Fix race condition when removing closed connections from the pool (#437)

Author: lovelydinosaur

httpcore/_async/connection_pool.py

@@ -273,7 +273,7 @@ async def response_closed(self, status: RequestStatus) -> None:
             # Update the state of the connection pool.
             self._requests.remove(status)
 
-            if connection.is_closed():
+            if connection.is_closed() and connection in self._pool:
                 self._pool.remove(connection)
 
             # Since we've had a response closed, it's possible we'll now be able

httpcore/_sync/connection_pool.py

@@ -273,7 +273,7 @@ def response_closed(self, status: RequestStatus) -> None:
             # Update the state of the connection pool.
             self._requests.remove(status)
 
-            if connection.is_closed():
+            if connection.is_closed() and connection in self._pool:
                 self._pool.remove(connection)
 
             # Since we've had a response closed, it's possible we'll now be able