A stdio MCP server that never completes `initialize` (e.g. emits a
non-JSON-RPC frame and then blocks on stdin) leaks a child process plus its
stdio pipes/pidfd on every discovery-retry cycle — unbounded, until the
gateway hits EMFILE and every new open()/spawn fails (#59349).
Root cause (confirmed by instrumenting the live repro, and different from the
issue's own hypothesis): the spawned child IS captured in `new_pids`, so the
report's "new_pids empty at finally" guess is not it. The real cause is that
`session.initialize()` hangs forever on the garbage stream. `connect_timeout`
only bounds the caller's `.result()` wait on the foreground thread — it does
NOT cancel the `_run_stdio` coroutine on the background MCP loop. So the
coroutine is stuck at `await session.initialize()` permanently, its cleanup
`finally` never runs, the child is never reaped, and it stays invisible to the
orphan-reaper (whose `_orphan_stdio_pids` set never gets populated).
Fix: wrap `session.initialize()` in `asyncio.wait_for(..., connect_timeout)`
so a stalled handshake fails instead of hanging. The TimeoutError unwinds
through the SDK context managers (closing the child's stdin -> EOF -> exit)
and lets the existing `finally` reap any straggler. Cross-platform — no
signals/pgid/proc.
Scope: stdio only. The HTTP path has the same `await session.initialize()`
shape but spawns no subprocess (so it can't cause this leak) and already has
httpx transport timeouts.
Verified: the reporter's repro goes from unbounded growth to draining to zero;
added a hermetic regression test (fake transport whose `initialize()` hangs,
asserts the connect is bounded by connect_timeout) that fails on the pre-fix
code and passes on the fix; 566 existing MCP tests pass; ruff clean.
Repro confirmed on macOS (pipe FDs); the Linux-specific pidfd growth in the
report should be equivalent — the reporter offered to validate on Linux.
Closes#59349
rainbowgits
1f6836cd81