Improve thread safety in _PyThread_AfterFork by preserving not-starte…

…d handles
python · colesbury · May 23, 2025 · May 22, 2025 · May 22, 2025 · May 22, 2025
commit c20f566f3ea7efa5bc1ef195075a2ea156740a9a
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
@@ -296,6 +296,11 @@ _PyThread_AfterFork(struct _pythread_runtime_state *state)
             continue;
         }
 
+        // keep not_start handles – they are safe to start in the child
+        if (handle->state == THREAD_HANDLE_NOT_STARTED){
+            continue;
+        }
+
         // Mark all threads as done. Any attempts to join or detach the
         // underlying OS thread (if any) could crash. We are the only thread;
         // it's safe to set this non-atomically.