bugs in ProactorEventLoop:

  • ConnectPipe() Race Condition
  • Race Condition in BaseSubprocessTransport._try_finish()
  • Close the transport on failure: ResourceWarning
  • Cleanup code handling pipes

Previous article: Proactor Cancellation From Hell. Next article: asyncio: WSARecv() cancellation causing data loss.

ConnectPipe() Race Condition

Once I succeeded to fix the root issue of the random asyncio crashes on Windows (Proactor Cancellation From Hell), I started to look at the ConnectPipe special case: asyncio issue #204: Investigate IocpProactor.accept_pipe() special case (don't register overlapped) (issue created at 25 Aug 2014).

At January 21, 2015, I opened bpo-23293: race condition related to IocpProactor.connect_pipe().

While fixing bpo-23095 (race condition when cancelling a _WaitHandleFuture), I saw that IocpProactor.connect_pipe() causes "GetQueuedCompletionStatus() returned an unexpected event" messages to be logged, but also to hang the test suite.

IocpProactor._register() contains the comment:

# Even if GetOverlappedResult() was called, we have to wait for the
# notification of the completion in GetQueuedCompletionStatus().
# Register the overlapped operation to keep a reference to the
# OVERLAPPED object, otherwise the memory is freed and Windows may
# read uninitialized memory.
# For an unknown reason, ConnectNamedPipe() behaves differently:
# the completion is not notified by GetOverlappedResult() if we
# already called GetOverlappedResult(). For this specific case, we
# don't expect notification (register is set to False).

IocpProactor.close() contains this comment:

# The operation was started with connect_pipe() which
# queues a task to Windows' thread pool.  This cannot
# be cancelled, so just forget it.

IocpProactor.connect_pipe() is implemented with QueueUserWorkItem() which starts a thread that cannot be interrupted. Because of that, this function requires special cases in _register() and close() methods of IocpProactor.

I proposed a solution to reimplement IocpProactor.connect_pipe() without a thread: asyncio issue #197: Rewrite IocpProactor.connect_pipe() with non-blocking calls to avoid non interruptible QueueUserWorkItem().

At January 22, 2015, I pushed commit 7ffa2c5f:

commit 7ffa2c5fdda8a9cc254edf67c4458b15db1252fa
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Thu Jan 22 22:55:08 2015 +0100

    Issue #23293, asyncio: Rewrite IocpProactor.connect_pipe()

The change adds _overlapped.ConnectPipe() which tries to connect to the pipe for asynchronous I/O (overlapped): call CreateFile() in a loop until it doesn't fail with ERROR_PIPE_BUSY. Use an increasing delay between 1 ms and 100 ms.

Race Condition in BaseSubprocessTransport._try_finish()

If the process exited before the _post_init() method was called, scheduling the call to _call_connection_lost() with call_soon() is wrong: connection_made() must be called before connection_lost().

Reuse the BaseSubprocessTransport._call() method to schedule the call to _call_connection_lost() to ensure that connection_made() and connection_lost() are called in the correct order.

At Dec 18, 2014, I pushed commit 1b9763d0. The explanation is long, but the change is basically a single line change, extract:

- self._loop.call_soon(self._call_connection_lost, None)
+ self._call(self._call_connection_lost, None)

Ordering properly callbacks in asyncio is challenging! The order matters for the semantics of asyncio: it is part of the design of the PEP 3156 -- Asynchronous IO Support Rebooted: the "asyncio" Module.

Close the transport on failure: ResourceWarning

At January 15, 2015, I pushed commit 4bf22e03, extract:

-  yield from transp._post_init()
+  try:
+      yield from transp._post_init()
+  except:
+      transp.close()
+      raise

Later, I will spend a lot of time (push many more changes) to ensure that resources are properly released (especially close transports on failure, similar to this change).

I will add many ResourceWarnings warnings in destructors when a transport, subprocess or event loop is not closed explicitly.

For example, notice the ResourceWarnings in the current destructor of _SelectorTransport:

class _SelectorTransport(transports._FlowControlMixin,

    def __del__(self, _warn=warnings.warn):
        if self._sock is not None:
            _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)

I even enhanced Python 3.6 to be able to provide the traceback where the leaked resource has been allocated thanks to my tracemalloc module. Example with filebug.py:

def func():
    f = open(__file__)
    f = None


Output with Python 3.6:

$ python3 -Wd -X tracemalloc=5 filebug.py
filebug.py:3: ResourceWarning: unclosed file <_io.TextIOWrapper name='filebug.py' mode='r' encoding='UTF-8'>
  f = None
Object allocated at (most recent call first):
  File "filebug.py", lineno 2
    f = open(__file__)
  File "filebug.py", lineno 5

The line where the warning is emitted is usually useless to understand the bug, whereas the traceback is very useful to identify the leaked resource.

See my ResourceWarning documentation.

Cleanup code handling pipes

Thanks to the new implementation of connect_pipe(), I was able to push changes to simplify the code and remove various hacks in code handling pipes.

commit 2b77c546:

commit 2b77c5467f376257ae22cbfbcb3a0e5e6349e92d
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Thu Jan 22 23:50:03 2015 +0100

    asyncio, Tulip issue 204: Fix IocpProactor.accept_pipe()

    Overlapped.ConnectNamedPipe() now returns a boolean: True if the pipe is
    connected (if ConnectNamedPipe() failed with ERROR_PIPE_CONNECTED), False if
    the connection is in progress.

    This change removes multiple hacks in IocpProactor.

commit 3d2256f6:

commit 3d2256f671b7ed5c769dd34b27ae597cbc69047c
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Mon Jan 26 11:02:59 2015 +0100

    Issue #23293, asyncio: Cleanup IocpProactor.close()

    The special case for connect_pipe() is not more needed. connect_pipe() doesn't
    use overlapped operations anymore.

commit a19b7b3f:

commit a19b7b3fcafe52b98245e14466ffc4d6750ca4f1
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Mon Jan 26 15:03:20 2015 +0100

    asyncio: Fix ProactorEventLoop.start_serving_pipe()

    If a client connected before the server was closed: drop the client (close the
    pipe) and exit.

commit e0fd157b:

commit e0fd157ba0cc92e435e7520b4ff641ca68d72244
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Mon Jan 26 15:04:03 2015 +0100

    Issue #23293, asyncio: Rewrite IocpProactor.connect_pipe() as a coroutine

    Use a coroutine with asyncio.sleep() instead of call_later() to ensure that the
    schedule call is cancelled.

    Add also a unit test cancelling connect_pipe().

commit 41063d2a:

commit 41063d2a59a24e257cd9ce62137e36c862e3ab1e
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Mon Jan 26 22:30:49 2015 +0100

    asyncio, Tulip issue 204: Fix IocpProactor.recv()

    If ReadFile() fails with ERROR_BROKEN_PIPE, the operation is not pending: don't
    register the overlapped.

    I don't know if WSARecv() can fail with ERROR_BROKEN_PIPE. Since
    Overlapped.WSARecv() already handled ERROR_BROKEN_PIPE, let me guess that it
    has the same behaviour than ReadFile().