Prevent race condition when launching EventMachine reactor

reactor_running? will be true just after the thread enters
EventMachine.run; reactor_thread only gets set after the internal
initialize_event_machine method has been called, the C extension is set
up, and it is entering its run loop.

actioncable/lib/action_cable/subscription_adapter/evented_redis.rb

@@ -68,10 +68,10 @@ def redis_connection_for_broadcasts
         end
 
         def ensure_reactor_running
-          return if EventMachine.reactor_running?
+          return if EventMachine.reactor_running? && EventMachine.reactor_thread
           @@mutex.synchronize do
             Thread.new { EventMachine.run } unless EventMachine.reactor_running?
-            Thread.pass until EventMachine.reactor_running?
+            Thread.pass until EventMachine.reactor_running? && EventMachine.reactor_thread
           end
         end
     end

actioncable/test/subscription_adapter/evented_redis_test.rb

@@ -23,6 +23,32 @@ def teardown
     $VERBOSE = @previous_verbose
   end
 
+  def test_slow_eventmachine
+    require "eventmachine"
+    require "thread"
+
+    lock = Mutex.new
+
+    EventMachine.singleton_class.class_eval do
+      alias_method :delayed_initialize_event_machine, :initialize_event_machine
+      define_method(:initialize_event_machine) do
+        lock.synchronize do
+          sleep 0.5
+          delayed_initialize_event_machine
+        end
+      end
+    end
+
+    test_basic_broadcast
+  ensure
+    lock.synchronize do
+      EventMachine.singleton_class.class_eval do
+        alias_method :initialize_event_machine, :delayed_initialize_event_machine
+        remove_method :delayed_initialize_event_machine
+      end
+    end
+  end
+
   def cable_config
     { adapter: "evented_redis", url: "redis://127.0.0.1:6379/12" }
   end