FIX: Make sure Redis fallback don't fall into a permanent readonly state.

lib/discourse_redis.rb

@@ -60,7 +60,7 @@ class DiscourseRedis
     end
 
     def running?
-      synchronize { @timer_task.running? }
+      @timer_task.running?
     end
 
     private
@@ -92,7 +92,10 @@ class DiscourseRedis
     end
 
     def resolve
-      return @slave_options if !@fallback_handler.master
+      if !@fallback_handler.master
+        @fallback_handler.verify_master unless @fallback_handler.running?
+        return @slave_options
+      end
 
       begin
         options = @options.dup

spec/components/discourse_redis_spec.rb

@@ -60,11 +60,14 @@ describe DiscourseRedis do
 
         Redis::Client.any_instance.expects(:call).raises(error).once
         expect { connector.resolve }.to raise_error(error)
+        fallback_handler.instance_variable_get(:@timer_task).shutdown
+        expect(fallback_handler.running?).to eq(false)
 
         config = connector.resolve
 
         expect(config[:host]).to eq(slave_host)
         expect(config[:port]).to eq(slave_port)
+        expect(fallback_handler.running?).to eq(true)
       ensure
         fallback_handler.master = true
       end