gh-157741: Prevent NULL 'environ' pointer after os.environ.clear()

[search1ofall-maker]

What issue does this PR fix?

Fixes #157741

Description of changes

In glibc, calling clearenv() sets the C environ pointer to NULL. Third-party C extensions and libraries (such as Tcl/Tk) access environ directly without checking for NULL, leading to a segmentation fault when os.environ.clear() is called.

This PR sets environ to a static empty array (empty_environ = { NULL }) immediately after calling clearenv() in os._clearenv, matching the safe fallback behavior used when HAVE_CLEARENV is not defined.

• Issue: os.environ.clear() now leaves the C environ pointer NULL, which crashes Tcl/Tk (tkinter) on the next Tcl_CreateInterp #157741

[python-cla-bot]

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[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[aisk]

There are multiple news entry files now, please remove the invalid one.

[search1ofall-maker]

There are multiple news entry files now, please remove the invalid one.

Cleaned up the duplicate news entry and regenerated clinic files. Thanks for catching that!

[search1ofall-maker]

Hi @aisk,

All CI checks are now passing!

To summarize the fix:

os._clearenv() now points environ to a static empty environment array (empty_environ) after clearenv() runs to prevent crashes in C libraries accessing environ.

empty_environ is declared as const with ignored.tsv updated to satisfy check-c-globals while avoiding NULL pointer dereferences.

Ready for your review whenever you have time. Thanks!

[search1ofall-maker]

Thanks @aisk! I've simplified empty_environ back to static char *empty_environ[] = {NULL};, removed the explicit cast, and sorted its entry alphabetically in ignored.tsv.

[aisk]

LGTM

[search1ofall-maker]

@aisk The failure in test_tkinter on macos-26-intel appears to be an unrelated flaky CI test (GUI/display issue on macOS runner). All core OS/environment tests passed cleanly. I think it is not because of my changes

[search1ofall-maker]

LGTM

Is it possible to merge it

[aisk]

Is it possible to merge it

Let's wait for @ericsnowcurrently or another core dev to review it. And please be patient before it gets merged, as there are a lot of pending PRs and issues in the backlog these days.

[search1ofall-maker]

All CI checks are now passing!
@ericsnowcurrently , @aisk