Multiple mdat boxes lead to panic

[alttagil]

I came across mp4 files with two mdat boxes: first is the real one and the next is empty. All players and wowza were playing it all right but mp4ff reads such file and rewrite file.Mdat with second empty one. I propose these changes to file.go for fixing it:

    case *MdatBox:
        if !f.isFragmented {
            if f.Mdat == nil || f.Mdat.Data == nil {
                f.Mdat = box
            }
        } else {

[tobbee]

@alttagil Thanks for the report. This is a weird case, but apparently allowed by the spec. There may also be multiple mdat boxes containing data, or references to data in another file. The goal of mp4ff is not to be the most general parser/processor possible, but this is an easy enough change that I can add something similar to the code change you suggest. I think, I'll rather use the size of mdat than a nil pointer in the test, though. I suppose your empty mdat box has the size properly set to 8?

[alttagil]

[ftyp] size=8+24
  major_brand = isom
  minor_version = 200
  compatible_brand = isom
  compatible_brand = iso2
  compatible_brand = avc1
  compatible_brand = mp41
[free] size=8+0
[mdat] size=8+20990219
[mdat] size=8+0
[moov] size=8+72942
  [mvhd] size=12+96
    timescale = 1000
    duration = 127318
    duration(ms) = 127318

here's the link to the file: https://www.dropbox.com/scl/fi/su2erq0990vih6tqdgti6/rabbit.mp4?rlkey=arn4nhci75we4tbx1zij3c2eh&st=vra0xays&dl=1

[tobbee]

I wrote a bit more complex fix where Mdat will point to a single empty mdat box, but it will be point to a non-empty one if such a box is added. A second non-empty box will return an error. This should hopefully avoid panics.

[alttagil]

it seems that your PR doesn't prevent rewriting non-empty box with empty one

[tobbee]

@alttagil Yeah, you're right. I didn't write a test to check the behaviour. Thanks for checking!

The code has now been fixed and there is a a new test that checks the various cases with mixed empty and non-empty mdat boxes.