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Title: tests: eliminate port-allocation races in SSE/StreamableHTTP tests by maxisbey · Pull Request #2264 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: tests: eliminate port-allocation races in SSE/StreamableHTTP tests by maxisbey · Pull Request #2264 · modelcontextprotocol/python-sdk

X Title: tests: eliminate port-allocation races in SSE/StreamableHTTP tests by maxisbey · Pull Request #2264 · modelcontextprotocol/python-sdk

Description: Motivation and Context Fixes #1573. Several test files use the pattern socket.bind(("", 0)) → get port → start uvicorn in multiprocessing.Process → wait_for_server() → connect. This has a TOCTOU window between port selection and bind: with pytest-xdist parallel workers, two tests can grab the same port, leading to connections landing on the wrong server (the 404 != 200 failure in test_response) or "address already in use" errors. Prior attempt #1528 used worker-specific port ranges but was closed unmerged. How Has This Been Tested? Full suite: 1136 passed, 100% coverage Stress: 5× runs with 14 parallel workers, all pass test_response flakefinder: 20/20 pass Runtime: ~4.7s for 104 affected tests (was ~25s+ with multiprocessing) Breaking Changes None. Test infrastructure only. Types of changes Bug fix (non-breaking change which fixes an issue) New feature (non-breaking change which adds functionality) Breaking change (fix or feature that would cause existing functionality to change) Documentation update Checklist I have read the MCP Documentation My code follows the repository's style guidelines New and existing tests pass locally I have added appropriate error handling I have added or updated documentation as needed Additional context Approach New helper run_server_in_thread (tests/test_helpers.py): runs uvicorn in a daemon thread with port=0. The kernel atomically assigns a free port at bind time, then we read it back from the bound socket via server.servers[0].sockets[0].getsockname(). No window between check and use. Why not httpx.ASGITransport everywhere? I tested this — httpx.ASGITransport buffers the full response body before returning (await self.app(scope, receive, send) blocks until the app returns). This works for StreamableHTTP POST→SSE cycles where each request completes, but deadlocks for: Legacy SSE (sse_client + SseServerTransport) — the GET /sse stream stays open indefinitely StreamableHTTP standalone GET stream — same problem for server-initiated notifications Any SSE security test expecting a 200 response (the stream never completes) So I used a hybrid: File Approach test_streamable_http_security.py httpx.ASGITransport (all POST-based, responses complete) test_http_unicode.py httpx.ASGITransport (POST→SSE cycles only) test_sse_security.py run_server_in_thread (several tests need 200 on long-lived stream) test_sse.py run_server_in_thread (legacy SSE requires real HTTP) test_streamable_http.py run_server_in_thread (fixtures only — test bodies unchanged) Net: −412 lines (495 added, 907 deleted). Side effects Server-side handler code now runs in-process (same PID, different thread), so coverage.py tracks it. Removed some # pragma: no cover annotations and cleaned up unused handlers that were never actually exercised. The event_store in event_server fixture is now the same object the server uses (not a pickled copy), so tests can inspect server-side state if needed. tests/shared/test_ws.py and tests/server/mcpserver/test_integration.py still use the old pattern — out of scope for this PR. AI Disclaimer

Open Graph Description: Motivation and Context Fixes #1573. Several test files use the pattern socket.bind(("", 0)) → get port → start uvicorn in multiprocessing.Process → wait_for_server() → connect. This has a...

X Description: Motivation and Context Fixes #1573. Several test files use the pattern socket.bind(("", 0)) → get port → start uvicorn in multiprocessing.Process → wait_for_server() → connect. Th...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2264

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