Fix PyInstaller shutdown errors on macOS

[Ankitsinghsisodya]

In frozen (PyInstaller) builds, disable multiprocessing by limiting workers to 1. This
prevents shutdown errors caused by worker processes attempting to import modules after
PyInstaller cleanup begins.

Fixes #4823

Description

When using the prebuilt Black binary on macOS (built with PyInstaller), users encounter
a FileNotFoundError for base_library.zip during shutdown. This happens because
ProcessPoolExecutor worker processes attempt to import modules after PyInstaller has
begun cleanup of the frozen environment.

The fix adds a check for sys.frozen in reformat_many() to limit workers to 1 in
frozen builds, which forces use of ThreadPoolExecutor instead of
ProcessPoolExecutor. This avoids the shutdown race condition while still allowing
Black to function correctly.

This approach mirrors the existing fallback behavior for environments that don't support
multiprocessing (like AWS Lambda or Termux).

Checklist - did you ...

• Implement any code style changes under the --preview style, following the
  stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

Note on tests: No automated tests can be added for this fix because the issue only
manifests in PyInstaller-frozen builds. The existing test
test_works_in_mono_process_only_environment verifies the mono-process fallback logic
works correctly.

[github-actions]

diff-shades results comparing this PR (c676452) to main (14713ac):

--preview style: no changes

--stable style: no changes

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What is this? | Workflow run | diff-shades documentation

[cobaltt7]

Hey @FrankPortman, are you able to test this fix? I've built the mac binary already (build logs)

(25.12.0 binary for comparison)

[Copilot]

Pull request overview

Fixes a shutdown-time import race in PyInstaller-frozen Black binaries (notably on macOS) by preventing ProcessPoolExecutor usage in frozen environments.

Changes:

• In reformat_many(), detect frozen builds via sys.frozen and force workers = 1 to avoid multiprocessing workers.
• Add a packaging changelog entry describing the PyInstaller shutdown fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/black/concurrency.py	Forces single-worker execution in frozen builds to avoid ProcessPoolExecutor shutdown/import issues.
CHANGES.md	Documents the PyInstaller/frozen-build shutdown fix under Packaging.

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[cobaltt7]

Fuzz error is unrelated to this PR