[FFmpeg-devel] [PATCH] avformat/demux: ensure avformat_find_stream_info updates internal stream contexts (PR #21224)

[James Almer via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #21224 opened by James Almer (jamrial)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21224
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21224.patch

read_frame_internal() may result in a stream being modified without also returning a packet from it. Given said function only bothered to update the internal stream context for the returned packet, the result would be a desync between the stream's AVCodecParameters and the internal AVCodecContext.
This change makes sure all streams are updated within the avformat_find_stream_info() loop.

Fixes #YWH-PGM40646-20

This PR supersedes #21215.


>From 27fa209cf5f2b995828926e863ce11f1e531881f Mon Sep 17 00:00:00 2001
From: James Almer <jamrial@gmail.com>
Date: Tue, 16 Dec 2025 19:20:40 -0300
Subject: [PATCH] avformat/demux: ensure avformat_find_stream_info updates
 internal stream contexts

read_frame_internal() may result in a stream being modified without also
returning a packet from it. Given said function only bothered to update the
internal stream context for the returned packet, the result would be a desync
between the stream's AVCodecParameters and the internal AVCodecContext.

This change makes sure all streams are updated within the
avformat_find_stream_info() loop.

Fixes #YWH-PGM40646-20

Signed-off-by: James Almer <jamrial@gmail.com>
---
 libavformat/demux.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/libavformat/demux.c b/libavformat/demux.c
index 934eb80553..b1918d2110 100644
--- a/libavformat/demux.c
+++ b/libavformat/demux.c
@@ -186,6 +186,8 @@ static int init_input(AVFormatContext *s, const char *filename,
                                   s, 0, s->format_probesize);
 }
 
+static int codec_close(FFStream *sti);
+
 static int update_stream_avctx(AVFormatContext *s)
 {
     int ret;
@@ -196,6 +198,14 @@ static int update_stream_avctx(AVFormatContext *s)
         if (!sti->need_context_update)
             continue;
 
+        if (avcodec_is_open(sti->avctx)) {
+            av_log(s, AV_LOG_DEBUG, "Demuxer context update while decoder is open, closing and trying to re-open\n");
+            ret = codec_close(sti);
+            sti->info->found_decoder = 0;
+            if (ret < 0)
+                return ret;
+        }
+
         /* close parser, because it depends on the codec */
         if (sti->parser && sti->avctx->codec_id != st->codecpar->codec_id) {
             av_parser_close(sti->parser);
@@ -2682,6 +2692,13 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
             break;
         }
 
+        /* read_frame_internal() in a previous iteration of this loop may
+         * have made changes to streams without returning a packet for them.
+         * Handle that here. */
+        ret = update_stream_avctx(ic);
+        if (ret < 0)
+            goto unref_then_goto_end;
+
         /* check if one codec still needs to be handled */
         for (i = 0; i < ic->nb_streams; i++) {
             AVStream *const st  = ic->streams[i];
-- 
2.49.1

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