Provide better diffs for MP4 failures.

If the user has the GPAC MP4 tools installed, this will use that to
provide a better, text-based diff of the MP4 files.  This makes looking
at test failures much better than just "binary files differ".

Change-Id: Ifdca54c02226a9f0fe9ddcf9c6b28e960e568111

packager/app/test/packager_test.py

@@ -133,6 +133,15 @@ class DiffFilesPolicy(object):
 
       output, error = self._GitDiff(expected_file, actual_file)
 
+      # If this is an MP4 file, get a better looking diff.
+      if ((output or error) and
+          os.path.splitext(actual_file)[1] in {'.mp4', '.m4s'}):
+        new_output, new_error = self._Mp4Diff(
+            out_dir, expected_file, actual_file)
+
+        output = new_output or output
+        error = new_error or error
+
       if output:
         failure_messages += [output.decode('utf8')]
 
@@ -160,6 +169,23 @@ class DiffFilesPolicy(object):
     p = subprocess.Popen(cmd, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
     return p.communicate()
 
+  def _Mp4Diff(self, out_dir, file_a, file_b):
+    dump_a = os.path.join(out_dir, os.path.basename(file_a) + '.dump.expected')
+    dump_b = os.path.join(out_dir, os.path.basename(file_b) + '.dump.actual')
+    try:
+      cmd = ['mp4dump', '--verbosity', '2', file_a]
+      with open(dump_a, 'w') as f:
+        subprocess.check_call(cmd, stdout=f)
+      cmd = ['mp4dump', '--verbosity', '2', file_b]
+      with open(dump_b, 'w') as f:
+        subprocess.check_call(cmd, stdout=f)
+    except (OSError, subprocess.CalledProcessError):
+      # If the program isn't available or returns an error, just ignore it and
+      # use the normal diff.
+      return None, None
+
+    return self._GitDiff(dump_a, dump_b)
+
   def _UpdateGold(self, out_dir, gold_dir):
     if os.path.exists(gold_dir):
       shutil.rmtree(gold_dir)