Merge pull request #26547 from palkan/fix/actioncable-confirmation-race-condition

Avoid race condition on subscription confirmation

2 files changed, 23 insertions, 17 deletions

diff --git a/actioncable/lib/action_cable/channel/base.rb b/actioncable/lib/action_cable/channel/base.rb
index 2e589a2cfa..a866044f95 100644
--- a/actioncable/lib/action_cable/channel/base.rb
+++ b/actioncable/lib/action_cable/channel/base.rb
@@ -144,13 +144,14 @@ module ActionCable
 
         # When a channel is streaming via pubsub, we want to delay the confirmation
         # transmission until pubsub subscription is confirmed.
-        @defer_subscription_confirmation = false
+        #
+        # The counter starts at 1 because it's awaiting a call to #subscribe_to_channel
+        @defer_subscription_confirmation_counter = Concurrent::AtomicFixnum.new(1)
 
         @reject_subscription = nil
         @subscription_confirmation_sent = nil
 
         delegate_connection_identifiers
-        subscribe_to_channel
       end
 
       # Extract the action name from the passed data and process it via the channel. The process will ensure
@@ -169,6 +170,17 @@ module ActionCable
         end
       end
 
+      # This method is called after subscription has been added to the connection
+      # and confirms or rejects the subscription.
+      def subscribe_to_channel
+        run_callbacks :subscribe do
+          subscribed
+        end
+
+        reject_subscription if subscription_rejected?
+        ensure_confirmation_sent
+      end
+
       # Called by the cable connection when it's cut, so the channel has a chance to cleanup with callbacks.
       # This method is not intended to be called directly by the user. Instead, overwrite the #unsubscribed callback.
       def unsubscribe_from_channel # :nodoc:
@@ -201,12 +213,18 @@ module ActionCable
           end
         end
 
+        def ensure_confirmation_sent
+          return if subscription_rejected?
+          @defer_subscription_confirmation_counter.decrement
+          transmit_subscription_confirmation unless defer_subscription_confirmation?
+        end
+
         def defer_subscription_confirmation!
-          @defer_subscription_confirmation = true
+          @defer_subscription_confirmation_counter.increment
         end
 
         def defer_subscription_confirmation?
-          @defer_subscription_confirmation
+          @defer_subscription_confirmation_counter.value > 0
         end
 
         def subscription_confirmation_sent?
@@ -230,18 +248,6 @@ module ActionCable
           end
         end
 
-        def subscribe_to_channel
-          run_callbacks :subscribe do
-            subscribed
-          end
-
-          if subscription_rejected?
-            reject_subscription
-          else
-            transmit_subscription_confirmation unless defer_subscription_confirmation?
-          end
-        end
-
         def extract_action(data)
           (data["action"].presence || :receive).to_sym
         end
diff --git a/actioncable/lib/action_cable/channel/streams.rb b/actioncable/lib/action_cable/channel/streams.rb
index 13deb62662..dbba333353 100644
--- a/actioncable/lib/action_cable/channel/streams.rb
+++ b/actioncable/lib/action_cable/channel/streams.rb
@@ -84,7 +84,7 @@ module ActionCable
 
         connection.server.event_loop.post do
           pubsub.subscribe(broadcasting, handler, lambda do
-            transmit_subscription_confirmation
+            ensure_confirmation_sent
             logger.info "#{self.class.name} is streaming from #{broadcasting}"
           end)
         end