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Fixes modelcontextprotocol#1933 When stdio_server() wraps sys.stdin.buffer/sys.stdout.buffer directly, closing the context manager also closes the real file descriptors, causing ValueError on subsequent I/O operations. Fix: Use os.dup() to duplicate file descriptors before wrapping them. This ensures closing the wrappers doesn't affect the real stdin/stdout. Added fallback for environments where fileno() is not available (e.g., BytesIO-backed streams in tests).
Contributor
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Thanks for the PR. Tracking this in #2040 instead. I'm closing this out as part of a backlog cleanup around the v2 release. Feel free to reopen if this is still relevant. |
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Fixes #1933
Problem
When
stdio_server()wrapssys.stdin.buffer/sys.stdout.bufferdirectly, closing the context manager also closes the real file descriptors, causingValueError: I/O operation on closed fileon subsequent I/O operations.Fix
Use
os.dup()to duplicate file descriptors before wrapping them. This ensures closing the wrappers does not affect the real stdin/stdout.Added fallback for environments where
fileno()is not available (e.g., BytesIO-backed streams in tests).Testing
test_stdio_server_uses_os_dupto verify the fix-raceflag