fix: pytester no longer destroys modules owning process global state

[RonnyPfannschmidt]

Closes #14841.

Pytester restores its sys.modules snapshot on teardown, which destroyed modules that had installed process global state while being imported. The state outlives the module object, and the next import installs it a second time from a second module instance — for multiprocessing that means an orphaned resource tracker process whose bookkeeping no longer matches the live one, which fails loudly at interpreter shutdown.

asyncio, concurrent, execnet, multiprocessing, random and rlcompleter are now preserved next to zope and readline, each with the evidence in a comment. Matching follows package boundaries instead of a name prefix, so zopelicious is no longer preserved by the zope entry — and the check is a single set lookup, ~10x cheaper on a path that runs once per module in sys.modules per test.

scripts/check-preserved-modules.py is where the list comes from. It probes each stdlib module (plus extras such as execnet) in its own subprocess: records atexit/os.register_at_fork/threading._register_atexit registrations with caller attribution, imports the module, replays the snapshot restore, and reports what was left behind. --python audits another interpreter, --check exits non-zero on anything uncovered. A full scan takes ~6s; the list above is the union over 3.10, 3.12, 3.13 and 3.14 on Linux.

It cannot see state installed by C extension modules, or state installed lazily on first use — the tracker process itself is only found because the module registers a fork handler while being imported.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes pytester’s sys.modules snapshot restoration so it no longer drops modules/packages that install process-global state during import (notably multiprocessing.resource_tracker), preventing duplicated global registrations and noisy interpreter-shutdown failures.

Changes:

• Replaces prefix-based preserve logic with package-boundary matching via a preserved top-level package set in Pytester.
• Expands the preserved set to cover additional stdlib/related packages known to install process-global state.
• Adds an audit script (scripts/check-preserved-modules.py) plus a dedicated tox env to maintain/validate the preserved list, and adds/extends tests + changelog entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tox.ini	Adds a tox environment to run the preserved-modules audit script.
testing/test_pytester.py	Extends snapshot preservation tests and adds a regression test for multiprocessing.resource_tracker surviving teardown.
src/_pytest/pytester.py	Introduces a preserved top-level package set and a package-boundary preservation predicate used by SysModulesSnapshot.
scripts/check-preserved-modules.py	New maintenance tool to probe modules in subprocesses and report which ones leave process-global state behind after a simulated snapshot-restore.
changelog/14841.bugfix.rst	Documents the bugfix and the new audit tooling.

Suppressed comments (1)

scripts/check-preserved-modules.py:149

• Same issue as above: the restore/dangling scan assumes every sys.modules value is a module and calls vars(module). If any entry is None (or otherwise non-module), the script will crash; skip non-modules here too.

    for modname, module in list(sys.modules.items()):
        for key, value in list(vars(module).items()):

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[bluetech]

The code changes LGTM.

But I'm not sure about adding the script, I don't know if it's worth it to maintain these 500 lines for an obscure thing like this. Maybe just stick it in the issue for reference if we ever need to run it again?

[bluetech]

Would remove the note about the script (not user-facing)

Suggested change

	``asyncio``, ``concurrent``, ``execnet``, ``multiprocessing``, ``random`` and ``rlcompleter`` are now preserved next to the ``readline`` and ``zope`` entries pytest already had. The list is audited by the new ``scripts/check-preserved-modules.py``, which probes the standard library for modules that cannot be dropped safely.
	``asyncio``, ``concurrent``, ``execnet``, ``multiprocessing``, ``random`` and ``rlcompleter`` are now preserved next to the ``readline`` and ``zope`` entries pytest already had.

[bluetech]

Would remove this as well, kinda doubt anyone would be affected by this...