bpo-31783: Fix a race condition creating workers during shutdown

Lib/concurrent/futures/thread.py

@@ -29,10 +29,14 @@
 
 _threads_queues = weakref.WeakKeyDictionary()
 _shutdown = False
+# Lock that ensures that new workers are not created while the interpreter is
+# shutting down. Must be held while mutating _threads_queues and _shutdown.
+_global_shutdown_lock = threading.Lock()
 
 def _python_exit():
     global _shutdown
-    _shutdown = True
+    with _global_shutdown_lock:
+        _shutdown = True
     items = list(_threads_queues.items())
     for t, q in items:
         q.put(None)
@@ -156,7 +160,7 @@ def __init__(self, max_workers=None, thread_name_prefix='',
         self._initargs = initargs
 
     def submit(self, fn, /, *args, **kwargs):
-        with self._shutdown_lock:
+        with self._shutdown_lock, _global_shutdown_lock:
             if self._broken:
                 raise BrokenThreadPool(self._broken)
 

Misc/NEWS.d/next/Library/2019-05-07-19-25-55.bpo-31783.lgLo69.rst

@@ -0,0 +1 @@
+Fix race condition in ThreadPoolExecutor when worker threads are created during interpreter shutdown.