Reap foreground job-control process groups

[austinywang]

Summary

• capture the PTY foreground process-group id before subprocess teardown
• signal and probe the session-leader and distinct foreground groups under one shared deadline
• add a regression with a SIGHUP-resistant foreground job group

Verification

• red regression on babe426: foreground group remained alive
• green on 47e9bd4: zig build test -Dtest-filter="subprocess stop"
• universal ReleaseFast GhosttyKit archive validated and published

cmux issue: manaflow-ai/cmux#9573

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Summary by cubic

Reap foreground job-control process groups when stopping a subprocess so foreground jobs don’t linger after the terminal closes. Fixes SIGHUP-resistant foreground groups remaining alive (see cmux issue ghostty-org#9573).

• Bug Fixes
  • Capture the PTY’s foreground process-group id before teardown.
  • Send SIGHUP then SIGKILL to both the primary and distinct foreground groups under a shared deadline.
  • Handle direct-child reaping and transient missing-group windows; ignore expected EPERM from killpg on Darwin.
  • Add a regression test that kills a SIGHUP-resistant foreground group.

^{Written for commit 47e9bd4. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Improved terminal process shutdown to reliably terminate child processes and their descendants.
  • Added handling for processes running in separate foreground terminal groups.
  • Made shutdown more resilient when processes exit or temporarily become unavailable.
  • Improved reporting of process groups involved during terminal cleanup.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 5edd8197-583d-4064-8e6b-b5f4a67b999d

📥 Commits

Reviewing files that changed from the base of the PR and between c0de068 and 47e9bd4.

📒 Files selected for processing (1)

• src/termio/Exec.zig

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📝 Walkthrough

Walkthrough

The POSIX subprocess teardown now detects the terminal foreground process group, coordinates SIGHUP and SIGKILL handling for two groups, preserves direct-child fallback behavior, and adds regression coverage for concurrently reaped leaders.

Changes

POSIX subprocess teardown

Layer / File(s)	Summary
Detect and pass foreground group src/termio/Exec.zig	The PTY foreground process-group ID is read through ioctl. Timeout-aware termination passes this ID and persistent process-group state into POSIX shutdown.
Coordinate multi-group shutdown src/termio/Exec.zig	Shutdown tracks the primary and foreground groups independently, retries transient group absence, shares SIGHUP/SIGKILL deadlines, and reports both group IDs with the direct-child ID.
Validate descendant termination src/termio/Exec.zig	A POSIX regression test covers a concurrently reaped session leader and a distinct terminal foreground process group.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant SubprocessStop as Subprocess stopping
  participant Pty as PTY
  participant CommandTermination as POSIX command termination
  participant ProcessGroups as Primary and foreground groups

  SubprocessStop->>Pty: Read foreground process-group ID
  Pty-->>SubprocessStop: Return group ID
  SubprocessStop->>CommandTermination: Start timeout-aware shutdown
  CommandTermination->>ProcessGroups: Send SIGHUP and probe both groups
  ProcessGroups-->>CommandTermination: Return group status
  CommandTermination->>ProcessGroups: Send SIGKILL after shared deadline

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Suggested reviewers: mitchellh, rockorager

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: reaping foreground job-control process groups during subprocess teardown.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-9573-descendant-reap

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