Handle additional unused 'mdat' properly

The spec allows having more than one 'mdat' boxes even if it is not
used.

This is happening on some mp4 files in the wild:
http://www.sample-videos.com/.

Fixes #298.

Change-Id: I729cc94bee095560b5c4b3ee8511323f25f7ad5a

packager/media/formats/mp4/mp4_media_parser.cc

@@ -257,12 +257,22 @@ bool MP4MediaParser::ParseBox(bool* err) {
     return false;
 
   if (reader->type() == FOURCC_mdat) {
-    // The code ends up here only if a MOOV box is not yet seen.
-    DCHECK(!moov_);
-
-    NOTIMPLEMENTED() << " Files with MDAT before MOOV is not supported yet.";
+    if (!moov_) {
+      // For seekable files, we seek to the 'moov' and load the 'moov' first
+      // then seek back (see LoadMoov function for details); we do not support
+      // having 'mdat' before 'moov' for non-seekable files. The code ends up
+      // here only if it is a non-seekable file.
+      NOTIMPLEMENTED() << " Non-seekable Files with 'mdat' box before 'moov' "
+                          "box is not supported.";
       *err = true;
       return false;
+    } else {
+      // This can happen if there are unused 'mdat' boxes, which is unusual
+      // but allowed by the spec. Ignore the 'mdat' and proceed.
+      LOG(INFO)
+          << "Ignore unused 'mdat' box - this could be as a result of extra "
+             "not usable 'mdat' or 'mdat' associated with unrecognized track.";
+    }
   }
 
   // Set up mdat offset for ReadMDATsUntil().

packager/media/formats/mp4/mp4_media_parser_unittest.cc

@@ -188,6 +188,15 @@ TEST_F(MP4MediaParserTest, TrailingMoov) {
   EXPECT_EQ(201u, num_samples_);
 }
 
+TEST_F(MP4MediaParserTest, TrailingMoovAndAdditionalMdat) {
+  // The additional mdat should just be ignored, so the parse is still
+  // successful with the same result.
+  EXPECT_TRUE(
+      ParseMP4File("bear-640x360-trailing-moov-additional-mdat.mp4", 1024));
+  EXPECT_EQ(2u, num_streams_);
+  EXPECT_EQ(201u, num_samples_);
+}
+
 TEST_F(MP4MediaParserTest, Flush) {
   // Flush while reading sample data, then start a new stream.
   InitializeParser(NULL);

packager/media/test/data/README

@@ -36,6 +36,7 @@ bear-640x360.mp4  - Same as above, but in a different resolution.
 bear-640x360-hevc.mp4  - Same content, but encoded with HEVC.
 bear-320x180.mp4  - Same as above, but in a different resolution.
 bear-640x360-trailing-moov.mp4  - Same content, but with moov box in the end.
+bear-640x360-trailing-moov-additional-mdat.mp4  - Same content, but with moov box in the end and an additional unused mdat, which should be ignored.
 bear-640x360-av_frag.mp4  - Same content, but in fragmented mp4.
 bear-640x360-aac_lc-silent_right.mp4  - Audio only, stereo, but right channel is silent, with AAC-LC profile.
 bear-640x360-aac_he-silent_right.mp4  - Same as above, but with AAC-HE profile.