bpo-37658

A research on bpo-37658 In some cases asyncio.wait_for can lead to socket leak.

Issues with the same reason:

• MagicStack/asyncpg#467 Leaked connections caused by timeout on pool.acquire
• MagicStack/asyncpg#547 Pool never closes if acquire with timeout is cancelled

Motivation

The currently merged fix caused even more serious problem: bpo-42130 AsyncIO's wait_for can hide cancellation in a rare race condition.

An example to reproduce reported problem

Steps:

• Start server: python server.py
• Run client: python client.py
• If needed (no exceptions seen in client's output), tweak timeout and/or raise number of attempts in test_cancel() function

On success the output should be like:

Fatal error on SSL transport
protocol: <asyncio.sslproto.SSLProtocol object at 0x10156faf0>
transport: <_SelectorSocketTransport closing fd=13>
Traceback (most recent call last):
File "~/.pyenv/versions/3.9.4/lib/python3.9/asyncio/selector_events.py", line 918, in write
    n = self._sock.send(data)
OSError: [Errno 9] Bad file descriptor

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "~/.pyenv/versions/3.9.4/lib/python3.9/asyncio/sslproto.py", line 684, in _process_write_backlog
    self._transport.write(chunk)
File "~/.pyenv/versions/3.9.4/lib/python3.9/asyncio/selector_events.py", line 924, in write
    self._fatal_error(exc, 'Fatal write error on socket transport')
File "~/.pyenv/versions/3.9.4/lib/python3.9/asyncio/selector_events.py", line 719, in _fatal_error
    self._force_close(exc)
File "~/.pyenv/versions/3.9.4/lib/python3.9/asyncio/selector_events.py", line 731, in _force_close
    self._loop.call_soon(self._call_connection_lost, exc)
File "~/.pyenv/versions/3.9.4/lib/python3.9/asyncio/base_events.py", line 746, in call_soon
    self._check_closed()
File "~/.pyenv/versions/3.9.4/lib/python3.9/asyncio/base_events.py", line 510, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

Circular references when using SSL/TLS

Steps:

• Install objgraph
• Start server: python server.py
• Run client: python ssl_circular_refs.py, expected result: the error message as above
• Uncomment the line culprit_types.reverse() and run client again, expected result: no error

As side affect this example produces te following graph of references between culprit objects:

The example is tuned to get reproducable order of destructor calls to show that we get the error with one order and don't with other.