fix(core): resolve PTY exhaustion and orphan MCP subprocess leaks

[spencer426]

Summary of Changes

Fixes #15945
Fixes #20941

This PR addresses systemic PTY exhaustion and file descriptor leaks related to long-running or orphaned shell subprocesses (such as MCP servers) on macOS and Linux platforms.

The previous code had the following failure points:

1. ShellExecutionService relied on pty.kill(), which did not cleanly release the PTY master file descriptors.
2. Terminal instances generated for headless tracking were never disposed.
3. killProcessGroup invoked process.kill(-pid) but left orphaned descendants (e.g., detached processes launched by mcp_polygon) running indefinitely without a TTY.

This PR resolves these by:

1. Guaranteeing execution of pty.destroy() and headlessTerminal.dispose() on the onExit hook and the spawn catch block within ShellExecutionService.
2. Implementing recursive process tree walking via pgrep in killProcessGroup for Unix-like systems, to ensure all detached child processes are appropriately killed.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses systemic issues related to PTY exhaustion and orphaned subprocesses in the core shell execution service. By ensuring that PTY masters are properly destroyed and headless terminals are disposed of after execution, the changes prevent resource leaks. Additionally, the process termination logic has been significantly hardened to perform recursive tree walking, ensuring that all detached child processes are correctly identified and terminated.

Highlights

• PTY Resource Management: Implemented explicit PTY destruction and headless terminal disposal in ShellExecutionService to prevent file descriptor leaks and memory exhaustion.
• Process Tree Cleanup: Enhanced killProcessGroup on Unix-like systems to recursively identify and terminate orphaned child processes using pgrep, ensuring no detached subprocesses remain.
• Robustness Improvements: Added comprehensive cleanup hooks and improved error handling during process termination to ensure resources are released even when processes are already dead.

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[github-actions]

Size Change: +1.36 kB (0%)

Total Size: 34 MB

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[gemini-code-assist]

Code Review

This pull request enhances PTY and process management by introducing centralized cleanup routines and recursive process tree walking to prevent resource leaks and ensure all descendant processes are terminated. Feedback focuses on critical timing issues where terminal resources are disposed of before execution results are captured, leading to empty output. Additionally, the process killing logic should be updated to target individual descendant PIDs regardless of the group kill's success to effectively reap detached orphans.