fix: detect stdin EOF on parent death for stdio transport#2258
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Bortlesboat wants to merge 4 commits intomodelcontextprotocol:mainfrom
Open
fix: detect stdin EOF on parent death for stdio transport#2258Bortlesboat wants to merge 4 commits intomodelcontextprotocol:mainfrom
Bortlesboat wants to merge 4 commits intomodelcontextprotocol:mainfrom
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Add a background monitor that uses select.poll() to detect POLLHUP on stdin's file descriptor. When the parent process dies and the pipe's write end closes, the monitor cancels the task group, triggering a clean shutdown. The anyio.wrap_file async iterator may not propagate EOF promptly because it runs readline() in a worker thread. The poll-based monitor detects the hang-up at the OS level independent of the worker thread. Only enabled on non-Windows platforms where select.poll() is available.
Move select import and TaskGroup import to module level, add explicit return type annotation, and add tests covering the win32 early-return, fileno failure path, POLLHUP detection, and POLLIN-only event handling.
The write_fd cleanup in finally blocks is defensive code for error cases that don't occur in the happy path. Mark with pragma: no cover to satisfy 100% coverage requirement.
Use await anyio.sleep() instead of while loops to wait for monitor cancellation. The while loop's False-condition branch was never taken because the scope always exits via cancellation, not loop termination.
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Fixes #2231
Summary
When an MCP server using
transport="stdio"has its parent process die, the server process is orphaned and continues running indefinitely. Theanyio.wrap_fileasync iterator may not propagate stdin EOF promptly because it runsreadline()in a worker thread.This adds a background monitor that uses
select.poll()to detectPOLLHUPon stdin's file descriptor. When the parent process dies and the pipe's write end closes, the monitor cancels the task group, triggering a clean shutdown.select.poll()is availableTest plan
test_stdio_serverpasses (uses custom stdin, monitor correctly skipped)