Wait for subscription confirmation before broadcasting

app/channels/application_cable/channel.rb

@@ -2,5 +2,25 @@
 
 module ApplicationCable
   class Channel < ActionCable::Channel::Base
+    @streaming_confirmation_callback = -> {}
+
+    def ensure_confirmation_sent
+      # Currently, we are required to overwrite this ActionCable method.
+      # Once called and the subscription confirmation is sent, we call the custom callback.
+      # See https://github.com/rails/rails/issues/25333.
+      super
+      @streaming_confirmation_callback.call if subscription_confirmation_sent?
+    end
+
+    def send_after_streaming_confirmed(&block)
+      if connection.server.config.pubsub_adapter.to_s == 'ActionCable::SubscriptionAdapter::Async'
+        # We can send messages immediately if we're using the async adapter.
+        yield block
+      else
+        # We need to wait for the subscription to be confirmed before we can send further messages.
+        # Otherwise, the client might not be ready to receive them.
+        @streaming_confirmation_callback = block
+      end
+    end
   end
 end

app/channels/synchronized_editor_channel.rb

@@ -6,11 +6,15 @@ class SynchronizedEditorChannel < ApplicationCable::Channel
 
     # We generate a session_id for the user and send it to the client
     @session_id = SecureRandom.uuid
-    connection.transmit identifier: @identifier, message: {action: :session_id, session_id: @session_id}
 
-    message = create_message('connection_change', 'connected')
-    Event::SynchronizedEditor.create_for_connection_change(message, current_user, programming_group)
-    ActionCable.server.broadcast(specific_channel, message)
+    # We need to wait for the subscription to be confirmed before we can send further messages
+    send_after_streaming_confirmed do
+      connection.transmit identifier: @identifier, message: {action: :session_id, session_id: @session_id}
+
+      message = create_message('connection_change', 'connected')
+      Event::SynchronizedEditor.create_for_connection_change(message, current_user, programming_group)
+      ActionCable.server.broadcast(specific_channel, message)
+    end
   end
 
   def unsubscribed