Test failure in test_executemany_server_failure_during_writes

Start with tests/test_execute.py, especially TestExecuteMany.test_executemany_server_failure_during_writes around the failing assertion at line 215. Reproduce the test with Python 3.12, PostgreSQL 15.4, and asyncio, then compare the observed position of 128 with the test's expected early stop. Done means the failure is understood and the test passes consistently without an unjustified platform-specific skip.

• asyncpg version: 0.29.0
• PostgreSQL version: 15.4
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: N/A, building the python-asyncpg package on Fedora Linux infrastructure
• Python version: 3.12.0
• Platform: Fedora Linux Rawhide, s390x architecture (only!)
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: No, I am the maintainer of the distribution package.
• If you built asyncpg locally, which version of Cython did you use?: 0.29.35
• Can the issue be reproduced under both asyncio and
  uvloop?: No, only asyncio. yes

This is new in 0.29.0. I don’t have any idea why it is happening.

I can’t reproduce this with PYTHONASYNCIODEBUG=1, nor can I reproduce it with USE_UVLOOP=1. I have now seen this on x86_64 with asyncio and PYTHONASYNCIODEBUG=1.

I don’t have interactive access to an s390x machine, but I am happy to run experiments or test proposed fixes by submitting package test builds on Fedora infrastructure. For the time being, I plan to simply skip this test on s390x.

=================================== FAILURES ===================================
________ TestExecuteMany.test_executemany_server_failure_during_writes _________
Traceback (most recent call last):
  File "/usr/lib64/python3.12/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/usr/lib64/python3.12/unittest/case.py", line 634, in run
    self._callTestMethod(testMethod)
  File "/usr/lib64/python3.12/unittest/case.py", line 589, in _callTestMethod
    if method() is not None:
       ^^^^^^^^
  File "/builddir/build/BUILDROOT/python-asyncpg-0.29.0-1.fc40.s390x/usr/lib64/python3.12/site-packages/asyncpg/_testbase/__init__.py", line 92, in wrapper
    self.loop.run_until_complete(coro)
  File "/usr/lib64/python3.12/asyncio/base_events.py", line 664, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/builddir/build/BUILD/asyncpg-0.29.0/tests/test_execute.py", line 215, in test_executemany_server_failure_during_writes
    self.assertLess(pos, 128, 'should stop early')
  File "/usr/lib64/python3.12/unittest/case.py", line 1257, in assertLess
    self.fail(self._formatMessage(msg, standardMsg))
  File "/usr/lib64/python3.12/unittest/case.py", line 715, in fail
    raise self.failureException(msg)
AssertionError: 128 not less than 128 : should stop early