IOLoop.current(): only create automatic loop on main thread

tornado/ioloop.py

@@ -30,6 +30,7 @@
 import numbers
 import os
 import sys
+import threading
 import time
 import math
 import random
@@ -257,7 +258,9 @@ def current(instance: bool = True) -> Optional["IOLoop"]:  # noqa: F811
         except RuntimeError:
             if not instance:
                 return None
-            # Create a new asyncio event loop for this thread.
+            if threading.current_thread() is not threading.main_thread():
+                raise
+            # Create a new asyncio event loop for the main thread.
             loop = asyncio.new_event_loop()
             asyncio.set_event_loop(loop)