Re: [FFmpeg-devel] [PATCH] avformat: add CRI USM demuxer

[Paul B Mahol <onemda@gmail.com>]

On Sun, Sep 10, 2023 at 2:54 PM Andreas Rheinhardt <
andreas.rheinhardt@outlook.com> wrote:

> Paul B Mahol:
> > On Sun, Sep 10, 2023 at 2:18 PM Andreas Rheinhardt <
> > andreas.rheinhardt@outlook.com> wrote:
> >
> >> Paul B Mahol:
> >>> On Sun, Sep 10, 2023 at 2:06 PM Andreas Rheinhardt <
> >>> andreas.rheinhardt@outlook.com> wrote:
> >>>
> >>>> Paul B Mahol:
> >>>>> On Sun, Sep 10, 2023 at 1:59 PM Andreas Rheinhardt <
> >>>>> andreas.rheinhardt@outlook.com> wrote:
> >>>>>
> >>>>>> Paul B Mahol:
> >>>>>>> On Wed, Sep 6, 2023 at 1:30 PM Andreas Rheinhardt <
> >>>>>>> andreas.rheinhardt@outlook.com> wrote:
> >>>>>>>
> >>>>>>>> Paul B Mahol:
> >>>>>>>>> On Wed, Sep 6, 2023 at 11:26 AM Andreas Rheinhardt <
> >>>>>>>>> andreas.rheinhardt@outlook.com> wrote:
> >>>>>>>>>
> >>>>>>>>>> Paul B Mahol:
> >>>>>>>>>>>
> >>>>>>>>>>> +    chunk_type = avio_rb32(pb);
> >>>>>>>>>>> +    chunk_size = avio_rb32(pb);
> >>>>>>>>>>
> >>>>>>>>>> You are not checking whether the chunk here exceeds its
> containing
> >>>>>>>> chunk.
> >>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> +    av_fast_malloc(&usm->header, &usm->header_size,
> >>>>>>>>>>> +                   chunk_size + AV_INPUT_BUFFER_PADDING_SIZE);
> >>>>>>>>>>> +    if (!usm->header)
> >>>>>>>>>>> +        return AVERROR(ENOMEM);
> >>>>>>>>>>
> >>>>>>>>>> The bytestream2 API does not rely on the buffer being padded at
> >> all.
> >>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> +    bytestream2_skip(&sgb, string_offset);
> >>>>>>>>>>
> >>>>>>>>>> This is unnecessary, because you seek with an absolute offset
> >>>> lateron
> >>>>>>>>>> anyway before using sgb.
> >>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> +        bytestream2_seek(&sgb, string_offset + offset,
> >> SEEK_SET);
> >>>>>>>>>>> +        while (bytestream2_get_bytes_left(&sgb) > 0) {
> >>>>>>>>>>> +            key[n] = bytestream2_get_byte(&sgb);
> >>>>>>>>>>> +            if (!key[n])
> >>>>>>>>>>> +                break;
> >>>>>>>>>>> +            if (n >= sizeof(key) - 1)
> >>>>>>>>>>> +                break;
> >>>>>>>>>>> +            n++;
> >>>>>>>>>>> +        }
> >>>>>>>>>>> +        key[n] = '\0';
> >>>>>>>>>>
> >>>>>>>>>> IMO this would be easier with strnlen(), avoiding sgb
> altogether.
> >>>>>>>>>> You would of course need to explicitly check that you are not
> >>>>>>>>>> overreading, but that is good practice anyway.
> >>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> +    chunk_start = avio_tell(pb);
> >>>>>>>>>>> +    avio_skip(pb, 1);
> >>>>>>>>>>> +    payload_offset = avio_r8(pb);
> >>>>>>>>>>> +    padding_size = avio_rb16(pb);
> >>>>>>>>>>> +    stream_index = avio_r8(pb);
> >>>>>>>>>>> +    avio_skip(pb, 2);
> >>>>>>>>>>> +    payload_type = avio_r8(pb);
> >>>>>>>>>>> +    frame_time = avio_rb32(pb);
> >>>>>>>>>>> +    frame_rate = avio_rb32(pb);
> >>>>>>>>>>> +    avio_skip(pb, 8);
> >>>>>>>>>>
> >>>>>>>>>> payload_offset and frame_time are set-but-unused; this might
> lead
> >> to
> >>>>>>>>>> compiler warnings.
> >>>>>>>>>>
> >>>>>>>>>>> +        if (usm->ch[is_audio][stream_index].used == 1) {
> >>>>>>>>>>> +            uint32_t pkt_size = chunk_size - (avio_tell(pb) -
> >>>>>>>>>> chunk_start);
> >>>>>>>>>>> +
> >>>>>>>>>>
> >>>>>>>>>> This is unnecessary: Unless we already had a read error,
> pkt_size
> >> is
> >>>>>>>>>> chunk_size - (1 + 1 + 2 + 1 + 2 + 1 + 4 + 4 + 8).
> >>>>>>>>>>
> >>>>>>>>>> (Notice that in case padding_size is > 0, it will be part of the
> >>>>>> packet
> >>>>>>>>>> with the current code; not sure if that is an issue.)
> >>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> +
> >>>>>>>>>>> +    avio_skip(pb, padding_size);
> >>>>>>>>>>> +    avio_skip(pb, chunk_size - (avio_tell(pb) - chunk_start));
> >>>>>>>>>>> +
> >>>>>>>>>>
> >>>>>>>>>> Simpler to just use avio_seek(pb, chunk_start + chunk_size,
> >>>> SEEK_SET);
> >>>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> But input might not be seekable.
> >>>>>>>>>
> >>>>>>>>
> >>>>>>>> And? You know that avio_skip(pb, offset) is just avio_seek(pb,
> >> offset,
> >>>>>>>> SEEK_CUR)?
> >>>>>>>>
> >>>>>>>
> >>>>>>> And? Do you know that SEEK_SET is different from SEEK_CUR with
> >> positive
> >>>>>>> argument.
> >>>>>>>
> >>>>>>
> >>>>>> You are using SEEK_CUR with -avio_tell(pb), which effectively makes
> it
> >>>>>> an absolute seek.
> >>>>>>
> >>>>>
> >>>>> Nope, I skip data only, not seeking backward ever.
> >>>>>
> >>>>
> >>>> avio_seek() internally translates any avio_seek() with SEEK_CUR to an
> >>>> absolute seek (as required by the seek callbacks) and does not care
> >>>> about whether it is seeking forward or backward.
> >>>>
> >>>
> >>> Irrelevant. Seeking needs enough cache to work on non-seekable input.
> >>>
> >>
> >> Given that avio_skip() is just a wrapper around avio_seek(), the
> >> behaviour will not change even when the input is non-seekable.
> >>
> >
> > What you are trying to sell?
> >
> > That seeking works with unseekable input?
> >
>
> Seeking forward works for unseekable input by reading data and
> discarding it (that is how avio_seek() works). But actually we don't
> need that, all we need is that avio_skip() is just a wrapper around
> avio_seek(), so that the two forms are equivalent.
>

Patches welcome, just do not break unseekable demuxing.


>
> - Andreas
>
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