Prevent `Queue#dequeue` from blowing the stack

In the case of many sequential jobs having been deleted, a recursive
implementation of `Queue#dequeue` is prone to blowing the stack in the
absence of tail-recursion support. Change the implementation from
recursive to iterative to work around this issue in CPython.

rq/queue.py

@@ -329,6 +329,7 @@ class Queue(object):
 
         Returns a job_class instance, which can be executed or inspected.
         """
+        while True:
             job_id = self.pop_job_id()
             if job_id is None:
                 return None
@@ -336,8 +337,7 @@ class Queue(object):
                 job = self.job_class.fetch(job_id, connection=self.connection)
             except NoSuchJobError as e:
                 # Silently pass on jobs that don't exist (anymore),
-            # and continue by reinvoking itself recursively
+                continue
-            return self.dequeue()
             except UnpickleError as e:
                 # Attach queue information on the exception for improved error
                 # reporting

tests/test_queue.py

@@ -173,6 +173,14 @@ class TestQueue(RQTestCase):
         # ...and assert the queue count when down
         self.assertEquals(q.count, 0)
 
+    def test_dequeue_deleted_jobs(self):
+        """Dequeueing deleted jobs from queues don't blow the stack."""
+        q = Queue()
+        for _ in range(1,1000):
+            job = q.enqueue(say_hello)
+            job.delete()
+        q.dequeue()
+
     def test_dequeue_instance_method(self):
         """Dequeueing instance method jobs from queues."""
         q = Queue()