Immediately close a WebSocket connection to a runner

app/controllers/submissions_controller.rb

@@ -140,6 +140,7 @@
         case event[:cmd]
           when :client_kill
             @testrun[:status] = :terminated_by_client
+            runner_socket&.close(:terminated_by_client)
             close_client_connection(client_socket)
             Rails.logger.debug('Client exited container.')
           when :result, :canvasevent, :exception

lib/runner/connection.rb

@@ -84,7 +84,12 @@
     return unless active?
 
     @status = status
-    @socket.close
+    # Close the WebSocket connection _immediately_
+    # by scheduling it for the next execution of the EventMachine reactor run.
+    # Otherwise, the message might be queued causing delays for users.
+    EventMachine.next_tick do
+      @socket.close
+    end
   end
 
   # Check if the WebSocket connection is currently established