[FFmpeg-devel] [PATCH 03/23] avformat/matroskaenc: Use random TrackUID
Michael Niedermayer
michael at niedermayer.cc
Sun Nov 24 21:28:25 EET 2019
On Sun, Nov 24, 2019 at 09:08:00AM +0000, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > On Wed, Nov 06, 2019 at 03:49:02AM +0100, Andreas Rheinhardt wrote:
> >> Up until now, the TrackUID of a Matroska track which is supposed to be
> >> random was not random at all: It always coincided with the TrackNumber
> >> which is usually the 1-based index of the corresponding stream in the
> >> array of AVStreams. This has been changed: It is now set via an AVLFG
> >> if AVFMT_FLAG_BITEXACT is not set. Otherwise it is set like it is set
> >> now (the only change happens if an explicit track number has been
> >> choosen via dash_track_number, because the system used in the normal
> >> situation is now used, too). In particular, no FATE tests need to be
> >> updated.
> >>
> >> This also fixes a bug in case the dash_track_number option was used:
> >> In this case the TrackUID was set to the track number, but the tags were
> >> written with a TagTrackUID simply based upon the index, so that the tags
> >> didn't apply to the track they ought to apply to.
> >>
> >> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> >> ---
> >> mkv_get_uid() might be overkill, but I simply wanted to be sure that
> >> there are no collisions.
> >>
> >> libavformat/matroskaenc.c | 65 ++++++++++++++++++++++++++++++++++-----
> >> 1 file changed, 57 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
> >> index de57e474be..b87d15b7ff 100644
> >> --- a/libavformat/matroskaenc.c
> >> +++ b/libavformat/matroskaenc.c
> >> @@ -94,6 +94,7 @@ typedef struct mkv_cues {
> >> typedef struct mkv_track {
> >> int write_dts;
> >> int has_cue;
> >> + uint32_t uid;
> >> int sample_rate;
> >> int64_t sample_rate_offset;
> >> int64_t last_timestamp;
> >> @@ -1199,8 +1200,7 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
> >> track = start_ebml_master(pb, MATROSKA_ID_TRACKENTRY, 0);
> >> put_ebml_uint (pb, MATROSKA_ID_TRACKNUMBER,
> >> mkv->is_dash ? mkv->dash_track_number : i + 1);
> >> - put_ebml_uint (pb, MATROSKA_ID_TRACKUID,
> >> - mkv->is_dash ? mkv->dash_track_number : i + 1);
> >> + put_ebml_uint (pb, MATROSKA_ID_TRACKUID, mkv->tracks[i].uid);
> >> put_ebml_uint (pb, MATROSKA_ID_TRACKFLAGLACING , 0); // no lacing (yet)
> >>
> >> if ((tag = av_dict_get(st->metadata, "title", NULL, 0)))
> >> @@ -1650,7 +1650,8 @@ static int mkv_write_tags(AVFormatContext *s)
> >> if (!mkv_check_tag(st->metadata, MATROSKA_ID_TAGTARGETS_TRACKUID))
> >> continue;
> >>
> >> - ret = mkv_write_tag(s, st->metadata, MATROSKA_ID_TAGTARGETS_TRACKUID, i + 1);
> >> + ret = mkv_write_tag(s, st->metadata, MATROSKA_ID_TAGTARGETS_TRACKUID,
> >> + mkv->tracks[i].uid);
> >> if (ret < 0) return ret;
> >> }
> >>
> >> @@ -1658,13 +1659,15 @@ static int mkv_write_tags(AVFormatContext *s)
> >> for (i = 0; i < s->nb_streams; i++) {
> >> AVIOContext *pb;
> >> AVStream *st = s->streams[i];
> >> + mkv_track *track = &mkv->tracks[i];
> >> ebml_master tag_target;
> >> ebml_master tag;
> >>
> >> if (st->codecpar->codec_type == AVMEDIA_TYPE_ATTACHMENT)
> >> continue;
> >>
> >> - mkv_write_tag_targets(s, MATROSKA_ID_TAGTARGETS_TRACKUID, i + 1, &tag_target);
> >> + mkv_write_tag_targets(s, MATROSKA_ID_TAGTARGETS_TRACKUID,
> >> + track->uid, &tag_target);
> >> pb = mkv->tags_bc;
> >>
> >> tag = start_ebml_master(pb, MATROSKA_ID_SIMPLETAG, 0);
> >> @@ -1863,10 +1866,6 @@ static int mkv_write_header(AVFormatContext *s)
> >> version = 4;
> >> }
> >>
> >> - mkv->tracks = av_mallocz_array(s->nb_streams, sizeof(*mkv->tracks));
> >> - if (!mkv->tracks) {
> >> - return AVERROR(ENOMEM);
> >> - }
> >> ebml_header = start_ebml_master(pb, EBML_ID_HEADER, MAX_EBML_HEADER_SIZE);
> >> put_ebml_uint (pb, EBML_ID_EBMLVERSION , 1);
> >> put_ebml_uint (pb, EBML_ID_EBMLREADVERSION , 1);
> >> @@ -2667,8 +2666,42 @@ static int webm_query_codec(enum AVCodecID codec_id, int std_compliance)
> >> return 0;
> >> }
> >>
