fix(concurrent): resolve ThreadPoolExecutor shutdown deadlock in signal handlers

Author: Synteri

Lib/concurrent/futures/thread.py

@@ -192,7 +192,7 @@ def __init__(self, max_workers=None, thread_name_prefix='',
         self._threads = set()
         self._broken = False
         self._shutdown = False
-        self._shutdown_lock = threading.Lock()
+        self._shutdown_lock = threading.RLock()
         self._thread_name_prefix = (thread_name_prefix or
                                     ("ThreadPoolExecutor-%d" % self._counter()))
 
@@ -213,6 +213,10 @@ def submit(self, fn, /, *args, **kwargs):
 
             self._work_queue.put(w)
             self._adjust_thread_count()
+            if self._shutdown or _shutdown:
+                f.cancel()
+                w.task = None  # Clear reference to task arguments to avoid memory leak
+                raise RuntimeError('cannot schedule new futures after shutdown')
             return f
     submit.__doc__ = _base.Executor.submit.__doc__
 
@@ -252,6 +256,15 @@ def _initializer_failed(self):
                     work_item.future.set_exception(self.BROKEN(self._broken))
 
     def shutdown(self, wait=True, *, cancel_futures=False):
+        # Detect if we are called reentrantly (e.g. from a signal handler on a thread
+        # already holding self._shutdown_lock). Fallback if RLock does not support _is_owned.
+        reentrant = False
+        if hasattr(self._shutdown_lock, '_is_owned'):
+            try:
+                reentrant = self._shutdown_lock._is_owned()
+            except Exception:
+                pass
+
         with self._shutdown_lock:
             self._shutdown = True
             if cancel_futures:
@@ -268,7 +281,10 @@ def shutdown(self, wait=True, *, cancel_futures=False):
             # Send a wake-up to prevent threads calling
             # _work_queue.get(block=True) from permanently blocking.
             self._work_queue.put(None)
-        if wait:
+        
+        # If we are reentrant, we cannot join threads synchronously because the current
+        # thread is interrupted and blocking it would cause a deadlock.
+        if wait and not reentrant:
             for t in self._threads:
                 t.join()
     shutdown.__doc__ = _base.Executor.shutdown.__doc__