test_timeline_copy is flaky because of race condition inside moto/s3

[alexanderlaw]

A number of test_timeline_copy failures, e. g.:
https://neon-github-public-dev.s3.amazonaws.com/reports/main/13040862411/index.html#/testresult/174be23113d55fb0 # [release-pg15-100000] / ARM64

test_runner/regress/test_wal_acceptor.py:2447: in test_timeline_copy
    sk.http_client().copy_timeline(
test_runner/fixtures/safekeeper/http.py:291: in copy_timeline
    res.raise_for_status()
/github/home/.cache/pypoetry/virtualenvs/non-package-mode-_pxWMzVK-py3.11/lib/python3.11/site-packages/requests/models.py:1024: in raise_for_status
    raise HTTPError(http_error_msg, response=self)
E   requests.exceptions.HTTPError: 500 Server Error: Internal Server Error for url: http://localhost:21530/v1/tenant/b2fcb6f628b105d018e72f58a7c6e8d5/timeline/2fb1c61e998b27a863bf82d06866b247/copy

...
2025-01-30 00:01:50.547 INFO [_internal.py:97] 127.0.0.1 - - [30/Jan/2025 00:01:50] "�[35m�[1mPUT /safekeeper-test-timeline-copy-release-pg15-100000-//b2fcb6f628b105d018e72f58a7c6e8d5/d834403b51e253bee4e5d6e79cbeaa2d/000000010000000000000001?x-id=CopyObject HTTP/1.1�[0m" 500 -
2025-01-30 00:01:50.578 ERROR [_internal.py:97] Error on request:
Traceback (most recent call last):
  File ".../site-packages/werkzeug/serving.py", line 370, in run_wsgi
    execute(self.server.app)
...
  File ".../site-packages/moto/s3/responses.py", line 1673, in _key_response_put
    self.backend.copy_object(
  File ".../site-packages/moto/s3/models.py", line 2792, in copy_object
    value=src_key.value,
          ^^^^^^^^^^^^^^
  File ".../site-packages/moto/s3/models.py", line 184, in value
    self._value_buffer.seek(0)
  File "/home/nonroot/.pyenv/versions/3.11.10/lib/python3.11/tempfile.py", line 825, in seek
    return self._file.seek(*args)
           ^^^^^^^^^^^^^^^^^^^^^^
ValueError: seek of closed file

and:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-10556/13029076128/index.html#/testresult/71e92c6c93540a42 # [release-pg17-100000] / ARM64
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-6560/13026466544/index.html#/testresult/f57c131cbb605e50 # [release-pg14-100000] / ARM64
https://neon-github-public-dev.s3.amazonaws.com/reports/main/12994277281/index.html#/testresult/1954bd3c905604dd # [release-pg16-100000] / X64
https://neon-github-public-dev.s3.amazonaws.com/reports/main/12748032596/index.html#/testresult/23d45078dcdc321e # [release-pg17-100000] / X64

demonstrate a race condition inside moto/s3/utils.py / setlist().

With
time.sleep(.1)
added just before the last line of the function:
https://github.com/getmoto/moto/blob/3012a95d69bf5a0f6e39695e2530bdff12982028/moto/s3/utils.py#L156-L168

I get
BUILD_TYPE=release poetry run pytest test_runner/regress/test_wal_acceptor.py::test_timeline_copy >test.log
failing with the same diagnostics on a first or second run.

Initially observed with moto 5.0.6, but then reproduced with 5.0.28 and on the current master.

