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

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

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.

- Andreas

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