flvdec: require need_context_update when changing codec id

Otherwise the codec context and codecpar might disagree on the codec id,
triggering asserts in av_parser_parse2.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/flvdec.c

@@ -291,7 +291,9 @@ static int flv_same_video_codec(AVCodecParameters *vpar, int flags)
 static int flv_set_video_codec(AVFormatContext *s, AVStream *vstream,
                                int flv_codecid, int read)
 {
+    int ret = 0;
     AVCodecParameters *par = vstream->codecpar;
+    enum AVCodecID old_codec_id = vstream->codecpar->codec_id;
     switch (flv_codecid) {
     case FLV_CODECID_H263:
         par->codec_id = AV_CODEC_ID_FLV1;
@@ -319,20 +321,28 @@ static int flv_set_video_codec(AVFormatContext *s, AVStream *vstream,
             else
                 avio_skip(s->pb, 1);
         }
-        return 1;     // 1 byte body size adjustment for flv_read_packet()
+        ret = 1;     // 1 byte body size adjustment for flv_read_packet()
+        break;
     case FLV_CODECID_H264:
         par->codec_id = AV_CODEC_ID_H264;
         vstream->need_parsing = AVSTREAM_PARSE_HEADERS;
-        return 3;     // not 4, reading packet type will consume one byte
+        ret = 3;     // not 4, reading packet type will consume one byte
+        break;
     case FLV_CODECID_MPEG4:
         par->codec_id = AV_CODEC_ID_MPEG4;
-        return 3;
+        ret = 3;
+        break;
     default:
         avpriv_request_sample(s, "Video codec (%x)", flv_codecid);
         par->codec_tag = flv_codecid;
     }
 
-    return 0;
+    if (!vstream->internal->need_context_update && par->codec_id != old_codec_id) {
+        avpriv_request_sample(s, "Changing the codec id midstream");
+        return AVERROR_PATCHWELCOME;
+    }
+
+    return ret;
 }
 
 static int amf_get_string(AVIOContext *ioc, char *buffer, int buffsize)
@@ -547,7 +557,9 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream,
                     st->codecpar->codec_id = AV_CODEC_ID_TEXT;
                 } else if (flv->trust_metadata) {
                     if (!strcmp(key, "videocodecid") && vpar) {
-                        flv_set_video_codec(s, vstream, num_val, 0);
+                        int ret = flv_set_video_codec(s, vstream, num_val, 0);
+                        if (ret < 0)
+                            return ret;
                     } else if (!strcmp(key, "audiocodecid") && apar) {
                         int id = ((int)num_val) << FLV_AUDIO_CODECID_OFFSET;
                         flv_set_audio_codec(s, astream, apar, id);
@@ -1098,7 +1110,10 @@ retry_duration:
             avcodec_parameters_free(&par);
         }
     } else if (stream_type == FLV_STREAM_TYPE_VIDEO) {
-        size -= flv_set_video_codec(s, st, flags & FLV_VIDEO_CODECID_MASK, 1);
+        int ret = flv_set_video_codec(s, st, flags & FLV_VIDEO_CODECID_MASK, 1);
+        if (ret < 0)
+            return ret;
+        size -= ret;
     } else if (stream_type == FLV_STREAM_TYPE_DATA) {
         st->codecpar->codec_id = AV_CODEC_ID_TEXT;
     }