Immediately close a WebSocket connection to a runner

The previous code mostly worked but there could be cases where stopping was delayed. This is especially problematic with the reservation of runners, since they could still be reserved (internally) without a visual indicator for the learners.

 The change in the SubmissionsController is optional, since a browser should also close the connection once `close_client_connection` is executed. However, this change might stop the runner earlier, which is especially useful in case of infinite loops.

Author: MrSerth

app/controllers/submissions_controller.rb

@@ -140,6 +140,7 @@ def run # rubocop:disable Metrics/PerceivedComplexity, Metrics/CyclomaticComplex
         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 @@ def close(status)
     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