cmux-tui: reap child when reaper thread spawn fails

[lawrencecchen]

If the PTY child reaper thread cannot be created, the child handle was previously dropped without kill or wait. Keep the child in recoverable shared ownership and synchronously kill and wait on thread creation failure.

No local Cargo/Rust build per task. Ran rustfmt (edition 2024) and git diff --check.

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Summary by cubic

Fixes a leak where a failed reaper thread spawn dropped the PTY child without kill or wait, leaving the process running and the PTY master open.

Bug Fixes

• Keeps the child in shared ownership so a failed spawn can kill and wait on it, then closes the PTY master and returns the spawn error.

^{Written for commit 4b9c086. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Improved cleanup when launching a local terminal process fails.
  • Ensures partially started processes are properly stopped and collected, preventing lingering background processes.
  • Failed launches now close associated terminal resources and report the original startup error cleanly.
  • Improves reliability by preventing incomplete process launches from remaining active after an error.

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Project	Deployment	Actions	Updated
cmux166	Ready	Preview	Sep 2, 2026 6:26pm UTC
cmux41	Ready	Preview	Sep 2, 2026 6:26pm UTC

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• cmux-tui/crates/cmux-tui-core/src/surface.rs

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📝 Walkthrough

Walkthrough

The local PTY spawn path stores the child process in a shared slot. The reaper takes ownership from the slot. If thread creation fails, the caller kills and waits for the child, closes the terminal master, and returns the error.

Changes

PTY child cleanup

Layer / File(s)	Summary
Child ownership and failure cleanup cmux-tui/crates/cmux-tui-core/src/surface.rs	The spawn path shares the child through Arc<Mutex<Option<Child>>>. The reaper takes ownership from the slot. The failure path kills and waits for the child and closes the local terminal master when thread creation fails.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to b8506

This localized change ensures a PTY child is killed and reaped, and its terminal resource is closed, when reaper-thread creation fails; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 13 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains what changed, why it changed, and the checks that ran. It does not include the required Demo Video section, Review Trigger block, or Checklist, and it does not use the templat…	Add the missing template sections: Demo Video with a video or an explicit not-applicable note, the Review Trigger block, and the completed Checklist. Use the Summary and Testing headings to document the change and verification steps.
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (13 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: reaping the child process when the reaper thread cannot be spawned.
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.
Cmux Swift Actor Isolation	✅ Passed	PASS. The pull-request range changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust file. The diff contains no Swift files or actor-isolation markers, so it cannot introduce or worsen the…
Cmux Swift Blocking Runtime	✅ Passed	PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust file. The combined diff contains no Swift files or production Swift changes. The Swift blocking-runtime check…
Cmux Browser Automation Off-Main	✅ Passed	PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs. The diff adds PTY child kill/wait cleanup and closes the PTY master on reaper-thread failure. It changes no browser …
Cmux Expensive Synchronous Load	✅ Passed	PASS: The pull-request diff changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust source file. The changes manage a PTY child and reaper thread. No Swift file or expensive Swift agent-hi…
Cmux Cache Substitution Correctness	✅ Passed	PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust. The custom check applies only to production Swift, TypeScript, and JavaScript changes. Therefore, the…
Cmux No Hacky Sleeps	✅ Passed	PASS: The pull request changes only Rust code in cmux-tui/crates/cmux-tui-core/src/surface.rs. The diff adds child cleanup and PTY master closure on reaper thread creation failure. It does not chang…
Cmux Algorithmic Complexity	✅ Passed	The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust production code. The custom check applies only to production Swift, TypeScript, JavaScript, shell, and runti…
Cmux Swift Concurrency	✅ Passed	PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust. The verified diff from the PR base contains no Swift files or Swift concurrency changes. Therefore th…
Cmux Swift @Concurrent	✅ Passed	PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust. The base-to-HEAD diff contains no Swift files and no @concurrent, nonisolated async, or Swift act…
Cmux Swift Package Boundaries	✅ Passed	PASS: The pull request does not introduce production Swift changes. The combined diff for commits e36b39f and b8506ce changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust source f…

Full details: Description check

Explanation

The description explains what changed, why it changed, and the checks that ran. It does not include the required Demo Video section, Review Trigger block, or Checklist, and it does not use the template headings.

