Make SubprocessServer shared cache purging idempotent

[shunping]

Avoid raising ValueError during atexit shutdown when a cached subprocess server was already cleaned up earlier. Right now, such exception is already ignored (from the log below).

Example traceback (https://github.com/apache/beam/actions/runs/25705764624/job/75475412939):

Exception ignored in atexit callback: <bound method StopOnExitJobServer.stop of <apache_beam.runners.portability.job_server.StopOnExitJobServer object at 0x7f60831f4da0>>
Traceback (most recent call last):
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/runners/portability/job_server.py", line 98, in stop
    self._job_server.stop()
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/runners/portability/job_server.py", line 132, in stop
    return self._server.stop()
           ^^^^^^^^^^^^^^^^^^^
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/utils/subprocess_server.py", line 254, in stop
    self.stop_process()
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/utils/subprocess_server.py", line 259, in stop_process
    self._cache.purge(self._owner_id)
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/utils/subprocess_server.py", line 94, in purge
    raise ValueError(f"{owner} not in {self._live_owners}")
ValueError: 267 not in set()
Exception ignored in atexit callback: <bound method StopOnExitJobServer.stop of <apache_beam.runners.portability.job_server.StopOnExitJobServer object at 0x7f603aeb8ef0>>
Traceback (most recent call last):
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/runners/portability/job_server.py", line 98, in stop
    self._job_server.stop()
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/runners/portability/job_server.py", line 132, in stop
    return self._server.stop()
           ^^^^^^^^^^^^^^^^^^^
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/utils/subprocess_server.py", line 254, in stop
    self.stop_process()
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/utils/subprocess_server.py", line 259, in stop_process
    self._cache.purge(self._owner_id)
  File "/runner/_work/beam/beam/sdks/python/test-suites/tox/py312/build/srcs/sdks/python/target/.tox-py312/py312/lib/python3.12/site-packages/apache_beam/utils/subprocess_server.py", line 94, in purge
    raise ValueError(f"{owner} not in {self._live_owners}")
ValueError: 2 not in set()

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where subprocess servers would raise a ValueError during atexit shutdown if the cache had already been cleaned up. By making the purge operation idempotent and logging a warning instead of failing, the system now handles redundant cleanup calls gracefully, ensuring cleaner process termination.

Highlights

• Idempotent Cache Purging: Modified the SubprocessServer cache purging logic to be idempotent, preventing ValueError exceptions during atexit shutdown.
• Logging Improvement: Replaced the exception raise with a warning log when an owner is already purged, improving observability during shutdown sequences.
• Test Coverage: Updated existing tests to verify idempotent behavior and added a new test case to ensure stop_process() succeeds even after the cache has been purged.

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[gemini-code-assist]

Code Review

This pull request modifies the purge method in subprocess_server.py to be idempotent, replacing a ValueError with a warning log when an owner is already purged. This change addresses issues where subprocesses are cleaned up multiple times during shutdown or test teardown. The test suite has been updated to reflect this behavior and includes a new test case for verification. Feedback was provided regarding the placement of the log message within a lock block, which could lead to performance issues or deadlocks.

[github-actions]

Assigning reviewers:

R: @jrmccluskey for label python.

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[jrmccluskey]

LGTM