Fix Windows graceful process shutdown with CTRL_C_EVENT

[felixweinberger]

Summary

• Enables graceful shutdown of Windows processes by using signal.CTRL_C_EVENT before forceful termination
• Fixes issue where cleanup code after yield in lifespan context managers was not executed on Windows
• Addresses GitHub Issue The cleanup procedure after "yield" in lifespan is unreachable on Windows #1027

Problem

On Windows, the cleanup procedure after yield in the lifespan context manager was not executed because process.terminate() forcefully killed processes without allowing cleanup code to run. This prevented proper resource cleanup and graceful shutdown of MCP servers on Windows.

Solution

Modified the Windows process termination logic to:

1. First attempt graceful shutdown using signal.CTRL_C_EVENT (Windows-specific signal)
2. Wait up to 2 seconds for the process to exit gracefully
3. Fall back to terminate() if graceful shutdown fails
4. Finally use kill() as a last resort

The fix is applied to both:

• FallbackProcess.__aexit__: Handles process cleanup when exiting context manager
• terminate_windows_process(): Utility function for terminating Windows processes

Technical Details

• Uses os.kill(pid, signal.CTRL_C_EVENT) to send interrupt signal on Windows
• Uses getattr(signal, "CTRL_C_EVENT") to avoid type errors on non-Windows platforms
• Maintains backward compatibility with existing behavior
• Includes proper exception handling for ProcessLookupError and OSError

Test Plan

• All existing stdio tests pass
• Code passes ruff formatting and linting
• Code passes pyright type checking
• Manual testing confirms cleanup code executes properly
• Windows-specific testing to verify CTRL_C_EVENT behavior
• Verify no regression in process termination reliability

Related Issues

• Fixes The cleanup procedure after "yield" in lifespan is unreachable on Windows #1027: Cleanup after yield in lifespan not executed on Windows
• Related to Fix Windows subprocess NotImplementedError (STDIO clients) #596: Fix Windows subprocess NotImplementedError
• Related to Bug: RuntimeError during cleanup on Windows – no running event loop #575: RuntimeError during cleanup on Windows

[ihrpr]

@felixweinberger is there a way to add a test for this?

[felixweinberger]

@felixweinberger is there a way to add a test for this?

Yes accidentally pushed as non-draft and planning to test on a Windows VM - working on this atm.