Docserver: Persist stat cache for versioned file systems

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

 # Copyright (c) 2012 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.
 
 import posixpath
 import traceback
 
 from future import Future
 from path_util import (
     AssertIsDirectory, AssertIsValid, IsDirectory, IsValid, SplitParent,
@@ skipping 142 matching lines @@
     raw StatInfo.
 
     This is a bandaid for a lack of an async Stat function. Stat() should be
     async by default but for now just let implementations override this if they
     like.
     '''
     return Future(callback=lambda: self.Stat(path))
 
   def GetIdentity(self):
     '''The identity of the file system, exposed for caching classes to
-    namespace their caches. this will usually depend on the configuration of
+    namespace their caches. This will usually depend on the configuration of
     that file system - e.g. a LocalFileSystem with a base path of /var is
     different to that of a SubversionFileSystem with a base path of /bar, is
     different to a LocalFileSystem with a base path of /usr.
     '''
     raise NotImplementedError(self.__class__)
 
+  def GetVersion(self):
+    '''The version of the file system, exposed for more granular caching.
+    This may be any serializable data, though generally it should be a revision
+    number or hash string. The default implementation returns None, indicating
+    that the FileSystem is not versioned.
+    '''
+    return None
+
   def Walk(self, root, depth=-1, file_lister=None):
     '''Recursively walk the directories in a file system, starting with root.
 
     Behaviour is very similar to os.walk from the standard os module, yielding
     (base, dirs, files) recursively, where |base| is the base path of |files|,
     |dirs| relative to |root|, and |files| and |dirs| the list of files/dirs in
     |base| respectively. If |depth| is specified and greater than 0, Walk will
     only recurse |depth| times.
 
     |file_lister|, if specified, should be a callback of signature
@@ skipping 42 matching lines @@
             self.GetIdentity() == other.GetIdentity())
 
   def __ne__(self, other):
     return not (self == other)
 
   def __repr__(self):
     return '<%s>' % type(self).__name__
 
   def __str__(self):
     return repr(self)