Docserver: Add FileSystem.Exists method and start sanity checking path format

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.
 
 import logging
 import posixpath
 import traceback
 
 from environment import IsPreviewServer
 from file_system import FileNotFoundError
@@ skipping 18 matching lines @@
     def CreateServerInstance(self):
       raise NotImplementedError(self.__class__)
 
   def __init__(self, request, delegate):
     Servlet.__init__(self, request)
     self._delegate = delegate
 
   def Get(self):
     ''' Render the page for a request.
     '''
-    # TODO(kalman): a consistent path syntax (even a Path class?) so that we
-    # can stop being so conservative with stripping and adding back the '/'s.
     path = self._request.path.lstrip('/')
     server_instance = self._delegate.CreateServerInstance()
 
     try:
       return self._GetSuccessResponse(path, server_instance)
     except FileNotFoundError:
       if IsPreviewServer():
         logging.error(traceback.format_exc())
       # Maybe it didn't find the file because its canonical location is
       # somewhere else; this is distinct from "redirects", which are typically
@@ skipping 53 matching lines @@
       if warnings:
         sep = '\n - '
         logging.warning('Rendering %s:%s%s' % (path, sep, sep.join(warnings)))
 
     content_type = content_and_type.content_type
     if isinstance(content, unicode):
       content = content.encode('utf-8')
       content_type += '; charset=utf-8'
 
     return Response.Ok(content, headers=_MakeHeaders(content_type))