Send subscription confirmation from server to the client to avoid race conditions.

Without this, it's very easy to send messages over a subscription even before the
redis pubsub has been fully initialized.

Now we delay calling the subscription#connected method on the client side until we
receive a subscription confirmation message from the server.

2 files changed, 31 insertions, 3 deletions

diff --git a/lib/action_cable/channel/base.rb b/lib/action_cable/channel/base.rb
index df87064195..c8292be183 100644
--- a/lib/action_cable/channel/base.rb
+++ b/lib/action_cable/channel/base.rb
@@ -73,6 +73,8 @@ module ActionCable
       include Naming
       include Broadcasting
 
+      SUBSCRIPTION_CONFIRMATION_INTERNAL_MESSAGE = 'confirm_subscription'
+
       on_subscribe   :subscribed
       on_unsubscribe :unsubscribed
 
@@ -120,6 +122,10 @@ module ActionCable
         @identifier = identifier
         @params     = params
 
+        # When a channel is streaming via redis pubsub, we want to delay the confirmation
+        # transmission until redis pubsub subscription is confirmed.
+        @defer_subscription_confirmation = false
+
         delegate_connection_identifiers
         subscribe_to_channel
       end
@@ -165,6 +171,15 @@ module ActionCable
         end
 
 
+      protected
+        def defer_subscription_confirmation!
+          @defer_subscription_confirmation = true
+        end
+
+        def defer_subscription_confirmation?
+          @defer_subscription_confirmation
+        end
+
       private
         def delegate_connection_identifiers
           connection.identifiers.each do |identifier|
@@ -177,6 +192,7 @@ module ActionCable
 
         def subscribe_to_channel
           run_subscribe_callbacks
+          transmit_subscription_confirmation unless defer_subscription_confirmation?
         end
 
 
@@ -213,6 +229,12 @@ module ActionCable
         def run_unsubscribe_callbacks
           self.class.on_unsubscribe_callbacks.each { |callback| send(callback) }
         end
+
+        def transmit_subscription_confirmation
+          logger.info "#{self.class.name} is transmitting the subscription confirmation"
+          connection.transmit ActiveSupport::JSON.encode(identifier: @identifier, type: SUBSCRIPTION_CONFIRMATION_INTERNAL_MESSAGE)
+        end
+
     end
   end
 end
diff --git a/lib/action_cable/channel/streams.rb b/lib/action_cable/channel/streams.rb
index 9fffdf1789..b5ffa17f72 100644
--- a/lib/action_cable/channel/streams.rb
+++ b/lib/action_cable/channel/streams.rb
@@ -69,12 +69,18 @@ module ActionCable
       # Start streaming from the named <tt>broadcasting</tt> pubsub queue. Optionally, you can pass a <tt>callback</tt> that'll be used
       # instead of the default of just transmitting the updates straight to the subscriber.
       def stream_from(broadcasting, callback = nil)
-        callback ||= default_stream_callback(broadcasting)
+        # Hold off the confirmation until pubsub#subscribe is successful
+        defer_subscription_confirmation!
 
+        callback ||= default_stream_callback(broadcasting)
         streams << [ broadcasting, callback ]
-        EM.next_tick { pubsub.subscribe broadcasting, &callback }
 
-        logger.info "#{self.class.name} is streaming from #{broadcasting}"
+        EM.next_tick do
+          pubsub.subscribe(broadcasting, &callback).callback do |reply|
+            transmit_subscription_confirmation
+            logger.info "#{self.class.name} is streaming from #{broadcasting}"
+          end
+        end
       end
 
       # Start streaming the pubsub queue for the <tt>model</tt> in this channel. Optionally, you can pass a