With extra debug logging (see attached a patch for the master: moto-debugging.patch.txt ), I'm observing the following events:

...
2025-02-05 11:24:13.913 INFO [_internal.py:97] 127.0.0.1 - - [05/Feb/2025 11:24:13] "PUT /safekeeper-test-timeline-copy-release-pg14-500000-//3efc93439b7dd564025818f7d63d14cb/e43b91266996653cfa36101cb42948f5/000000010000000000000001?x-id=PutObject HTTP/1.1" 200 -

### moto replies to PutObject query for 3efc.../e43b.../00000001000000000000000

...
2025-02-05 11:24:14.14  INFO [models.py:2238] !!!put_object| <Thread(Thread-56 (process_request_thread), started daemon 136614384764608)>: /3efc93439b7dd564025818f7d63d14cb/e43b91266996653cfa36101cb42948f5/000000010000000000000001 False, [<moto.s3.models.FakeKey object at 0x7c400f352510>]
...
2025-02-05 11:24:14.16  INFO [models.py:2240]     !!!put_object| bucket.key: /3efc93439b7dd564025818f7d63d14cb/e43b91266996653cfa36101cb42948f5/000000010000000000000001
2025-02-05 11:24:14.16  INFO [models.py:2242]         !!!put_object| valuse: <moto.s3.models.FakeKey object at 0x7c400f356f00> / <tempfile.SpooledTemporaryFile object at 0x7c400f356080>

### put_object() finds the previous version of the object value (3efc.../e43b.../00000001000000000000000) and disposes it (closes the underlying SpooledTemporaryFile 7c400f356080):

2025-02-05 11:24:14.16  INFO [utils.py:169] !!!setlist| disposing existing_version: <Thread(Thread-56 (process_request_thread), started daemon 136614384764608)> / /3efc93439b7dd564025818f7d63d14cb/e43b91266996653cfa36101cb42948f5/000000010000000000000001, <moto.s3.models.FakeKey object at 0x7c400f356f00> / <tempfile.SpooledTemporaryFile object at 0x7c400f356080>
...
2025-02-05 11:24:14.103 INFO [responses.py:1809] !!!copy_object| <Thread(Thread-62 (process_request_thread), started daemon 136614565119680)> / ParseResult(scheme='', netloc='', path='safekeeper-test-timeline-copy-release-pg14-500000-//3efc93439b7dd564025818f7d63d14cb/e43b91266996653cfa36101cb42948f5/000000010000000000000001', params='', query='', fragment='') / safekeeper-test-timeline-copy-release-pg14-500000- / /3efc93439b7dd564025818f7d63d14cb/e43b91266996653cfa36101cb42948f5/000000010000000000000001 / None / <tempfile.SpooledTemporaryFile object at 0x7c400f356080>
2025-02-05 11:24:14.103 INFO [responses.py:1814] !!!copy_object| + <moto.s3.models.FakeBucket object at 0x7c400f353080> / <moto.s3.models.FakeKey object at 0x7c400f356f00> / <tempfile.SpooledTemporaryFile object at 0x7c400f356080>

### copy_object() attempts to copy object 3efc.../e43b.../000000010000000000000001, which still has underlying file 7c400f356080,
### though it should be replaced by put_object(), and fails:

2025-02-05 11:24:14.103 INFO [_internal.py:97] 127.0.0.1 - - [05/Feb/2025 11:24:14] "^[[35m^[[1mPUT /safekeeper-test-timeline-copy-release-pg14-500000-//3efc93439b7dd564025818f7d63d14cb/2f9f886d9a886a0d21b5299c28c8e5c8/000000010000000000000001?x-id=CopyObject HTTP/1.1^[[0m" 500 -
...
  File ".../site-packages/moto/s3/models.py", line 2977, in copy_object
    value=src_key.value,
          ^^^^^^^^^^^^^^
  File ".../site-packages/moto/s3/models.py", line 194, in value
    self._value_buffer.seek(0)
  File "/usr/lib/python3.12/tempfile.py", line 967, in seek
    return self._file.seek(*args)
           ^^^^^^^^^^^^^^^^^^^^^^
ValueError: seek of closed file

Full log of the failed run

While testing, I also saw errors "ValueError: read of closed file", which doesn't affect the test result, that is the test finishes successfully despite these errors.