improve background tests process cleanup

Author: T4rk1n

dash/testing/application_runners.py

@@ -121,6 +121,28 @@ def __init__(self, **kwargs):
         self._old_threads = list(threading._active.keys())  # type: ignore[reportAttributeAccessIssue]; pylint: disable=W0212
 
     def kill(self):
+        # First, try to gracefully shut down ThreadPoolExecutor workers.
+        # In Python 3.12+, Werkzeug's threaded mode uses ThreadPoolExecutor,
+        # and we need to signal workers to exit cleanly before injecting SystemExit.
+        # This prevents issues with Python's atexit handler trying to join killed threads.
+        try:
+            # pylint: disable=import-outside-toplevel
+            from concurrent.futures import thread as _thread_module
+
+            if hasattr(_thread_module, "_threads_queues"):
+                # Signal all thread pool workers to shut down
+                # pylint: disable=protected-access
+                for t, q in list(_thread_module._threads_queues.items()):
+                    if t.ident not in self._old_threads:
+                        try:
+                            q.put(None)  # Shutdown sentinel
+                        except Exception:  # pylint: disable=broad-except
+                            pass
+                # Give workers a moment to exit cleanly
+                time.sleep(0.1)
+        except Exception:  # pylint: disable=broad-except
+            pass
+
         # Kill all the new threads.
         for thread_id in list(threading._active):  # type: ignore[reportAttributeAccessIssue]; pylint: disable=W0212
             if thread_id in self._old_threads:
@@ -130,7 +152,8 @@ def kill(self):
                 ctypes.c_long(thread_id), ctypes.py_object(SystemExit)
             )
             if res == 0:
-                raise ValueError(f"Invalid thread id: {thread_id}")
+                # Thread might have already exited, which is fine
+                continue
             if res > 1:
                 ctypes.pythonapi.PyThreadState_SetAsyncExc(
                     ctypes.c_long(thread_id), None
@@ -208,8 +231,11 @@ def run():
 
     def stop(self):
         self.thread.kill()  # type: ignore[reportOptionalMemberAccess]
-        self.thread.join()  # type: ignore[reportOptionalMemberAccess]
-        wait.until_not(self.thread.is_alive, self.stop_timeout)  # type: ignore[reportOptionalMemberAccess]
+        # Give threads a moment to process the SystemExit
+        time.sleep(0.1)
+        self.thread.join(timeout=self.stop_timeout)  # type: ignore[reportOptionalMemberAccess]
+        # Don't use wait.until_not here - join() with timeout is sufficient
+        # and avoids potential issues with thread state checking
         self.started = False