Adds settings.cfg for gce-backend.

appengine/gce-backend/config.py

 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Utilities for reading GCE Backend configuration."""
 
 import collections
 import logging
 
 from components import utils
 utils.fix_protobuf_package()
 
 from google import protobuf
 from google.appengine.ext import ndb
 
 import metrics
 
 from components import config
 from components import datastore_utils
+from components import utils
 from components.config import validation
 
 from proto import config_pb2
 
 
 TEMPLATES_CFG_FILENAME = 'templates.cfg'
 MANAGERS_CFG_FILENAME = 'managers.cfg'
+SETTINGS_CFG_FILENAME = 'settings.cfg'
 
 
 class Configuration(datastore_utils.config.GlobalConfig):
   """Configuration for this service."""
   # Text-formatted proto.config_pb2.InstanceTemplateConfig.
   template_config = ndb.TextProperty(default='')
   # Text-formatted proto.config_pb2.InstanceGroupManagerConfig.
   manager_config = ndb.TextProperty(default='')
   # Revision of the configs.
   revision = ndb.StringProperty(default='')
@@ skipping 96 matching lines @@
       context.error(
           'zone %s is not unique in template %s.',
           manager.zone,
           manager.template_base_name,
       )
       valid = False
     else:
       zones[manager.template_base_name].add(manager.zone)
 
   metrics.config_valid.set(valid, fields={'config': MANAGERS_CFG_FILENAME})
+
+
+@validation.self_rule(SETTINGS_CFG_FILENAME, config_pb2.SettingsCfg)
+def validate_settings_config(config, context):
+  pass
+
+
+def _get_settings():
+  """Returns (rev, cfg) where cfg is a parsed SettingsCfg message.
+
+  The config is cached in the datastore.
+  """
+  rev, cfg = config.get_self_config(
+      SETTINGS_CFG_FILENAME, config_pb2.SettingsCfg, store_last_good=True)
+  cfg = cfg or config_pb2.SettingsCfg()
+  return rev, cfg
+
+
+@utils.cache_with_expiration(60)
+def settings():
+  """Loads settings from an NDB-based cache or a default one if not present."""
+  return _get_settings()[1]