The salvaged fix only ran the CLI in its own session on POSIX and kept
plain subprocess.run on Windows — the one platform where the wedge in
#106244 is actually reproducible: CPython's run() retries an unbounded
communicate() after kill() there, so a grandchild holding the capture
pipes blocks the worker forever. On POSIX run() wait()s the PID and
returns promptly; the grandchild merely leaks (live-reproduced on Linux).
One code path for both platforms:
- _group_popen_kwargs: start_new_session=True on POSIX,
CREATE_NEW_PROCESS_GROUP + hide flags on Windows (replaces the
hide-only _windows_popen_kwargs).
- _kill_cli_process_group: os.killpg SIGKILL on POSIX, taskkill /T /F
on Windows (same kwarg set as the sibling taskkill sites).
- The Popen decodes with encoding="utf-8", errors="replace" like every
other subprocess call in this file (windows footgun rule).
- Drain test patches the kill helper instead of os.killpg so it runs on
every host.
Windows behaviour is not live-verifiable on this Linux host.
Teknium
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