test.test_asyncio.test_server.TestServer2.test_abort_clients consistently fails on Linux 6.10.x

[hroncok]

Bug report

Bug description:

Hello, we run the testsuite of the optimized and debug builds of Python in Fedora CI. Since the addition in 4159644 the test has constantly failed like this on Fedora Rawhide / Fedora Linux 41 (the development version). It passes on Fedora 40 and 39.

======================================================================
FAIL: test_abort_clients (test.test_asyncio.test_server.TestServer2.test_abort_clients)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib64/python3.13/unittest/async_case.py", line 93, in _callTestMethod
    if self._callMaybeAsync(method) is not None:
       ~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/usr/lib64/python3.13/unittest/async_case.py", line 115, in _callMaybeAsync
    return self._asyncioRunner.run(
           ~~~~~~~~~~~~~~~~~~~~~~~^
        func(*args, **kwargs),
        ^^^^^^^^^^^^^^^^^^^^^^
        context=self._asyncioTestContext,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/usr/lib64/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/usr/lib64/python3.13/asyncio/base_events.py", line 721, in run_until_complete
    return future.result()
           ~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.13/test/test_asyncio/test_server.py", line 249, in test_abort_clients
    self.assertNotEqual(s_wr.transport.get_write_buffer_size(), 0)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 0 == 0

----------------------------------------------------------------------

I was unable to reproduce this outside of Fedora CI. Perhaps this has to do with how the network is configured, no idea.

This is the output of python3.13 -m test.pythoninfo:

Python debug information
========================

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build.NDEBUG: ignore assertions (macro defined)
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build.Py_TRACE_REFS: No (sys.getobjects() missing)
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build.WITH_DTRACE: Yes
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build.WITH_MIMALLOC: Yes
build.WITH_PYMALLOC: Yes
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builtins.float.double_format: IEEE, little-endian
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curses.ncurses_version: curses.ncurses_version(major=6, minor=4, patch=20240127)
datetime.datetime.now: 2024-07-22 16:05:29.258162
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time.get_clock_info(monotonic): namespace(implementation='clock_gettime(CLOCK_MONOTONIC)', monotonic=True, adjustable=False, resolution=1e-09)
time.get_clock_info(perf_counter): namespace(implementation='clock_gettime(CLOCK_MONOTONIC)', monotonic=True, adjustable=False, resolution=1e-09)
time.get_clock_info(process_time): namespace(implementation='clock_gettime(CLOCK_PROCESS_CPUTIME_ID)', monotonic=True, adjustable=False, resolution=1e-09)
time.get_clock_info(thread_time): namespace(implementation='clock_gettime(CLOCK_THREAD_CPUTIME_ID)', monotonic=True, adjustable=False, resolution=1e-09)
time.get_clock_info(time): namespace(implementation='clock_gettime(CLOCK_REALTIME)', monotonic=False, adjustable=True, resolution=1e-09)
time.time: 1721664329.3427575
time.timezone: 0
time.tzname: ('UTC', 'UTC')
tkinter.TCL_VERSION: 8.6
tkinter.TK_VERSION: 8.6
tkinter.info_patchlevel: 8.6.14
zlib.ZLIB_RUNTIME_VERSION: 1.3.1.zlib-ng
zlib.ZLIB_VERSION: 1.3.1.zlib-ng

We invoke the installed tests like this:

python3.13 -m test -wW -j0 -i test_check_probes

I'd like to debug this and see if something is wrong with the test or perhaps in Fedora 41. But I don't know where to start.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-122136: test_asyncio: Don't fail if the kernel buffers more data than advertised #123423
• [3.13] gh-122136: test_asyncio: Don't fail if the kernel buffers more data than advertised (GH-123423) #123443

[hroncok]

I wanted to cc @CendioOssman in the report but forgot.

[CendioOssman]

What the test tries to do is to fill the kernel socket buffers so that select() stops signalling that a send() is possible.

What we do to achieve this is set an explicit send and receive buffer size, to force the kernel to stop dynamically resizing them. We then ask the kernel what the real size is (since it will do funky stuff like double the requested size), and then write that much data in the buffer. The assertNotEqual() then checks that we've been forced to do some local buffering, as the kernel is full. If this hasn't happened, the test would otherwise give a false positive.

No idea why this isn't working in Fedora's CI. And it's difficult to debug if it only happens there. :/

Perhaps you do some creative fiddling with the network settings that forces dynamic buffer scaling to stay on?

Or is there some virtualization/containerization that results in the system lying to us?

[CendioOssman]

To debug, could you add some output of getsockopt(SO_SNDBUF), getsockopt(SO_RCVBUF), ioctl(SIOCINQ), and ioctl(SIOCOUTQ)?

[hroncok]

Thank you! The system is 100% virtualized/containerized. It runs on the Tetsing Farm and I don't know much about it, but perhaps @thrix might know what is happening or how to easily reproduce this locally.

[hroncok]

To debug, could you add some output of getsockopt(SO_SNDBUF), getsockopt(SO_RCVBUF), ioctl(SIOCINQ), and ioctl(SIOCOUTQ)?

Is this C? My Python has no socket.getsockopt, even thou it is documented at https://docs.python.org/3/library/socket.html#socket.socket.getsockopt

[hroncok]

Oh, I have getsockopt on socket objects, but it takes 2 arguments (level and option).

[hroncok]

So I can do something like his and run it on the CI (outputs from my machine):

>>> import socket
>>> s = socket.socket()
>>> s.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF)
16384
>>> s.getsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF)
131072

But for ioctl, I am out of my depth. I've tried something like fcntl.ioctl(s, termios.TIOCOUTQ) (as TIOCOUTQ is SIOCOUTQ in my /usr/include/linux/sockios.h), but I get OSError: [Errno 14] Bad address.

[mcepl]

We seem to have exactly the same here when packaging 3.13.0rc1 on openSUSE/Tumbleweed,
build log. Is it possible that the chrooted environment (which is what both us and Fedora have) misses something expected by this test?

[mgorny]

I'm also seeing this sometimes when testing on Gentoo amd64, in systemd-nspawn container. However, the test is only flaky here — generally it fails when the system is busy, but passes when Python's test suite has all the CPU to itself.

[hroncok]

This now started failing in the Fedora build system as well.