fix(opencode): robust process exit detection for child processes

[NachoFLizaur]

Issue for this PR

Closes #14769
Possible related issue that might get fixed by this: #15675 (it seems related based on the description)

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The bash tool and Process.spawn rely solely on Bun's proc.once("exit") to detect when a child process finishes. In containerized environments, this event is not reliably delivered (due to Bun), causing the completion promise to hang forever, tool calls get permanently stuck at status=running.

The fix removes the runtime dependency on Bun for process exit detection, replacing it with a multi-layer strategy that uses OS-level primitives:

1. close event: independent signal path from exit in Bun's internals
2. stdio end events (bash tool only): uses the pipe I/O subsystem, independent of process lifecycle monitoring
3. Polling watchdog (1s interval): checks proc.exitCode and process.kill(pid, 0) to detect dead processes via OS-level signals, completely bypassing Bun's event system
4. Server-level watchdog (5s interval in prompt.ts): external safety net that force-completes processes stuck past their timeout

All detection paths funnel through a single done() function guarded by a resolved boolean, ensuring exactly-once Promise resolution.

Also improves Shell.killTree() with an alive() check before escalating from SIGTERM to SIGKILL.

How did you verify your code works?

Polling watchdog isolation tests directly prove exit detection works WITHOUT exit/close events firing (simulates the exact Bun failure mode).

Regression tests verify normal bash tool execution is unaffected by the additional detection layers. Shell.killTree tests verify SIGTERM->alive check->SIGKILL escalation.

I also tested it in containerized opencode serve mode, bash tool calls now complete reliably instead of hanging.

Screenshots / recordings

No UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my searches, I found one potentially related PR:

Related PR Found:

• fix: resolve memory leaks and zombie processes from missing cleanup handlers #13186 - "fix: resolve memory leaks and zombie processes from missing cleanup handlers"
  • fix: resolve memory leaks and zombie processes from missing cleanup handlers #13186
  • This PR addresses similar concerns around process cleanup and preventing zombie processes, which is related to the exit detection and proper process lifecycle management that PR fix(opencode): robust process exit detection for child processes #15757 is improving.

All other search results only returned the current PR (#15757) itself. The searches did not find any open PRs that are direct duplicates of this process exit detection fix. PR #13186 is adjacent in addressing process-related issues but focuses on memory leaks and cleanup handlers rather than the specific exit detection problem.

[NachoFLizaur]

fixed description according to template

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