grit: Add fallback_to_default_layout attribute to <output>

grit/gather/chrome_scaled_image.py

 #!/usr/bin/env python
 # 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.
 
 '''Gatherer for <structure type="chrome_scaled_image">.
 '''
 
 import os
 import struct
@@ skipping 82 matching lines @@
   def _FindInputFile(self):
     output_context = self.grd_node.GetRoot().output_context
     match = self.split_context_re_.match(output_context)
     if not match:
       raise exception.MissingMandatoryAttribute(
           'All <output> nodes must have an appropriate context attribute'
           ' (e.g. context="touch_200_percent")')
     req_layout, req_scale = match.group(1), int(match.group(2))
 
     layouts = [req_layout]
-    if 'default' not in layouts:
+    try_default_layout = self.grd_node.GetRoot().fallback_to_default_layout
+    if try_default_layout and 'default' not in layouts:
       layouts.append('default')
+
+    # TODO(tdanderson): Search in descending order of all image scales
+    #                   instead of immediately falling back to 100.
+    #                   See crbug.com/503643.
     scales = [req_scale]
     try_low_res = self.grd_node.FindBooleanAttribute(
         'fallback_to_low_resolution', default=False, skip_self=False)
     if try_low_res and 100 not in scales:
       scales.append(100)
+
     for layout in layouts:
       for scale in scales:
         dir = '%s_%s_percent' % (layout, scale)
         path = os.path.join(dir, self.rc_file)
         if os.path.exists(self.grd_node.ToRealPath(path)):
           return path, scale, req_scale
-    # If we get here then the file is missing, so fail.
+
+    if not try_default_layout:
+      # The file was not found in the specified output context and it was
+      # explicitly indicated that the default context should not be searched
+      # as a fallback, so return an empty path.
+      return None, 100, req_scale
+
+    # The file was found in neither the specified context nor the default
+    # context, so raise an exception.
     dir = "%s_%s_percent" % (_MakeBraceGlob(layouts),
                              _MakeBraceGlob(map(str, scales)))
     raise exception.FileNotFound(
         'Tried ' + self.grd_node.ToRealPath(os.path.join(dir, self.rc_file)))
 
   def GetInputPath(self):
     path, scale, req_scale = self._FindInputFile()
     return path
 
   def Parse(self):
     pass
 
   def GetTextualIds(self):
     return [self.extkey]
 
   def GetData(self, *args):
     path, scale, req_scale = self._FindInputFile()
+    if path is None:
+      return None
+
     data = util.ReadFile(self.grd_node.ToRealPath(path), util.BINARY)
     data = _RescaleImage(data, scale, req_scale)
     data = _MoveSpecialChunksToFront(data)
     return data
 
   def Translate(self, *args, **kwargs):
     return self.GetData()