Re: [FFmpeg-devel] [PATCH] avformat/mpegts: Add allow_codec_changes option to AVFormatContext

[Pavel Koshevoy <pkoshevoy-at-gmail.com@ffmpeg.org>]

On Sun, Jun 22, 2025 at 1:26 AM Andreas Rheinhardt <
andreas.rheinhardt@outlook.com> wrote:

> Pavel Koshevoy:
> > Make runtime AVStream.codecpar codec_id updates optional and disabled
> > by default, so that avformat API clients can enable this feature
> explicitly
> > when they add support for runtime codec changes.
> > ---
> >  doc/APIchanges              |  3 +++
> >  doc/formats.texi            |  6 ++++++
> >  libavformat/avformat.h      | 10 ++++++++++
> >  libavformat/mpegts.c        |  4 +++-
> >  libavformat/options_table.h |  1 +
> >  libavformat/version.h       |  2 +-
> >  tests/fate/demux.mak        |  2 +-
> >  7 files changed, 25 insertions(+), 3 deletions(-)
> >
> > diff --git a/doc/APIchanges b/doc/APIchanges
> > index 91710bb27d..29b3389094 100644
> > --- a/doc/APIchanges
> > +++ b/doc/APIchanges
> > @@ -2,6 +2,9 @@ The last version increases of all libraries were on
> 2025-03-28
> >
> >  API changes, most recent first:
> >
> > +2025-06-17 - xxxxxxxxxx - lavf 62.2.100 - avformat.h
> > +  Add AVFormatContext.allow_codec_changes AVOption.
> > +
> >  2025-05-21 - xxxxxxxxxx - lavu 60.3.100 - avassert.h
> >    Add av_unreachable() and av_assume() macros.
> >
> > diff --git a/doc/formats.texi b/doc/formats.texi
> > index 876a9e92b3..d18613f8bd 100644
> > --- a/doc/formats.texi
> > +++ b/doc/formats.texi
> > @@ -228,6 +228,12 @@ would require too many resources due to a large
> number of streams.
> >  Skip estimation of input duration if it requires an additional probing
> for PTS at end of file.
> >  At present, applicable for MPEG-PS and MPEG-TS.
> >
> > +@item allow_codec_changes @var{bool} (@emph{input})
> > +Allow AVStream.codecpar codec_type and codec_id to change at runtime
> > +(for example when MPEG-TS PMT ES stream_type changes at runtime).
> > +This is disabled by default, because most clients of avformat API
> > +do not support random mid-stream codec changes.
> > +
> >  @item duration_probesize @var{integer} (@emph{input})
> >  Set probing size, in bytes, for input duration estimation when it
> actually requires
> >  an additional probing for PTS at end of file (at present: MPEG-PS and
> MPEG-TS).
> > diff --git a/libavformat/avformat.h b/libavformat/avformat.h
> > index b6c63e2237..c6956626b5 100644
> > --- a/libavformat/avformat.h
> > +++ b/libavformat/avformat.h
> > @@ -1884,6 +1884,16 @@ typedef struct AVFormatContext {
> >       * @see skip_estimate_duration_from_pts
> >       */
> >      int64_t duration_probesize;
> > +
> > +    /**
> > +     * Allow AVStream.codecpar codec_type and codec_id to change at
> runtime
> > +     * (for example when MPEG-TS PMT ES stream_type changes at runtime).
> > +     * This is disabled by default, because most clients of avformat API
> > +     * do not support random mid-stream codec changes.
> > +     * - muxing: unused
> > +     * - demuxing: set by user
> > +     */
> > +    int allow_codec_changes;
> >  } AVFormatContext;
> >
> >  /**
> > diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
> > index deb69a0548..0c78e870bb 100644
> > --- a/libavformat/mpegts.c
> > +++ b/libavformat/mpegts.c
> > @@ -2510,7 +2510,9 @@ static void pmt_cb(MpegTSFilter *filter, const
> uint8_t *section, int section_len
> >          if (!st)
> >              goto out;
> >