Full details: Cmux Swift Actor Isolation

Explanation

PASS. The pull-request range changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust file. The diff contains no Swift files or actor-isolation markers, so it cannot introduce or worsen the specified Swift 6 isolation mistakes.

Full details: Cmux Swift Blocking Runtime

Explanation

PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust file. The combined diff contains no Swift files or production Swift changes. The Swift blocking-runtime check is therefore not applicable, even though the Rust change uses child termination and waiting.

Full details: Cmux Browser Automation Off-Main

Explanation

PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs. The diff adds PTY child kill/wait cleanup and closes the PTY master on reaper-thread failure. It changes no browser socket command, WebKit/AppKit routing, worker router, or policy tests. The browser automation check is therefore not applicable.

Full details: Cmux Expensive Synchronous Load

Explanation

PASS: The pull-request diff changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust source file. The changes manage a PTY child and reaper thread. No Swift file or expensive Swift agent-history load is added or moved, so this Swift-specific check is not applicable.

Full details: Cmux Cache Substitution Correctness

Explanation

PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust. The custom check applies only to production Swift, TypeScript, and JavaScript changes. Therefore, the cache-substitution condition is not applicable.

Full details: Cmux No Hacky Sleeps

Explanation

PASS: The pull request changes only Rust code in cmux-tui/crates/cmux-tui-core/src/surface.rs. The diff adds child cleanup and PTY master closure on reaper thread creation failure. It does not change TypeScript, JavaScript, shell, or build/runtime script code, and it introduces no covered sleep or timer.

Full details: Cmux Algorithmic Complexity

Explanation

The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust production code. The custom check applies only to production Swift, TypeScript, JavaScript, shell, and runtime code, so its algorithmic-complexity failure conditions do not apply.

Full details: Cmux Swift Concurrency

Explanation

PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust. The verified diff from the PR base contains no Swift files or Swift concurrency changes. Therefore the Swift-specific failure conditions do not apply.

Full details: Cmux Swift `@Concurrent`

Explanation

PASS: The pull request changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, which is Rust. The base-to-HEAD diff contains no Swift files and no @concurrent, nonisolated async, or Swift actor-isolation changes. The Swift-specific check is not applicable.

Full details: Cmux Swift Package Boundaries

Explanation

PASS: The pull request does not introduce production Swift changes. The combined diff for commits e36b39f and b8506ce changes only cmux-tui/crates/cmux-tui-core/src/surface.rs, a Rust source file, with no .swift or Package.swift paths. The Swift package-boundaries check is therefore inapplicable.

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Actionable comments posted: 1

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Inline comments:
In `@cmux-tui/crates/cmux-tui-core/src/surface.rs`:
- Around line 2582-2585: Update the reaper-thread spawn failure path to call
close_local_terminal_master_after_exit(&surface) immediately after child.wait(),
ensuring the ConPTY master is closed before returning the error.

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Review profile: ASSERTIVE

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Reviewing files that changed from the base of the PR and between 4e67fe9 and 9352678.

📒 Files selected for processing (1)

• cmux-tui/crates/cmux-tui-core/src/surface.rs

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[lawrencecchen]

Closing this pull request as duplicate of PR #11414.

At the current heads, PR #11414 (72abb6e3d7928e37c92c4810498bf312d810cff8) contains the same local PTY reaper-thread failure behavior as this PR (4b9c086e4c9e869b46eb9cf9454a85149110ca25): it owns the child through thread creation, kills and waits when creation fails, and closes the PTY master. PR #11414 also includes the startup guard and wait handling. Current main still lacks this behavior, so the fix remains tracked there. No unique change from this pull request remains.