Add guard to broadcast.

2 files changed, 22 insertions, 1 deletions

diff --git a/actioncable/lib/action_cable/subscription_adapter/subscriber_map.rb b/actioncable/lib/action_cable/subscription_adapter/subscriber_map.rb
index 37eed09793..4ec513e3ba 100644
--- a/actioncable/lib/action_cable/subscription_adapter/subscriber_map.rb
+++ b/actioncable/lib/action_cable/subscription_adapter/subscriber_map.rb
@@ -32,7 +32,11 @@ module ActionCable
       end
 
       def broadcast(channel, message)
-        list = @sync.synchronize { @subscribers[channel].dup }
+        list = @sync.synchronize do
+          return if !@subscribers.key?(channel)
+          @subscribers[channel].dup
+        end
+
         list.each do |subscriber|
           invoke_callback(subscriber, message)
         end
diff --git a/actioncable/test/subscription_adapter/subscriber_map_test.rb b/actioncable/test/subscription_adapter/subscriber_map_test.rb
new file mode 100644
index 0000000000..5965ac2b90
--- /dev/null
+++ b/actioncable/test/subscription_adapter/subscriber_map_test.rb
@@ -0,0 +1,17 @@
+require 'test_helper'
+
+class SubscriberMapTest < ActionCable::TestCase
+  test "broadcast should not change subscribers" do
+    setup_subscription_map
+    origin = @subscription_map.instance_variable_get(:@subscribers).dup
+
+    @subscription_map.broadcast('not_exist_channel', '')
+
+    assert_equal origin, @subscription_map.instance_variable_get(:@subscribers)
+  end
+
+  private
+    def setup_subscription_map
+      @subscription_map = ActionCable::SubscriptionAdapter::SubscriberMap.new
+    end
+end