Don't fail if job dependency is modified during enqueue process.

rq/exceptions.py

@@ -15,9 +15,6 @@ class UnpickleError(Exception):
         super(UnpickleError, self).__init__(message, inner_exception)
         self.raw_data = raw_data
 
+
 class DequeueTimeout(Exception):
     pass
-
-
-class EnqueueError(Exception):
-    pass

rq/queue.py

@@ -1,7 +1,7 @@
 import times
 from .connections import resolve_connection
 from .job import Job, Status
-from .exceptions import (DequeueTimeout, EnqueueError, InvalidJobOperationError, 
+from .exceptions import (DequeueTimeout, InvalidJobOperationError,
                          NoSuchJobError, UnpickleError)
 from .compat import total_ordering
 
@@ -145,20 +145,21 @@ class Queue(object):
                          dependency=after)
         
         # If job depends on an unfinished job, register itself on it's
-        # parent's waitlist instead of enqueueing it
+        # parent's waitlist instead of enqueueing it.
+        # If WatchError is raised in the process, that means something else is
+        # modifying the dependency. In this case we simply retry
         if after is not None:
             with self.connection.pipeline() as pipe:
-                try:
-                    pipe.watch(after.key)
-                    if after.status != Status.FINISHED:
-                        job.register_dependency()
-                        job.save()
-                        return job
-                except WatchError:
-                    raise EnqueueError(
-                        'Parent job (%s) modified during enqueue process. ' +
-                        'Bailing out to avoid race conditions' % after.id
-                    )
+                while True:
+                    try:
+                        pipe.watch(after.key)
+                        if after.status != Status.FINISHED:
+                            job.register_dependency()
+                            job.save()
+                            return job
+                        break
+                    except WatchError:
+                        continue
             
         return self.enqueue_job(job, timeout=timeout)