Wait on callback pipes with poll - #78
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select() is undefined for a descriptor above FD_SETSIZE, so a VM with more than 1024 open files could not bring up a thread worker and every thread callback after that failed with "Failed to spawn thread handler". The ready-wait also releases the GIL, and the coordinator logs a failed ready signal instead of leaving Python to time out.
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The macOS CI flake in
py_reentrant_SUITEwas not a flake: the thread-callback pipes waited withselect(), which cannot watch a descriptor above 1024, and a test run that had just exercised the isolated suites had that many files open. The first worker created past that point timed out on its ready signal and, since it stayed in the pool, took every later thread callback down with it. The waits now usepoll(), the ready-wait no longer holds the GIL, and the coordinator reports a failed ready write with its reason. A new case forces descriptors above the limit and runs several thread callbacks at once; with the fix reverted it reproduces the CI failure exactly.