Unlocked lock

In December 2017, Yury Selivanov pushed the long awaited start_tls() function.

A newly added test failed on Windows. Later, the test started to fail randomly on Linux as well. In fact, it was a well hidden race condition in the asynchronous handshake of SSLProtocol which will take 5 months of work to be identified and fixed. The bug wasn't a recent regression, but only spotted thanks to newly added tests.

Even after this bug has been fixed, the same test still failed randomly on Windows! Once I found how to reproduce the bug, I understood that it's a very scary bug: WSARecv() cancellation randomly caused data loss! Again, it was a very well hidden bug which likely existing since the early days of the ProactorEventLoop implementation.

Previous article: Asyncio: Proactor ConnectPipe() Race Condition. Next article: asyncio: WSASend() memory leak.

New start_tls() function

The "starttls" feature have been requested since creation of asyncio. At October 24, 2013, Guido van Rossum created asyncio issue #79:

Glyph [Lefkowitz] and Antoine [Pitrou] really want a API to upgrade an existing Transport/Protocol pair to SSL/TLS, without having to create a new protocol.

At March 23, 2015, Giovanni Cannata created bpo-23749 which is basically the same feature request. I replied:

asyncio got a new SSL implementation which makes possible to implement STARTTLS. Are you interested to implement it?

Elizabeth Myers, Antoine Pitrou, Guido van Rossum and Yury Selivanov designed the feature. Yury wrote a prototype in 2015 for PostgreSQL. In 2017, Barry Warsaw wrote his own implementation for SMTP.

At the end of 2017, four year after Guido van Rossum created the feature request, Yury Selivanov implemented the feature and pushed the commit f111b3dc:

commit f111b3dcb414093a4efb9d74b69925e535ddc470
Author: Yury Selivanov <yury@magic.io>
Date:   Sat Dec 30 00:35:36 2017 -0500

    bpo-23749: Implement loop.start_tls() (#5039)

SSLProtocol Race Condition

Test fails on AppVeyor (Windows): temporary fix

At December 30, 2017, just after Yury pushed his implementation of start_tls() (the same day), Antoine Pitrou reported bpo-32458: it seems test_asyncio fails sporadically on AppVeyor:

ERROR: test_start_tls_server_1 (test.test_asyncio.test_sslproto.ProactorStartTLS)
Traceback (most recent call last):
  File "C:\projects\cpython\lib\test\test_asyncio\test_sslproto.py", line 284, in test_start_tls_server_1
    asyncio.wait_for(main(), loop=self.loop, timeout=10))
  File "C:\projects\cpython\lib\asyncio\base_events.py", line 440, in run_until_complete
    return future.result()
  File "C:\projects\cpython\lib\asyncio\tasks.py", line 398, in wait_for
    raise futures.TimeoutError()

Yury Selivanov wrote:

I'm leaving on a two-weeks vacation today. To avoid risking breaking the workflow, I'll mask this tests on AppVeyor. I'll investigate this when I get back.

and skipped the test as a temporary fix, commit 0c36bed1:

commit 0c36bed1c46d07ef91d3e02e69e974e4f3ecd31a
Author: Yury Selivanov <yury@magic.io>
Date:   Sat Dec 30 15:40:20 2017 -0500

    bpo-32458: Temporarily mask start-tls proactor test on Windows (#5054)

Bug reproduced on Linux

At May 23, 2018, five month after the bug have been reported, I wrote:

test_start_tls_server_1() just failed on my Linux. It likely depends on the system load.

Christian Heimes added:

[On Linux,] It's failing reproducible with OpenSSL 1.1.1 and TLS 1.3 enabled. I haven't seen it failing with TLS 1.2 yet.

At May 28, 2018, I found a reliable way to reproduce the issue on Linux:

Open 3 terminals and run these commands in parallel:

  1. ./python -m test test_asyncio -m test_start_tls_server_1 -F
  2. ./python -m test -j16 -r
  3. ./python -m test -j16 -r

It's a race condition which doesn't depend on the OS, but on the system load.

Root issue identified

Once I found how to reproduce the bug, I was able to investigate it. I created bpo-33674.

I found a race condition in SSLProtocol of asyncio/sslproto.py. Sometimes, _sslpipe.feed_ssldata() is called before _sslpipe.shutdown().

  • SSLProtocol.connection_made() -> SSLProtocol._start_handshake(): self._loop.call_soon(self._process_write_backlog)
  • SSLProtoco.data_received(): direct call to self._sslpipe.feed_ssldata(data)
  • Later, self._process_write_backlog() calls self._sslpipe.do_handshake()

The first write is delayed by call_soon(), whereas the first read is a direct call to the SSL pipe.


diff --git a/Lib/asyncio/sslproto.py b/Lib/asyncio/sslproto.py
index 2bfa45dd15..4a5dbb38a1 100644
--- a/Lib/asyncio/sslproto.py
+++ b/Lib/asyncio/sslproto.py
@@ -592,7 +592,7 @@ class SSLProtocol(protocols.Protocol):
         # (b'', 1) is a special value in _process_write_backlog() to do
         # the SSL handshake
         self._write_backlog.append((b'', 1))
-        self._loop.call_soon(self._process_write_backlog)
+        self._process_write_backlog()
         self._handshake_timeout_handle = \

Yury Selivanov wrote:

The fix is correct and the bug is now obvious: data_received() occurs pretty much any time after connection_made() call; if call_soon() is used in connection_made(), data_received() may find the protocol in an incorrect state.

