Docserver: achieve online vs offline (cron vs instance) behaviour at the object

chrome/common/extensions/docs/server2/render_servlet.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from fnmatch import fnmatch
 import logging
 import mimetypes
 import os
 import traceback
 
 from appengine_wrappers import IsDevServer
 from branch_utility import BranchUtility
 from file_system import FileNotFoundError
 from server_instance import ServerInstance
 from servlet import Servlet, Response
 import svn_constants
+from third_party.json_schema_compiler.memoize import memoize
 
 _DEFAULT_CHANNEL = 'stable'
 
 _ALWAYS_ONLINE = IsDevServer()
 
 def _IsBinaryMimetype(mimetype):
   return any(mimetype.startswith(prefix)
              for prefix in ['audio', 'image', 'video'])
 
-def AlwaysOnline(fn):
-  '''A function decorator which forces the rendering to be always online rather
-  than the default offline behaviour. Useful for testing.
-  '''
-  def impl(*args, **optargs):
-    global _ALWAYS_ONLINE
-    was_always_online = _ALWAYS_ONLINE
-    try:
-      _ALWAYS_ONLINE = True
-      return fn(*args, **optargs)
-    finally:
-      _ALWAYS_ONLINE = was_always_online
-  return impl
+@memoize
+def _GetOrCreateServerInstance(channel):
+  return ServerInstance.CreateOffline(channel)
 
 class RenderServlet(Servlet):
   '''Servlet which renders templates.
   '''
   def Get(self, server_instance=None):
+    # A note about |server_instance|:
+    #
+    # AppEngine instances should never need to call out to SVN. That should
+    # only ever be done by the cronjobs, which then write the result into
+    # DataStore, which is as far as instances look. To enable this, crons can
+    # pass a custom (presumably online) ServerInstance into Get().
+    #
+    # Why? SVN is slow and a bit flaky. Cronjobs failing is annoying but
+    # temporary. Instances failing affects users, and is really bad.
+    #
+    # Anyway - to enforce this, we actually don't give instances access to SVN.
+    # If anything is missing from datastore, it'll be a 404. If the cronjobs
+    # don't manage to catch everything - uhoh. On the other hand, we'll figure
+    # it out pretty soon, and it also means that legitimate 404s are caught
+    # before a round trip to SVN.
+
     path_with_channel, headers = (self._request.path.lstrip('/'),
                                   self._request.headers)
 
     # Redirect "extensions" and "extensions/" to "extensions/index.html", etc.
     if (os.path.splitext(path_with_channel)[1] == '' and
         path_with_channel.find('/') == -1):
       path_with_channel += '/'
     if path_with_channel.endswith('/'):
       return Response.Redirect(path_with_channel + 'index.html')
 
     channel, path = BranchUtility.SplitChannelNameFromPath(path_with_channel)
 
     if channel == _DEFAULT_CHANNEL:
       return Response.Redirect('/%s' % path)
 
     if channel is None:
       channel = _DEFAULT_CHANNEL
 
-    # AppEngine instances should never need to call out to SVN. That should
-    # only ever be done by the cronjobs, which then write the result into
-    # DataStore, which is as far as instances look. To enable this, crons can
-    # pass a custom (presumably online) ServerInstance into Get().
-    #
-    # Why? SVN is slow and a bit flaky. Cronjobs failing is annoying but
-    # temporary. Instances failing affects users, and is really bad.
-    #
-    # Anyway - to enforce this, we actually don't give instances access to SVN.
-    # If anything is missing from datastore, it'll be a 404. If the cronjobs
-    # don't manage to catch everything - uhoh. On the other hand, we'll figure
-    # it out pretty soon, and it also means that legitimate 404s are caught
-    # before a round trip to SVN.
     if server_instance is None:
-      # The ALWAYS_ONLINE thing is for tests and preview.py that shouldn't need
-      # to run the cron before rendering things.
-      constructor = (ServerInstance.CreateOnline if _ALWAYS_ONLINE else
-                     ServerInstance.GetOrCreateOffline)
-      server_instance = constructor(channel)
+      server_instance = _GetOrCreateServerInstance(channel)
 
     canonical_path = server_instance.path_canonicalizer.Canonicalize(path)
     if path != canonical_path:
       return Response.Redirect(canonical_path if channel is None else
                                '%s/%s' % (channel, canonical_path))
 
     templates = server_instance.template_data_source_factory.Create(
         self._request, path)
 
     content = None
@@ skipping 28 matching lines @@
       return Response.NotFound(templates.Render('404'), headers=headers)
 
     if not content:
       logging.error('%s had empty content' % path)
 
     headers.update({
       'content-type': content_type,
       'cache-control': 'max-age=300',
     })
     return Response.Ok(content, headers=headers)