[Fix #25381] Avoid race condition on subscription confirmation

1 files changed, 18 insertions, 8 deletions

diff --git a/actioncable/lib/action_cable/channel/base.rb b/actioncable/lib/action_cable/channel/base.rb
index 2e589a2cfa..1d9038b76a 100644
--- a/actioncable/lib/action_cable/channel/base.rb
+++ b/actioncable/lib/action_cable/channel/base.rb
@@ -144,7 +144,9 @@ 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
+        #
+        # We use atomic fixnum to track the number of waiting tasks to avoid race conditions
+        @defer_subscription_confirmation_counter = Concurrent::AtomicFixnum.new(1)
 
         @reject_subscription = nil
         @subscription_confirmation_sent = nil
@@ -153,6 +155,14 @@ module ActionCable
         subscribe_to_channel
       end
 
+      # This method is called after subscription has been added to the channel.
+      # Send confirmation here to avoid race conditions when client tries to perform actions
+      # right after receiving confirmation.
+      def registered!
+        @defer_subscription_confirmation_counter.decrement
+        transmit_subscription_confirmation unless defer_subscription_confirmation?
+      end
+
       # Extract the action name from the passed data and process it via the channel. The process will ensure
       # that the action requested is a public method on the channel declared by the user (so not one of the callbacks
       # like #subscribed).
@@ -202,17 +212,21 @@ module ActionCable
         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.positive?
         end
 
         def subscription_confirmation_sent?
           @subscription_confirmation_sent
         end
 
+        def registered?
+          @registered
+        end
+
         def reject
           @reject_subscription = true
         end
@@ -235,11 +249,7 @@ module ActionCable
             subscribed
           end
 
-          if subscription_rejected?
-            reject_subscription
-          else
-            transmit_subscription_confirmation unless defer_subscription_confirmation?
-          end
+          reject_subscription if subscription_rejected?
         end
 
         def extract_action(data)