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

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

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

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