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Title: tests: resolve TOCTOU port race conditions for streamable-HTTP/SSE tests by Ar-maan05 · Pull Request #2705 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: tests: resolve TOCTOU port race conditions for streamable-HTTP/SSE tests by Ar-maan05 · Pull Request #2705 · modelcontextprotocol/python-sdk

X Title: tests: resolve TOCTOU port race conditions for streamable-HTTP/SSE tests by Ar-maan05 · Pull Request #2705 · modelcontextprotocol/python-sdk

Description: This PR resolves intermittent test failures and flakiness in the CI matrix (specifically under pytest -n auto parallelism) caused by a time-of-check/time-of-use (TOCTOU) port race when allocating ports for test servers. Closes #2704 Root Cause Previously, test server port fixtures bound to 127.0.0.1:0 to find a free port, then closed the socket immediately to return the port number. Between this allocation and the server starting in a separate multiprocessing.Process to bind that port, another parallel worker could be assigned the same port. This caused two primary issues: OSError: [Errno 98] Address already in use (failing to start the server) Crossed/interleaved connections where a client connects to the wrong test server instance. Solution I converted the server processes to bind their socket ephemerally (127.0.0.1:0) and called listen() inside the child process itself. The socket remains open and in a listening state throughout the process lifecycle. The child process reports its dynamically allocated port back to the parent process using a unidirectional multiprocessing.Pipe. • This fully closes the TOCTOU gap because the socket is never closed between check and use. • It removes the need for wait_for_server polling (since the socket is already listening before the fixture yields). • It remains fully cross-platform compatible as it does not require inheriting or pickling socket descriptors across process boundaries (only the pipe connection and integers are sent across process boundaries). Affected Files Migrated • tests/shared/test_streamable_http.py • tests/shared/test_sse.py • tests/server/test_sse_security.py • tests/server/test_streamable_http_security.py • tests/client/test_http_unicode.py Other Cleanups • Added a reusable running_server context manager in tests/test_helpers.py. • Removed the unused wait_for_server helper from tests/test_helpers.py to maintain 100% test coverage. • Updated affected test cases to consume these fixtures directly as URL strings, avoiding unnecessary server startups. Verification Automated Tests uv run --frozen --no-default-groups --group dev pytest • Result: 1271 passed (1172 passed, 98 skipped, 1 xfailed). Linting & Formatting uv run --frozen --no-default-groups --group dev ruff check . uv run --frozen --no-default-groups --group dev pyright • Result: 0 errors, 0 warnings. Test Coverage uv run --frozen --no-default-groups --group dev coverage report • Result: 100.00% Coverage (19,675 statements, 0 missed).

Open Graph Description: This PR resolves intermittent test failures and flakiness in the CI matrix (specifically under pytest -n auto parallelism) caused by a time-of-check/time-of-use (TOCTOU) port race when allocating p...

X Description: This PR resolves intermittent test failures and flakiness in the CI matrix (specifically under pytest -n auto parallelism) caused by a time-of-check/time-of-use (TOCTOU) port race when allocating p...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2705

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