Workers started with QUEUE=* do not cause error when there are no queues

* This should rarely happen (if at all) in production, but in
  development, we ran into annoying situations if Redis were cleared and
  we tried to start a catch-all Resque worker (via Foreman).
* The worker that is started will not process jobs, but at least it will
  not die immediately with a bad exit code (which would stop all
  processes in Foreman, for instance).

Author: alindeman

lib/resque/multi_queue.rb

@@ -46,11 +46,15 @@ def pop(non_block = false)
         end
       else
         queue_names = @queues.map {|queue| queue.redis_name }
-        synchronize do
-          value = @redis.blpop(*(queue_names + [1])) until value
-          queue_name, payload = value
+        if queue_names.any?
+          synchronize do
+            value = @redis.blpop(*(queue_names + [1])) until value
+            queue_name, payload = value
           queue = @queue_hash["#{@redis.namespace}:#{queue_name}"]
-          [queue, queue.decode(payload)]
+            [queue, queue.decode(payload)]
+          end
+        else
+          Kernel.sleep # forever
         end
       end
     end
@@ -61,12 +65,17 @@ def pop(non_block = false)
     # the timeout expires.
     def poll(timeout)
       queue_names = @queues.map {|queue| queue.redis_name }
-      queue_name, payload = @redis.blpop(*(queue_names + [timeout]))
-      return unless payload
+      if queue_names.any?
+        queue_name, payload = @redis.blpop(*(queue_names + [timeout]))
+        return unless payload
 
-      synchronize do
-        queue = @queue_hash["#{@redis.namespace}:#{queue_name}"]
-        [queue, queue.decode(payload)]
+        synchronize do
+          queue = @queue_hash["#{@redis.namespace}:#{queue_name}"]
+          [queue, queue.decode(payload)]
+        end
+      else
+        Kernel.sleep(timeout)
+        nil
       end
     end
   end

test/multi_queue_test.rb

@@ -1,6 +1,6 @@
 require "test_helper"
 
-describe "Resque::MulitQueue" do
+describe "Resque::MultiQueue" do
   let(:redis) { Resque.redis }
   let(:coder) { Resque::MultiJsonCoder.new }
 
@@ -15,6 +15,11 @@
     assert_nil queue.poll(1)
   end
 
+  it "poll is a no-op when queues are empty" do
+    queue = Resque::MultiQueue.new([], redis)
+    assert_nil queue.poll(1)
+  end
+
   it "blocks on pop" do
     foo   = Resque::Queue.new 'foo', redis, coder
     bar   = Resque::Queue.new 'bar', redis, coder
@@ -92,4 +97,11 @@
 
     assert_equal processed_queues, queues
   end
+
+  it "blocking pop is a no-op if queues are empty" do
+    queue = Resque::MultiQueue.new([], redis)
+    assert_raises Timeout::Error do
+      Timeout.timeout(2) { queue.pop }
+    end
+  end
 end