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

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

On Tue, Jun 17, 2025, 07:55 Pavel Koshevoy <pkoshevoy@gmail.com> wrote:

> Ping.  I think I've addressed the concerns raised with the previous
> patch.
> Please let me know if I may commit this... it is a segfault fix, so it
> probably should get some review priority.
>
> Thank you,
>     Pavel
>
>
> On Sun, Jun 15, 2025, 14:50 Pavel Koshevoy <pkoshevoy@gmail.com> 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/formats.texi            |  6 ++++++
>>  libavformat/avformat.h      | 10 ++++++++++
>>  libavformat/demux.c         |  8 +++++++-
>>  libavformat/mpegts.c        |  4 +++-
>>  libavformat/options_table.h |  1 +
>>  tests/fate/demux.mak        |  2 +-
>>  6 files changed, 28 insertions(+), 3 deletions(-)
>>
>> 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;
>>
>
Oops, I forgot to bump up the minor version.




>>  /**
>> 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/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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