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

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

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.
+
 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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