Properly reject invalid ActionCable subscriptions

Previously, we were not properly rejecting the submission, so that the channel name was still evaluated (leading to errors). Now, we handle these cases as well.

Fixes CODEOCEAN-V2

app/channels/pg_matching_channel.rb

@@ -3,7 +3,8 @@
 class PgMatchingChannel < ApplicationCable::Channel
   def subscribed
     set_and_authorize_exercise
-    stream_from specific_channel
+
+    stream_from specific_channel unless subscription_rejected?
   end
 
   def unsubscribed
@@ -13,10 +14,6 @@ class PgMatchingChannel < ApplicationCable::Channel
     stop_all_streams
   end
 
-  def specific_channel
-    "pg_matching_channel_exercise_#{@exercise.id}"
-  end
-
   def waiting_for_match
     @current_waiting_user = PairProgrammingWaitingUser.find_or_initialize_by(user: current_user, exercise: @exercise)
     @current_waiting_user.status_waiting!
@@ -40,8 +37,14 @@ class PgMatchingChannel < ApplicationCable::Channel
     ActionCable.server.broadcast(specific_channel, {action: 'joined_pg', users: pg.users.map(&:to_page_context)})
   end
 
+  def specific_channel
+    "pg_matching_channel_exercise_#{@exercise.id}"
+  end
+
   def set_and_authorize_exercise
     @exercise = Exercise.find(params[:exercise_id])
     reject unless ExercisePolicy.new(current_user, @exercise).implement?
+  rescue ActiveRecord::RecordNotFound
+    reject
   end
 end