Avoid generation gets hanging when user specifies multiple event loops (#5162)

Co-authored-by: Yineng Zhang <me@zhyncs.com>

python/sglang/srt/managers/tokenizer_manager.py

@@ -267,7 +267,7 @@ class TokenizerManager(TokenizerCommunicatorMixin):
             )
 
         # Request states
-        self.no_create_loop = False
+        self._chosen_loop = None
         self.rid_to_state: Dict[str, ReqState] = {}
         self.asyncio_tasks = set()
 
@@ -1170,11 +1170,14 @@ class TokenizerManager(TokenizerCommunicatorMixin):
         return background_tasks
 
     def auto_create_handle_loop(self):
-        if self.no_create_loop:
+        if self._chosen_loop is not None:
+            assert (
+                asyncio.get_event_loop() == self._chosen_loop
+            ), f"Please ensure only one event loop is ever used with SGLang. Previous loop: {self._chosen_loop}, current loop: {asyncio.get_event_loop()}"
             return
 
-        self.no_create_loop = True
         loop = asyncio.get_event_loop()
+        self._chosen_loop = loop
         self.asyncio_tasks.add(
             loop.create_task(print_exception_wrapper(self.handle_loop))
         )