Kudos Victor for debugging this.

I pushed commit be00a558:

commit be00a5583a2cb696335c527b921d1868266a42c6
Author: Victor Stinner <vstinner@redhat.com>
Date:   Tue May 29 01:33:35 2018 +0200

    bpo-33674: asyncio: Fix SSLProtocol race (GH-7175)

    Fix a race condition in SSLProtocol.connection_made() of
    asyncio.sslproto: start immediately the handshake instead of using
    call_soon(). Previously, data_received() could be called before the
    handshake started, causing the handshake to hang or fail.

... the change is basically a single line change:

- self._loop.call_soon(self._process_write_backlog)
+ self._process_write_backlog()

I closed bpo-32458 and Yury Selivanov closed bpo-33674.

Not a regression

The SSLProtocol race condition wasn't new: it existed since January 2015, commit 231b404c:

commit 231b404cb026649d4b7172e75ac394ef558efe60
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Wed Jan 14 00:19:09 2015 +0100

    Issue #22560: New SSL implementation based on ssl.MemoryBIO

    The new SSL implementation is based on the new ssl.MemoryBIO which is only
    available on Python 3.5. On Python 3.4 and older, the legacy SSL implementation
    (using SSL_write, SSL_read, etc.) is used. The proactor event loop only
    supports the new implementation.

    The new asyncio.sslproto module adds _SSLPipe, SSLProtocol and
    _SSLProtocolTransport classes. _SSLPipe allows to "wrap" or "unwrap" a socket
    (switch between cleartext and SSL/TLS).

    Patch written by Antoine Pitrou. sslproto.py is based on gruvi/ssl.py of the
    gruvi project written by Geert Jansen.

    This change adds SSL support to ProactorEventLoop on Python 3.5 and newer!

    It becomes also possible to implement STARTTTLS: switch a cleartext socket to

This is the new cool asynchronous SSL implementation written by Antoine Pitrou and Geert Jansen. It took 3 years and new functional tests to discover the race condition.

WSARecv() cancellation causing data loss

Yet another very boring buildbot test failure

At May 30, 2018, the day after I fixed SSLProtocol race condition, I created bpo-33694.

test_asyncio.test_start_tls_server_1() got multiple fixes recently (see bpo-32458 and bpo-33674)... but it still fails on Python on x86 Windows7 3.x at revision bb9474f1fb2fc7c7ed9f826b78262d6a12b5f9e8 which contains all these fixes.

The test fails even when test_asyncio is re-run alone (not when other tests run in parallel).

Example of failure:

ERROR: test_start_tls_server_1 (test.test_asyncio.test_sslproto.ProactorStartTLSTests)
Traceback (most recent call last):
  File "...\lib\test\test_asyncio\test_sslproto.py", line 467, in test_start_tls_server_1
  File "...\lib\asyncio\base_events.py", line 566, in run_until_complete
    raise RuntimeError('Event loop stopped before Future completed.')
RuntimeError: Event loop stopped before Future completed.

The test fails also on x86 Windows7 3.7. Moreover, 3.7 got an additional failure:

ERROR: test_pipe_handle (test.test_asyncio.test_windows_utils.PipeTests)
Traceback (most recent call last):
  File "...\lib\test\test_asyncio\test_windows_utils.py", line 73, in test_pipe_handle
    raise RuntimeError('expected ERROR_INVALID_HANDLE')
RuntimeError: expected ERROR_INVALID_HANDLE

Unable to reproduce the bug

Yury Selivanov failed to reproduce the issue in Windows 7 VM (on macOS) using:

  1. run test_asyncio
  2. run test_asyncio.test_sslproto
  3. run test_asyncio.test_sslproto -m test_start_tls_server_1

Andrew Svetlov added:

I used SNDBUF to enforce send buffer overloading. It is not required by sendfile tests but I thought that better to have non-mocked way to test such situations. We can remove the socket buffers size manipulation at all without any problem.

But Yury Selivanov replied:

When I tried to do that I think I was having more failures with that test. But really up to you.

Next days, I reported more and more similar failures on Windows buildbots and AppVeyor (our Windows CI).

Root issue identified: pause_reading()

Since this bug became more and more frequent, I decided to work on it. Yury and Andrew failed to reproduce it.

At June 7, 2018, I managed to reproduce the bug on Linux by inserting a sleep at the right place... I understood one hour later that my patch is wrong: "it introduces a bug in the test".

