[3.13] gh-115634: Fix ProcessPoolExecutor deadlock with max_tasks_per_child (GH-140900)#152928
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…ks_per_child (pythonGH-140900) The idle worker semaphore counts task completions, not idle workers, so it can hold a stale token released by a worker that later exited upon reaching its max_tasks_per_child limit. The worker replacement path consumed such tokens and skipped spawning a replacement, deadlocking the remaining queued tasks once no workers were left. Replace dead workers based on len(self._processes) without consulting the semaphore. The submit() path is unchanged, preserving on-demand spawning and idle worker reuse. Replace the documentation note added in pythonGH-140897 with a versionchanged entry now that the bug is fixed. Based on a fix proposed by Tabrez Mohammed. (cherry picked from commit b706767)
This was referenced Jul 3, 2026
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The idle worker semaphore counts task completions, not idle workers, so it can hold a stale token released by a worker that later exited upon reaching its max_tasks_per_child limit. The worker replacement path consumed such tokens and skipped spawning a replacement, deadlocking the remaining queued tasks once no workers were left.
Replace dead workers based on len(self._processes) without consulting the semaphore. The submit() path is unchanged, preserving on-demand spawning and idle worker reuse.
Replace the documentation note added in GH-140897 with a versionchanged entry now that the bug is fixed.
Based on a fix proposed by Tabrez Mohammed.
(cherry picked from commit b706767)