Clear _shutdown flag on server startup across all backends

After a shutdown sets _streaming._shutdown, a hot-reloaded restart
in the same process keeps it set, so all new streams exit immediately
and the page shows connection errors. Clear it on startup in each
backend: lifespan.startup for FastAPI, run() for Quart and Flask.

Author: T4rk1n

dash/backends/_fastapi.py

@@ -40,6 +40,7 @@
     STREAM_HEADERS,
     STREAM_MIMETYPE,
     StreamedCallbackResponse,
+    _shutdown as _streaming_shutdown,
     keepalive_seconds,
     marker_ndjson_aiter,
     to_json,
@@ -248,7 +249,9 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
 
             async def _receive_with_shutdown():
                 msg = await receive()
-                if msg.get("type") == "lifespan.shutdown":
+                if msg.get("type") == "lifespan.startup":
+                    _streaming_shutdown.clear()
+                elif msg.get("type") == "lifespan.shutdown":
                     shutdown_active_streams()
                 return msg
 

dash/backends/_flask.py

@@ -40,6 +40,7 @@
     STREAM_HEADERS,
     STREAM_MIMETYPE,
     StreamedCallbackResponse,
+    _shutdown as _streaming_shutdown,
     andjson_lines,
     keepalive_seconds,
     ndjson_lines,
@@ -184,6 +185,7 @@ def has_request_context(self) -> bool:
         return has_request_context()
 
     def run(self, dash_app: Dash, host: str, port: int, debug: bool, **kwargs: Any):
+        _streaming_shutdown.clear()
         self.server.run(host=host, port=port, debug=debug, **kwargs)
 
     def make_response(

dash/backends/_quart.py

@@ -44,6 +44,7 @@
     STREAM_HEADERS,
     STREAM_MIMETYPE,
     StreamedCallbackResponse,
+    _shutdown as _streaming_shutdown,
     keepalive_seconds,
     marker_ndjson_aiter,
     to_json,
@@ -298,6 +299,7 @@ def run(self, dash_app: Dash, host: str, port: int, debug: bool, **kwargs: _t.An
 
         loop = asyncio.new_event_loop()
         asyncio.set_event_loop(loop)
+        _streaming_shutdown.clear()
 
         # Initialize shutdown event for WebSocket handlers
         self._ws_shutdown_event = asyncio.Event()