fix(ci): make pre-push single-flight runner start on hosts without POSIX signals (#9724)

[kodjima33]

Fixes #9724.

Bug

On Windows, every git push fails before any pre-push check runs:

AttributeError: module 'signal' has no attribute 'SIGHUP'

scripts/pre-push-singleflight routes through .github/scripts/preflight_runner.py, whose run_owned() built its handler map from (signal.SIGINT, signal.SIGTERM, signal.SIGHUP) unconditionally. Windows Python does not define SIGHUP, so the wrapper exits 1 and Git rejects the push.

Root cause / failure class

The runner assumes POSIX process APIs are always present. Fixing only the signal tuple would leave two more landmines on the same code path, so this PR closes the class:

1. Signal registration — owned_signals() registers only signals the host actually defines, and picks up SIGBREAK where it exists (Windows' console-interrupt signal).
2. Signal forwarding — signal_child() uses os.killpg where process groups exist (POSIX children get their own session via start_new_session), and falls back to child.terminate() elsewhere.
3. Liveness probe — process_exists() used os.kill(pid, 0). On Windows os.kill does not send a signal; it calls TerminateProcess. The moment the runner became reachable on Windows, the stale-lock probe would have killed the live lock holder. Windows now probes with OpenProcess/GetExitCodeProcess (access-denied ⇒ alive, mirroring the POSIX PermissionError branch).

Regression test

.github/scripts/test_preflight_runner.py (9 tests) exercises signal selection against Windows- and POSIX-shaped signal modules, forwarding on hosts with and without process groups, and asserts process_exists never reaches os.kill on Windows — no real signal needed to interrupt the test process, per the issue. Registered in .github/checks-manifest.yaml (preflight-runner-portability, local + ci lanes), so it is diff-scoped-selected whenever the runner changes.

Verification

• python3 .github/scripts/test_preflight_runner.py → 9 tests OK
• python3 .github/scripts/test_run_checks.py → 9 tests OK (manifest contract guard)
• make preflight on this branch → 9 checks passed, with preflight-runner-portability SELECTED by the diff
• Real run: preflight_runner.py --name wd -- sh -c 'echo "==> phase-one"; echo hello; exit 3' streamed the phase line, wrote result.json, exited 3
• A/B against HEAD: SIGINT to the runner kills the child process group in both the old and the new code — POSIX forwarding behavior is unchanged
• Old expression against a Windows-shaped signal module raises AttributeError: 'SIGHUP'; owned_signals() returns (SIGINT, SIGTERM, SIGBREAK)

Not verified on a real Windows host (no Windows machine available) — the Windows paths are covered by the hermetic tests above and by CPython's documented os.kill/OpenProcess semantics.

🤖 automated by hourly watchdog; opened for review, not merged.

[tianmind-studio]

Blocking on a real Windows host: bash scripts/pre-push-singleflight fork still exits before any pre-push check starts. After the signal fix succeeds, the wrapper hands the extensionless Bash script scripts/pre-push directly to Python's subprocess.Popen; Windows CreateProcess cannot launch it, and CPython 3.11.15 raises FileNotFoundError: [WinError 2] at run_owned.

Please pass the active Bash executable explicitly (for example, -- "$BASH" scripts/pre-push "$@" in scripts/pre-push-singleflight) and cover that wrapper contract. As written, this changes the first Windows failure but does not restore git push.

[tianmind-studio]

Thanks for the signal/process work here. I reproduced the next Windows failure on this head and opened #9804 directly against your branch; it passes the active Bash executable to the runner, makes the existing POSIX simulations runnable on Windows, and adds the wrapper to the portability check trigger.