[FFmpeg-devel] [PATCH] avformat/mov: Properly consider if the file is self-initializing when marking sidx reading complete

[Martin Storsjö]

On Wed, 10 Mar 2021, Derek Buitenhuis wrote:

> Files with the "dash" major brand are guaranteed to only have a single
> initialization range for the whole file. We can check this and stop
> appropriately - which is useful, as the existing check to see if the
> offset reaches the end of the file is not sufficient. For example,
> YouTube creates valid self-initializing ISOBMFF files that contain
> a single 'uuid' box appended at the end, after the last 'moof', and
> this check does not catch that, which causes, for example, probing
> to traverse the entire file (every single 'moof'), which is extremely
> slow over a network, which is the main place these self-initializing
> fragmented ISOBMFF files would be used. This is the same behavior that
> was addressed in 2ff3c466eca66bb8eb32bb41a4ce70fe285e3ea0 for live-style
> fragmented files.
>
> Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
> ---
> libavformat/mov.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 1c07cff6b5..67eae24e02 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -5052,6 +5052,7 @@ static int mov_read_sidx(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>     AVStream *ref_st = NULL;
>     MOVStreamContext *sc, *ref_sc = NULL;
>     AVRational timescale;
> +    AVDictionaryEntry *major_brand = av_dict_get(c->fc->metadata, "major_brand", NULL, AV_DICT_MATCH_CASE);
>
>     version = avio_r8(pb);
>     if (version > 1) {
> @@ -5120,7 +5121,11 @@ static int mov_read_sidx(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>     sc->has_sidx = 1;
>
>     // See if the remaining bytes are just an mfra which we can ignore.
> -    is_complete = offset == stream_size;
> +    //
> +    // Also check to see if the file is Self-initializing, which guarantees
> +    // only a single initialization range is present for the whole file.
> +    // See: ISO-IEC 23009-1 Section 6.3.5.
> +    is_complete = offset == stream_size || (major_brand && !strncmp(major_brand->value, "dash", 4));
>     if (!is_complete && (pb->seekable & AVIO_SEEKABLE_NORMAL)) {
>         int64_t ret;
>         int64_t original_pos = avio_tell(pb);
> -- 
> 2.30.0

LGTM, I guess the only question that remains is whether "dash" in the 
compatible brands can imply the same too. (Our own muxer never sets "dash" 
as the major brand, only as a compatible brand, and that only if we only 
produce one global sidx.)

// Martin