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gh-158287: Fix blocked asyncio event loop on macOS when child process is stopped - #158962
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…rocess is stopped On macOS, waitid(os.P_PID, pid, os.WEXITED | os.WNOWAIT) returns on SIGSTOP with si_code == CLD_STOPPED even though WSTOPPED was not requested. In _ThreadedChildWatcher, consume the stop notification and continue waiting for actual process termination rather than calling _reap_and_notify with os.waitpid(pid, 0) which blocks the event loop thread.
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Fixes gh-158287.
On macOS (Darwin),
os.waitid(os.P_PID, expected_pid, os.WEXITED | os.WNOWAIT)unexpectedly returns when a child process is stopped (SIGSTOP), reportingsi_code == CLD_STOPPEDdespiteWSTOPPEDnot being requested (a known Darwin kernel quirk, also observed in dotnet/runtime#133930).In
_ThreadedChildWatcher._do_waitpid, treating this as process exit schedules_reap_and_notify, whereos.waitpid(expected_pid, 0)blocks the main event loop thread indefinitely because the child has not terminated.This change consumes the stop notification via
waitid(os.P_PID, expected_pid, consume_flags)and continues waiting until actual process termination. Also adds a regression test toSubprocessThreadedWatcherTests.