First few changes for the image diffs feature.

data/database.py

@@ -150,10 +150,19 @@ class RepositoryTag(BaseModel):
     )
 
 
+class QueueItem(BaseMode):
+  queue_name = CharField(index=True)
+  body = TextField()
+  available_after = DateTimeField(default=datetime.now, index=True)
+  available = BooleanField(default=True, index=True)
+  processing_expires = DateTimeField(null=True, index=True)
+
+
 def initialize_db():
   create_model_tables([User, Repository, Image, AccessToken, Role,
                        RepositoryPermission, Visibility, RepositoryTag,
-                       EmailConfirmation, FederatedLogin, LoginService])
+                       EmailConfirmation, FederatedLogin, LoginService,
+                       QueueItem])
   Role.create(name='admin')
   Role.create(name='write')
   Role.create(name='read')

data/queue.py

@@ -0,0 +1,55 @@
+from datetime import datetime, timedelta
+
+from database import QueueItem
+
+
+class WorkQueue(object):
+  def __init__(self, queue_name):
+    self.queue_name = queue_name
+
+  def put(message, available_after=0):
+    """
+    Put an item, if it shouldn't be processed for some number of seconds,
+    specify that amount as available_after.
+    """
+
+    params = {
+      'queue_name': self.queue_name,
+      'body': message,
+    }
+
+    if available_after:
+      available_date = datetime.now() + timedelta(seconds=available_after)
+      params['available_after'] = available_date
+
+    QueueItem.create(**params)
+
+  def get(processing_time=300):
+    """
+    Get an available item and mark it as unavailable for the default of five
+    minutes.
+    """
+    now = datetime.now()
+    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.
+    avaial = QueueItem.select().where(QueueItem.queue_name = self.queue_name,
+                                      QueueItem.available_after <= now,
+                                      available_or_expired)
+
+    found = list(avail.limit(1).order_by(QueueItem.available_after))
+
+    if found:
+      item = found[0]
+      item.available = False
+      item.processing_expires = now + timedelta(seconds=processing_time)
+      item.save()
+
+      return item
+
+    return None
+
+  def complete(completed_item):
+    item.delete_instance()