Docserver: Add more skip_not_found support and cache "not found"s

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 66 matching lines @@
       - If |skip_not_found| is True, the resulting object will not contain any
         mapping for that path.
       - Otherwise, and by default, a FileNotFoundError is raised. This is
         guaranteed to only happen once the Future has been resolved (Get()
         called).
 
     For any other failure, raises a FileSystemError.
     '''
     raise NotImplementedError(self.__class__)
 
-  def ReadSingle(self, path):
+  def ReadSingle(self, path, skip_not_found=False):
     '''Reads a single file from the FileSystem. Returns a Future with the same
     rules as Read(). If |path| is not found raise a FileNotFoundError on Get().
     '''
     AssertIsValid(path)
-    read_single = self.Read([path])
-    return Future(callback=lambda: read_single.Get()[path])
+    read_single = self.Read([path], skip_not_found=skip_not_found)
+    return Future(callback=lambda: read_single.Get().get(path, None))
 
   def Exists(self, path):
     '''Returns a Future to the existence of |path|; True if |path| exists,
     False if not. This method will not throw a FileNotFoundError unlike
     the Read* methods, however it may still throw a FileSystemError.
 
     There are several ways to implement this method via the interface but this
     method exists to do so in a canonical and most efficient way for caching.
     '''
     AssertIsValid(path)
@@ skipping 90 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)