> >> +static uint32_t mkv_get_uid(const mkv_track *tracks, int i, AVLFG *c)
> >> +{
> >> + uint32_t uid;
> >> +
> >> + for (int j = 0, k; j < 5; j++) {
> >> + uid = av_lfg_get(c);
> >> + if (!uid)
> >> + continue;
> >> + for (k = 0; k < i; k++) {
> >> + if (tracks[k].uid == uid) {
> >> + /* Was something wrong with our random seed? */
> >> + av_lfg_init(c, av_get_random_seed());
> >> + break;
> >> + }
> >> + }
> >> + if (k == i)
> >> + return uid;
> >> + }
> >> +
> >> + /* Test the numbers from 1 to i. */
> >> + for (int j = 1, k; j < i + 1; j++) {
> >> + for (k = 0; k < i; k++) {
> >> + if (tracks[k].uid == j)
> >> + break;
> >> + }
> >> + if (k == i)
> >> + return j;
> >> + }
> >> + /* Return i + 1. It can't coincide with another uid. */
> >> + return i + 1;
> >> +}
> >> +
> >> static int mkv_init(struct AVFormatContext *s)
> >> {
> >> + MatroskaMuxContext *mkv = s->priv_data;
> >> + AVLFG c;
> >> int i;
> >>
> >> if (s->nb_streams > MAX_TRACKS) {
> >> @@ -2697,7 +2730,23 @@ static int mkv_init(struct AVFormatContext *s)
> >> s->internal->avoid_negative_ts_use_pts = 1;
> >> }
> >>
> >> + mkv->tracks = av_mallocz_array(s->nb_streams, sizeof(*mkv->tracks));
> >> + if (!mkv->tracks) {
> >> + return AVERROR(ENOMEM);
> >> + }
> >> +
> >
> >> + if (!(s->flags & AVFMT_FLAG_BITEXACT))
> >> + av_lfg_init(&c, av_get_random_seed());
> >
> > would there be a disadvantage if the configuration of a stream / its content
> > is used a seed ?
> > That way it would be more deterministic
> >
> I see several disadvantages:
> 1. If one duplicates a stream (i.e. muxes it twice in the same file),
> they would get the same uid (unless one takes additional measures).
If you took the configuration of all streams as a single seed that would not
happen
> 2. If the packet content is used, one can determine the uid only after
> having received a packet. This would mean that one has to postpone
> writing the Tracks element until one has a packet and the same goes
> for the tags given that the uid is used to convey what they apply to.
> One could solve this problem in the seekable, non-livestreaming case
> by reserving size to be overwritten later, but this is nontrivial and
> I'd like to avoid that.
> 3. If the configuration record is used, then you won't get anything
> remotely unique: Using the same encoder settings will likely produce
> files with identical uids.
>
> Furthermore, this approach means that I can reuse the code for both
> attachments as well as tracks. (See the next patch in this series.)
fair enough, if its considered that avoiding some complexity is more
important than deterministic output
Either way if you need X bits of entropy in the muxer there should be
X/32 calls to av_get_random_seed(). as it is there are 4 independant
places calling av_get_random_seed() after the patch each using 32bit
to initialize an independant LFG.
seed collisions are not at all impossible at 32bits
also av_get_random_seed() can be expensive on some platforms so
i think there should be a single place calling it as needed and not
throwing the obtained entropy away.
Also, why 32bit ? arent these "uint" UIDs 64bit ?
and, this
+ /* Test the numbers from 1 to i. */
+ for (int j = 1, k; j < i + 1; j++) {
+ for (k = 0; k < i; k++) {
+ if (tracks[k].uid == j)
+ break;
+ }
+ if (k == i)
+ return j;
+ }
just smells "wrong"
if you want a simple non repeating UID, just count 0,1,2,3 for each
and put that in the unused upper 32bit for example
or you can even ditch LFG and use something that findamentgally generates
non repeating output.
UID[i] = seed + i
comes to mind
or if theres a reason why that is not good then you can also do something
like
UID[i] = (seed + i*(seed2|1)) & 0xFFFFFFFFFFFFFFFF;
but maybe theres a reason behind the more complex UID generation that
iam missing ?
Thanks
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
The bravest are surely those who have the clearest vision
of what is before them, glory and danger alike, and yet
notwithstanding go out to meet it. -- Thucydides
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