support ftypmp42 format

[PKiman]

Hi,

I found an issue with audio recordings created by Android devices.
iOS is fine.

A sample file has been added to test.
readMetadata.duration is null.

test/mp4/track_ftypmp42.m4a

It appears that the meta box size differs.
Files with ftypmp42 header have 4 bytes meta box size.
Files with ftypM4A header have 8 bytes meta box size.

At the moment only duration works. Other meta data are null for ftypmp42 files e.g. sampleRate.

https://xhelmboyx.tripod.com/formats/mp4-layout.txt

[ClementBeal]

Hi!

Thank you for the PR! I reviewed it and I don't notice any issue and it works! I'll try to find more mp4 to check.

With MusicBrainz, I set some metadata like the title or the album name and it works as well.

To be sure, the problem is the parsing of the ftyp was wrong, isn't it?

[PKiman]

Yes, there are different specs for ftyp.
Some specs are ISO normed, some are proprietary (Apple).
https://www.ftyps.com

This command was useful for debugging and understanding how type box parsing works.

ffprobe -v trace test/mp4/track_ftypmp42.m4a 

[file @ 0x7fdce5c04540] Setting default whitelist 'file,crypto,data'
Probing mov,mp4,m4a,3gp,3g2,mj2 score:100 size:2048
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] Format mov,mp4,m4a,3gp,3g2,mj2 probed with size=2048 and score=100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] type:'ftyp' parent:'root' sz: 24 8 15439
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] ISO: File Type Major Brand: mp42
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] type:'moov' parent:'root' sz: 885 32 15439
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] type:'mvhd' parent:'moov' sz: 108 8 877
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] time scale = 10000
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] type:'meta' parent:'moov' sz: 118 116 877
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fdce5c04100] type:'hdlr' parent:'meta' sz: 33 8 110

https://www.file-recovery.com/m4a-signature-format.htm
https://dev.to/alfg/a-quick-dive-into-mp4-57fo

[PKiman]

Obviously implementations use magic byte patterns and apply different specifications.
There is no general method for parsing an mp4 file.

https://github.com/discord/lilliput/blob/b13db14b20fd82e580836108c73235686b6ab820/lilliput.go#L24

[ClementBeal]

Let's resolve the merge conflict and then I merge your PR!