Re: [FFmpeg-devel] [PATCH] avformat/demux: Add allow_codec_changes option to AVFormatContext (v2)

[Marton Balint <cus@passwd.hu>]

On Tue, 17 Jun 2025, Pavel Koshevoy wrote:

> 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.
>
> Accordingly, codec_close should not assume that the codec_id cannot change.
> This fixes 'ffprobe 1_poc.mp4' segfault introduced with
> commit 0021484d05f9b0f032fa319399de6e24eea0c04f.
> ---
> doc/APIchanges              |  3 +++
> doc/formats.texi            |  6 ++++++
> libavformat/avformat.h      | 10 ++++++++++
> libavformat/demux.c         |  8 +++++++-
> libavformat/mpegts.c        |  4 +++-
> libavformat/options_table.h |  1 +
> libavformat/version.h       |  2 +-
> tests/fate/demux.mak        |  2 +-
> 8 files changed, 32 insertions(+), 4 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.

Considering that this is a simple boolean option, maybe you should use 
AVFormatContext->flags instead, so no new field need to be added, only a 
new flag.

Thanks,
Marton

> +
> 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/demux.c b/libavformat/demux.c
> index ecd4f40da9..3749ab67a3 100644
> --- a/libavformat/demux.c
> +++ b/libavformat/demux.c
> @@ -1292,9 +1292,15 @@ static int codec_close(FFStream *sti)
> {
>     AVCodecContext *avctx_new = NULL;
>     AVCodecParameters *par_tmp = NULL;
> +    const AVCodec *new_codec = NULL;
>     int ret;
>
> -    avctx_new = avcodec_alloc_context3(sti->avctx->codec);
> +    new_codec =
> +      (sti->avctx->codec_id != sti->pub.codecpar->codec_id) ?
> +      avcodec_find_decoder(sti->pub.codecpar->codec_id) :
> +      sti->avctx->codec;
> +
> +    avctx_new = avcodec_alloc_context3(new_codec);
>     if (!avctx_new) {
>         ret = AVERROR(ENOMEM);
>         goto fail;
> 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},
> {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
> -- 
> 2.43.0
>
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