More tolerant about malformed GIF files

Source/core/platform/image-decoders/gif/GIFImageReader.cpp

 /* -*- Mode: C; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* ***** BEGIN LICENSE BLOCK *****
  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
  *
  * The contents of this file are subject to the Mozilla Public License Version
  * 1.1 (the "License"); you may not use this file except in compliance with
  * the License. You may obtain a copy of the License at
  * http://www.mozilla.org/MPL/
  *
  * Software distributed under the License is distributed on an "AS IS" basis,
@@ skipping 466 matching lines @@
             break;
         }
 
         case GIFGlobalColormap: {
             m_isGlobalColormapDefined = true;
             GETN(1, GIFImageStart);
             break;
         }
 
         case GIFImageStart: {
             if (*currentComponent == ';') { // terminator.

[Peter Kasting, 2013/05/29 04:30:17]

Nit: You can nuke this block entirely and let it be handled by the catchall code
below, if you update the comment below to begin like:

Having handled the extension and image separator tokens above, the only other
valid token is ';', which terminates the file.  In the case of other characters,
the GIF87a spec tells us...

[Alpha Left Google, 2013/05/29 19:38:58]

Done.

                 GETN(0, GIFDone);
                 break;
             }
 
             if (*currentComponent == '!') { // extension.
                 GETN(2, GIFExtension);
                 break;
             }
 
+            if (*currentComponent == ',') { // image separator.
+                GETN(9, GIFImageHeader);
+                break;
+            }
+
             // If we get anything other than ',' (image separator), '!'
             // (extension), or ';' (trailer), there is extraneous data
             // between blocks. The GIF87a spec tells us to keep reading
             // until we find an image separator, but GIF89a says such
-            // a file is corrupt. We follow GIF89a and bail out.
-            if (*currentComponent != ',')
-                return false;
-
-            GETN(9, GIFImageHeader);
+            // a file is corrupt. We follow Mozilla's implementation and
+            // proceed as if the file were correctly terminated, so the
+            // GIF will display.
+            GETN(0, GIFDone);
             break;
         }
 
         case GIFExtension: {
             size_t bytesInBlock = currentComponent[1];
             GIFState es = GIFSkipBlock;
 
             switch (*currentComponent) {
             case 0xf9:
                 es = GIFControlExtension;
@@ skipping 300 matching lines @@
     rowPosition = 0;
     rowsRemaining = m_frameContext->height;
 
     // Clearing the whole suffix table lets us be more tolerant of bad data.
     suffix.fill(0);
     for (int i = 0; i < clearCode; i++)
         suffix[i] = i;
     stackp = 0;
     return true;
 }