> > -        if (pes && pes->stream_type != stream_type)
> > +        if (pes && (!pes->stream_type ||
> > +                    (pes->stream_type != stream_type &&
> > +                     ts->stream->allow_codec_changes)))
> >              mpegts_set_stream_info(st, pes, stream_type, prog_reg_desc);
> >
> >          add_pid_to_program(prg, pid);
> > diff --git a/libavformat/options_table.h b/libavformat/options_table.h
> > index e2e690fd2a..e949d60c3e 100644
> > --- a/libavformat/options_table.h
> > +++ b/libavformat/options_table.h
> > @@ -106,6 +106,7 @@ static const AVOption avformat_options[] = {
> >  {"skip_estimate_duration_from_pts", "skip duration calculation in
> estimate_timings_from_pts", OFFSET(skip_estimate_duration_from_pts),
> AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, D},
> >  {"max_probe_packets", "Maximum number of packets to probe a codec",
> OFFSET(max_probe_packets), AV_OPT_TYPE_INT, { .i64 = 2500 }, 0, INT_MAX, D
> },
> >  {"duration_probesize", "Maximum number of bytes to probe the durations
> of the streams in estimate_timings_from_pts", OFFSET(duration_probesize),
> AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, D},
> > +{"allow_codec_changes", "Allow AVStream.codecpar codec_type and
> codec_id to change at runtime", OFFSET(allow_codec_changes),
> AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, D},
>
> I am not certain that this should be settable via AVOptions. The latter
> would e.g. allow users of the FFmpeg tool to set it, although I don't
> think that the cli could handle it. E.g. (non-)thread-safe-callbacks
> could also not be set via AVOptions.
>
> >  {NULL},
> >  };
> >
> > diff --git a/libavformat/version.h b/libavformat/version.h
> > index a7c80dc564..904e7f06aa 100644
> > --- a/libavformat/version.h
> > +++ b/libavformat/version.h
> > @@ -31,7 +31,7 @@
> >
> >  #include "version_major.h"
> >
> > -#define LIBAVFORMAT_VERSION_MINOR   1
> > +#define LIBAVFORMAT_VERSION_MINOR   2
> >  #define LIBAVFORMAT_VERSION_MICRO 100
> >
> >  #define LIBAVFORMAT_VERSION_INT
> AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \
> > diff --git a/tests/fate/demux.mak b/tests/fate/demux.mak
> > index ead5ad4b10..4a39e950b9 100644
> > --- a/tests/fate/demux.mak
> > +++ b/tests/fate/demux.mak
> > @@ -158,7 +158,7 @@ FATE_SAMPLES_DEMUX-$(CONFIG_XWMA_DEMUXER) +=
> fate-xwma-demux
> >  fate-xwma-demux: CMD = crc -i $(TARGET_SAMPLES)/xwma/ergon.xwma -c:a
> copy
> >
> >  FATE_FFPROBE_DEMUX-$(CONFIG_MPEGTS_DEMUXER) += fate-ts-demux
> > -fate-ts-demux: CMD = ffprobe_demux
> $(TARGET_SAMPLES)/ac3/mp3ac325-4864-small.ts
> > +fate-ts-demux: CMD = ffprobe_demux
> $(TARGET_SAMPLES)/ac3/mp3ac325-4864-small.ts -allow_codec_changes 1
> >
> >  FATE_FFPROBE_DEMUX-$(CONFIG_MPEGTS_DEMUXER) += fate-ts-timed-id3-demux
> >  fate-ts-timed-id3-demux: CMD = ffprobe_demux
> $(TARGET_SAMPLES)/mpegts/id3.ts
>
> Should this be restricted to codec and codec_id changes? Right now
> AVStream.codecpar is documented to be "filled by libavformat on stream
> creation or in avformat_find_stream_info()" and only then. If I take
> your documentation literally, then e.g. the dimensions are not allowed
> to change when the codec changes. IMO everything in codecpar should be
> allowed to be updated if this flag is set.
>
> - Andreas
>

New patch submitted here:
https://ffmpeg.org/pipermail/ffmpeg-devel/2025-June/345640.html

Pavel.
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