cc: Ensure that eviction tiling range is sane.

cc/resources/picture_layer_tiling_set.cc

 // Copyright 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.
 
 #include "cc/resources/picture_layer_tiling_set.h"
 
 #include <limits>
 
 namespace cc {
 
@@ skipping 321 matching lines @@
   TilingRange high_res_range(0, 0);
   TilingRange low_res_range(tilings_.size(), tilings_.size());
   for (size_t i = 0; i < tilings_.size(); ++i) {
     const PictureLayerTiling* tiling = tilings_[i];
     if (tiling->resolution() == HIGH_RESOLUTION)
       high_res_range = TilingRange(i, i + 1);
     if (tiling->resolution() == LOW_RESOLUTION)
       low_res_range = TilingRange(i, i + 1);
   }
 
+  TilingRange range(0, 0);
   switch (type) {
     case HIGHER_THAN_HIGH_RES:
-      return TilingRange(0, high_res_range.start);
+      range = TilingRange(0, high_res_range.start);
+      break;
     case HIGH_RES:
-      return high_res_range;
+      range = high_res_range;
+      break;
     case BETWEEN_HIGH_AND_LOW_RES:
-      return TilingRange(high_res_range.end, low_res_range.start);
+      // TODO(vmpstr): This code assumes that high res tiling will come before
+      // low res tiling, however there are cases where this assumption is
+      // violated. As a result, it's better to be safe in these situations,
+      // since otherwise we can end up accessing a tile that doesn't exist. See
+      // crbug.com/429397 for high res tiling appearing after low res tiling.
+      if (high_res_range.start <= low_res_range.start)
+        range = TilingRange(high_res_range.end, low_res_range.start);
+      else
+        range = TilingRange(low_res_range.end, high_res_range.start);
+      break;
     case LOW_RES:
-      return low_res_range;
+      range = low_res_range;

[reveman, 2014/11/01 12:25:34]

missing a break statement here

[vmpstr, 2014/11/01 18:08:49]

Done.

     case LOWER_THAN_LOW_RES:
-      return TilingRange(low_res_range.end, tilings_.size());
+      range = TilingRange(low_res_range.end, tilings_.size());

[reveman, 2014/11/01 12:25:34]

and I think you should add a break statement here too even though that's not
technically needed

[vmpstr, 2014/11/01 18:08:49]

Done.

   }
 
-  NOTREACHED();
-  return TilingRange(0, 0);
+  DCHECK_LE(range.start, range.end);
+  return range;
 }
 
 }  // namespace cc