Suppress task cancel traceback (#2812)

sanic/server/websockets/impl.py

@@ -532,12 +532,11 @@ class WebsocketImplProtocol:
             raise WebsocketClosed(
                 "Cannot receive from websocket interface after it is closed."
             )
+        assembler_get: Optional[asyncio.Task] = None
         try:
             self.recv_cancel = asyncio.Future()
-            tasks = (
+            assembler_get = asyncio.create_task(self.assembler.get(timeout))
-                self.recv_cancel,
+            tasks = (self.recv_cancel, assembler_get)
-                asyncio.ensure_future(self.assembler.get(timeout)),
-            )
             done, pending = await asyncio.wait(
                 tasks,
                 return_when=asyncio.FIRST_COMPLETED,
@@ -551,6 +550,11 @@ class WebsocketImplProtocol:
             else:
                 self.recv_cancel.cancel()
                 return done_task.result()
+        except asyncio.CancelledError:
+            # recv was cancelled
+            if assembler_get:
+                assembler_get.cancel()
+            raise
         finally:
             self.recv_cancel = None
             self.recv_lock.release()
@@ -584,16 +588,15 @@ class WebsocketImplProtocol:
                 "Cannot receive from websocket interface after it is closed."
             )
         messages = []
+        assembler_get: Optional[asyncio.Task] = None
         try:
             # Prevent pausing the transport when we're
             # receiving a burst of messages
             self.can_pause = False
             self.recv_cancel = asyncio.Future()
             while True:
-                tasks = (
+                assembler_get = asyncio.create_task(self.assembler.get(0))
-                    self.recv_cancel,
+                tasks = (self.recv_cancel, assembler_get)
-                    asyncio.ensure_future(self.assembler.get(timeout=0)),
-                )
                 done, pending = await asyncio.wait(
                     tasks,
                     return_when=asyncio.FIRST_COMPLETED,
@@ -616,6 +619,11 @@ class WebsocketImplProtocol:
                 # next message to pass into the Assembler
                 await asyncio.sleep(0)
             self.recv_cancel.cancel()
+        except asyncio.CancelledError:
+            # recv_burst was cancelled
+            if assembler_get:
+                assembler_get.cancel()
+            raise
         finally:
             self.recv_cancel = None
             self.can_pause = True