queue: Make join not wait if there are no unfinished tasks

This fixes GitHub issue #54

eventlet/queue.py

@@ -418,7 +418,8 @@ class Queue(LightQueue):
         that the item was retrieved and all work on it is complete. When the count of
         unfinished tasks drops to zero, :meth:`join` unblocks.
         '''
-        self._cond.wait()
+        if self.unfinished_tasks > 0:
+            self._cond.wait()
 
 
 class PriorityQueue(Queue):

tests/queue_test.py

@@ -253,6 +253,10 @@ class TestQueue(LimitedTestCase):
         assert channel.unfinished_tasks == 0, channel.unfinished_tasks
         gt.wait()
 
+    def test_join_doesnt_block_when_queue_is_already_empty(self):
+        queue = eventlet.Queue()
+        queue.join()
+
 
 def store_result(result, func, *args):
     try: