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]
+    try_default_layout = True
+    allow_empty_path = False

[flackr, 2015/06/19 19:42:13]

I think the evaluation of this could be condensed a bit, simply:
try_default_layout = self.grd_node.GetRoot().fallback_to_default_layout !=
'false';
allow_empty_path is basically !try_default_layout, so I'd omit it altogether.

[tdanderson, 2015/06/22 23:33:20]

Done.

+    if self.grd_node.GetRoot().fallback_to_default_layout == 'false':  # awkward
+      try_default_layout = False
+
     if 'default' not in layouts:
-      layouts.append('default')
+      if try_default_layout:

[flackr, 2015/06/19 19:42:13]

No need for else, so merge with above, i.e.:
if 'default' not in layouts and try_default_layout:

[tdanderson, 2015/06/22 23:33:20]

Done.

+        layouts.append('default')
+      else:
+        allow_empty_path = True
+    # Terry - raise exception for 'elif not try_default_layout' ?
+
+    # TODO(tdanderson): Search in descending order of all image scales
+    #                   instead of immediately falling back to 100. crbug.com/x
     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 we get here then the file is missing.
+    if allow_empty_path:
+      return None, 100, req_scale
+
+    # Raise an exception if the file is missing and we do not permit returning
+    # an empty path.
     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()