Fix stdio transport closing sys.stdin.buffer/sys.stdout.buffer on exit#2722
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TheChyeahhh wants to merge 1 commit into
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Fix stdio transport closing sys.stdin.buffer/sys.stdout.buffer on exit#2722TheChyeahhh wants to merge 1 commit into
TheChyeahhh wants to merge 1 commit into
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When using transport='stdio', anyio.wrap_file() wraps a TextIOWrapper around sys.stdin.buffer/sys.stdout.buffer. When the server exits and the async file is closed, TextIOWrapper.close() propagates to close the underlying buffer, making subsequent stdio operations fail with: ValueError: underlying buffer has been closed This adds a _DetachingTextIOWrapper that detaches from the buffer on close, preventing the propagation. The original comment in the code stated 'Purposely not using context managers for these, as we don't want to close standard process handles' — this fix aligns the implementation with that intent. Fixes modelcontextprotocol#1933
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Description
Fixes #1933
When using
transport="stdio",anyio.wrap_file()wraps aTextIOWrapperaroundsys.stdin.buffer/sys.stdout.buffer. When the server exits, the async file close propagates throughTextIOWrapper.close()and closes the underlying buffer, making subsequent stdio operations fail withValueError: underlying buffer has been closed.Root Cause
The code comment at line 37-38 says:
But
anyio.wrap_file()creates a context manager that callsclose()on theTextIOWrapper, which in turn closes the underlyingsys.stdin.buffer— exactly what the comment says should not happen.Fix
Added
_DetachingTextIOWrapper— a thin subclass ofTextIOWrapperthat overridesclose()to detach the underlying buffer first, preventing close propagation. The buffer survives, and subsequent stdio operations work as expected after the server exits.Changes
src/mcp/server/stdio.py— +13/-2 lines