Implement reading of media track info from WebM and MP4 containers

media/formats/mp4/box_definitions.cc

Index: media/formats/mp4/box_definitions.cc
diff --git a/media/formats/mp4/box_definitions.cc b/media/formats/mp4/box_definitions.cc
index a792c90d9ba8e27ef17724bdb5f717966c9470be..684b92fdf74320cb406a6c231d56f7c4b2661660 100644
--- a/media/formats/mp4/box_definitions.cc
+++ b/media/formats/mp4/box_definitions.cc
@@ -375,8 +375,23 @@ FourCC HandlerReference::BoxType() const { return FOURCC_HDLR; }
 
 bool HandlerReference::Parse(BoxReader* reader) {
   FourCC hdlr_type;
-  RCHECK(reader->SkipBytes(8) && reader->ReadFourCC(&hdlr_type));
-  // Note: remaining fields in box ignored
+  RCHECK(reader->ReadFullBoxHeader() && reader->SkipBytes(4) &&
+         reader->ReadFourCC(&hdlr_type) && reader->SkipBytes(12));
+
+  // Now we should be at the beginning of the |name| field of HDLR box. The
+  // |name| is a zero-terminated ASCII string in ISO BMFF, but it was a
+  // Pascal-style counted string in older QT/Mov formats. So we'll read the
+  // remaining box bytes first, then if the last one is zero, we strip the last
+  // zero byte, otherwise we'll string the first byte (containing the length of
+  // the Pascal-style string).

[wolenetz, 2016/03/05 03:26:20]

A pascal-style string that is shorter than (remaining box bytes + 1) might need
to be truncated to remove potential garbage chars at the end. Should we do any
verification / truncation around suspected pascal-style strings here, too?

[servolk, 2016/03/07 23:45:01]

Sure, I've added an RCHECK to ensure that the length of pascal-style string is
correct.

+  std::vector<uint8_t> name_bytes;
+  RCHECK(reader->ReadVec(&name_bytes, reader->size() - reader->pos()));
+  if (name_bytes.back() == 0)
+    name_bytes.pop_back();
+  else
+    name_bytes.erase(name_bytes.begin());
+  name = std::string(name_bytes.begin(), name_bytes.end());
+
   if (hdlr_type == FOURCC_VIDE) {
     type = kVideo;
   } else if (hdlr_type == FOURCC_SOUN) {
@@ -632,7 +647,8 @@ MediaHeader::MediaHeader()
     : creation_time(0),
       modification_time(0),
       timescale(0),
-      duration(0) {}
+      duration(0),
+      language_code(0) {}
 MediaHeader::~MediaHeader() {}
 FourCC MediaHeader::BoxType() const { return FOURCC_MDHD; }
 
@@ -640,18 +656,29 @@ bool MediaHeader::Parse(BoxReader* reader) {
   RCHECK(reader->ReadFullBoxHeader());
 
   if (reader->version() == 1) {
-    RCHECK(reader->Read8(&creation_time) &&
-           reader->Read8(&modification_time) &&
-           reader->Read4(&timescale) &&
-           reader->Read8(&duration));
+    RCHECK(reader->Read8(&creation_time) && reader->Read8(&modification_time) &&
+           reader->Read4(&timescale) && reader->Read8(&duration) &&
+           reader->Read2(&language_code));
   } else {
     RCHECK(reader->Read4Into8(&creation_time) &&
            reader->Read4Into8(&modification_time) &&
-           reader->Read4(&timescale) &&
-           reader->Read4Into8(&duration));
+           reader->Read4(&timescale) && reader->Read4Into8(&duration) &&
+           reader->Read2(&language_code));
+  }
+  // Skip playback quality information
+  return reader->SkipBytes(2);
+}
+
+std::string MediaHeader::language() const {
+  if (language_code == 0x7fff || language_code < 0x400) {

[wolenetz, 2016/03/05 03:26:21]

A couple questions about this line:
1) Where do these two magic numbers come from?
2) Is the 16th bit always expected to be zero, or should we mask it off here
before comparing with the magic numbers?

[servolk, 2016/03/07 23:45:01]

1) I took those numbers from
https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/QTFFChap...
which is linked from
https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/QTFFChap....
Ideally we'll want to read and use BCP47 language tag from ELNG atom in .mp4
(see https://codereview.chromium.org/1718773002/). But we should probably keep
this as well, as a fallback for better backward compatibility with old streams,
that don't have the ELNG atom.
2) I think it should always be zero, but there's no harm in masking it.

[wolenetz, 2016/03/10 19:53:29]

Acknowledged.

+    return "und";
   }
-  // Skip language information
-  return reader->SkipBytes(4);
+  char lang_chars[4];
+  lang_chars[3] = 0;
+  lang_chars[2] = 0x60 + (language_code & 0x1f);

[wolenetz, 2016/03/05 03:26:21]

nit: constrain each of lang_chars[0..2] to be within the range of lower case
characters, and if that constraint isn't met, default to "und"?

[servolk, 2016/03/07 23:45:01]

Done.

+  lang_chars[1] = 0x60 + ((language_code >> 5) & 0x1f);
+  lang_chars[0] = 0x60 + ((language_code >> 10) & 0x1f);
+  return lang_chars;
 }
 
 MediaInformation::MediaInformation() {}