Query for and take a queue item in a transaction.

data/queue.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timedelta
 
-from database import QueueItem
+from data.database import QueueItem, db
 
 
 class WorkQueue(object):
@@ -34,8 +34,7 @@ class WorkQueue(object):
     available_or_expired = ((QueueItem.available == True) |
                             (QueueItem.processing_expires <= now))
 
-    # TODO the query and the update should be atomic, but for now we only
-    # have one worker.
+    with db.transaction():
       avail = QueueItem.select().where(QueueItem.queue_name == self.queue_name,
                                        QueueItem.available_after <= now,
                                        available_or_expired,