On the other hand, I found the root cause: calling pause_reading() and resume_reading() on the transport is not safe. Sometimes, we loose data. See the ugly hack described in the TODO comment below:

class _ProactorReadPipeTransport(_ProactorBasePipeTransport,
    """Transport for read pipes."""
    def pause_reading(self):
        if self._closing or self._paused:
        self._paused = True

        if self._read_fut is not None and not self._read_fut.done():
            # TODO: This is an ugly hack to cancel the current read future
            # *and* avoid potential race conditions, as read cancellation
            # goes through `future.cancel()` and `loop.call_soon()`.
            # We then use this special attribute in the reader callback to
            # exit *immediately* without doing any cleanup/rescheduling.
            self._read_fut.__asyncio_cancelled_on_pause__ = True

            self._read_fut = None
            self._reschedule_on_resume = True

        if self._loop.get_debug():
            logger.debug("%r pauses reading", self)

If you remove the "ugly hack", the test no longer hangs...

Extract of _ProactorReadPipeTransport.set_transport():

if self.is_reading():
    # reset reading callback / buffers / self._read_fut

This method cancels the pending overlapped WSARecv(), and then creates a new overlapped WSARecv().

Even after CancelIoEx(old overlapped), the IOCP loop still gets an event for the completion of the cancelled overlapped WSARecv(). Problem: since the Python future is cancelled, the event is ignored and so 176 bytes of data are lost.

I'm surprised that an overlapped WSARecv() cancelled by CancelIoEx() still returns data when IOCP polls for events.

Something else. The bug occurs when CancelIoEx() (on the current overlapped WSARecv()) fails internally with ERROR_NOT_FOUND. According to overlapped.c, it means:

/* CancelIoEx returns ERROR_NOT_FOUND if the I/O completed in-between */

HasOverlappedIoCompleted() returns 0 in that case.

The problem is that currently, Overlapped.cancel() also returns None in that case, and later the asyncio IOCP loop ignores the completion event and so drops incoming received data.

Release blocker bug?

Yury, Andrew, Ned: I set the priority to release blocker because I'm scared by what I saw. The START TLS has a race condition in its ProactorEventLoop implementation. But the bug doesn't see to be specific to START TLS, but rather to transport.set_transport(), and even more generally to transport.pause_reading() / transport.resume_reading(). The bug is quite severe: we loose data and it's really hard to know why (I spent a few hours to add many many print and try to reproduce on a very tiny reliable unit test). As an asyncio user, I expect that transports are 100% reliable, and I would first look into my code (like looking into start_tls() implementation in my case).

If the bug was very specific to start_tls(), I would suggest to "just" "disable" start_tls() on ProactorEventLoop (sorry, Windows!). But since the data loss seems to concern basically any application using ProactorEventLoop, I don't see any simple workaround.

My hope is that a fix can be written shortly to not block the 3.7.0 final release for too long :-(

Yury, Andrew: Can you please just confirm that it's a regression and that a release blocker is justified?

Functional test reproducing the bug

I wrote race.py script: simple echo client and server sending packets in both directions. Pause/resume reading the client transport every 100 ms to trigger the bug.

Using ProactorEventLoop and 2000 packets of 16 KiB, I easily reproduce the bug.

So again, it's nothing related to start_tls(), start_tls() was just one way to spot the bug.

The bug is in Proactor transport: the cancellation of overlapped WSARecv() sometime drops packets. The bug occurs when CancelIoEx() fails with ERROR_NOT_FOUND which means that the I/O (WSARecv()) completed.

One solution would be to not cancel WSARecv() on pause_reading(): wait until the current WSARecv() completes, store data somewhere but don't pass it to protocol.data_received(), and don't schedule a new WSARecv(). Once reading is resumed: call protocol.data_received() and schedule a new WSARecv().

That would be a workaround. I don't know how to really fix WSARecv() cancellation without loosing data. A good start would be to modify Overlapped.cancel() to return a boolean to notice if the overlapped I/O completed even if we just cancelled it. Currently, the corner case (CancelIoEx() fails with ERROR_NOT_FOUND) is silently ignored, and then the IOCP loop silently ignores the event of completed I/O...

Fix the bug: no longer cancel WSARecv()

At June 8, 2018, I pushed commit 79790bc3:

commit 79790bc35fe722a49977b52647f9b5fe1deda2b7
Author: Victor Stinner <vstinner@redhat.com>
Date:   Fri Jun 8 00:25:52 2018 +0200

    bpo-33694: Fix race condition in asyncio proactor (GH-7498)

    The cancellation of an overlapped WSARecv() has a race condition
    which causes data loss because of the current implementation of
    proactor in asyncio.

    No longer cancel overlapped WSARecv() in _ProactorReadPipeTransport
    to work around the race condition.

    Remove the optimized recv_into() implementation to get simple
    implementation of pause_reading() using the single _pending_data

    Move _feed_data_to_bufferred_proto() to protocols.py.

    Remove set_protocol() method which became useless.

I fixed the root issue (in Python 3.7 and future Python 3.8).

I used my race.py script to validate that the issue is fixed for real.


I fixed one race condition in the asynchronous handshake of SSLProtocol.

I found and fixed a data loss bug caused by WSARecv() cancellation.

Lessons learnt from these two bugs:

  • You should write an extensive test suite for your code.
  • You should keep an eye on your continuous integration (CI): any tiny test failure can hide a very severe bug.