Add the ability to select for update within transactions to fix some write after read hazards. Fix a bug in extend_processing.

data/database.py

@@ -29,6 +29,16 @@ SCHEME_RANDOM_FUNCTION = {
   'postgresql+psycopg2': fn.Random,
 }
 
+def real_for_update(query):
+  return query.for_update()
+
+def null_for_update(query):
+  return query
+
+SCHEME_SPECIALIZED_FOR_UPDATE = {
+  'sqlite': null_for_update,
+}
+
 class CallableProxy(Proxy):
   def __call__(self, *args, **kwargs):
     if self.obj is None:
@@ -68,6 +78,7 @@ class UseThenDisconnect(object):
 db = Proxy()
 read_slave = Proxy()
 db_random_func = CallableProxy()
+db_for_update = CallableProxy()
 
 
 def validate_database_url(url, connect_timeout=5):
@@ -105,6 +116,8 @@ def configure(config_object):
 
   parsed_write_uri = make_url(write_db_uri)
   db_random_func.initialize(SCHEME_RANDOM_FUNCTION[parsed_write_uri.drivername])
+  db_for_update.initialize(SCHEME_SPECIALIZED_FOR_UPDATE.get(parsed_write_uri.drivername,
+                                                             real_for_update))
 
   read_slave_uri = config_object.get('DB_READ_SLAVE_URI', None)
   if read_slave_uri is not None: