[Bug 1969817] [NEW] Uncaught exception when connector is cancelled

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Olivier Gayot (ogayot):

[Impact]

 * When cancelling a connector task, the associated resolver task (if
not finished) is not cancelled and continues running.

Unfortunately, if the resolver task eventually raises an exception
(e.g., socket.gaierror), the exception will go directly to the exception
handler because none will be awaiting the task anymore.

This results in applications crashing with exceptions such as:

Task exception was never retrieved
future: <Task finished name='Task-3' coro=<TCPConnector._resolve_host() done, defined at /usr/lib/python3/dist-packages/aiohttp/connector.py:774> exception=gaierror(-2, 'Name or service not known')>
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/aiohttp/connector.py", line 829, in _resolve_host
    addrs = await \
  File "/usr/lib/python3/dist-packages/aiohttp/resolver.py", line 29, in resolve
    infos = await self._loop.getaddrinfo(
  File "/usr/lib/python3.8/asyncio/base_events.py", line 825, in getaddrinfo
    return await self.run_in_executor(
  File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/lib/python3.8/socket.py", line 918, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -2] Name or service not known

 * This aiohttp bug is the root cause of a crash in Subiquity: https://bugs.launchpad.net/ubuntu-power-systems/+bug/1969393
Currently, we build the Subiquity snap based on deb packages (including python3-aiohttp) from focal. We temporarily moved from python3-aiohttp to python3-requests in Subiquity to work around this crash.
 * Other applications based on python3-aiohttp can be affected and crash despite the absence of other bug reports.

 * The patch in the debdiff makes sure that resolver tasks end up being
awaited when the associated connector tasks gets cancelled.

[Test Plan]

 * The following chunk of code can be executed to reproduce the issue:
https://paste.ubuntu.com/p/XrnfqVHtBh/ (it takes ~60 seconds to
execute).

    * On python3-aiohttp 3.6 (focal), the exception handler wakes up
with:

    Task exception was never retrieved
    future: <Task finished name='Task-3' coro=<TCPConnector._resolve_host() done, defined at /usr/lib/python3/dist-packages/aiohttp/connector.py:774> exception=gaierror(-2, 'Name or service not known')>

    * When aiohttp is patched, nothing should happens.

 * Manually testing the patched library against Subiquity to make sure it solves https://bugs.launchpad.net/ubuntu-power-systems/+bug/1969393.
This was done on my end using this PPA: https://launchpad.net/~ogayot/+archive/ubuntu/focal-bugfix . Tests were green.

[Where problems could occur]

 * Since the patch affects a python library, any application that
depends on this library (aka. python3-aiohttp) on focal would be
affected by the upload.

 * In the unlikely event that this patch introduces a regression,
applications that depend on python3-aiohttp (i.e., in focal/universe)
can crash or raise exceptions.

 * If any package in focal/main has python3-aiohttp as a Build-Depends,
a regression could cause said package to FTBFS.

[Other Info]

 * The debdiff brings a backport of an upstream patch that is present in
aiohttp 3.7 and newer versions:

https://github.com/aio-libs/aiohttp/pull/5050

 * Upstream bug report: https://github.com/aio-libs/aiohttp/issues/4330

** Affects: python-aiohttp (Ubuntu)
     Importance: Undecided
         Status: New

-- 
Uncaught exception when connector is cancelled
https://bugs.launchpad.net/bugs/1969817
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