websocket: Add warning if client connection isn't closed cleanly

This gives a warning that is not dependent on GC for the issue
in tornadoweb#3257.

tornado/websocket.py

@@ -20,6 +20,7 @@
 import struct
 import tornado
 from urllib.parse import urlparse
+import warnings
 import zlib
 
 from tornado.concurrent import Future, future_set_result_unless_cancelled
@@ -1410,6 +1411,15 @@ def __init__(
             104857600,
         )
 
+    def __del__(self) -> None:
+        if self.protocol is not None:
+            # Unclosed client connections can sometimes log "task was destroyed but
+            # was pending" warnings if shutdown strikes at the wrong time (such as
+            # while a ping is being processed due to ping_interval). Log our own
+            # warning to make it a little more deterministic (although it's still
+            # dependent on GC timing).
+            warnings.warn("Unclosed WebSocketClientConnection", ResourceWarning)
+
     def close(self, code: Optional[int] = None, reason: Optional[str] = None) -> None:
         """Closes the websocket connection.