[FFmpeg-devel] [PATCH] matroska : Fix writing packets to a non-seekable AVIOContext

[Aaron Colwell]

On Tue, Mar 6, 2012 at 10:24 PM, Michael Niedermayer <michaelni at gmx.at>wrote:

>
> I think the problem is that ffserver produces avio_tell() == 0 values
> and that causes
> "if (mkv->cluster_pos &&"
> to fail
> but iam guessing here a bit from your description of what the problem
> is so i could be wrong
>
> If that is correct then to fix it, i think the best would be either to
> make ffserver produce normal advancing url_tell() values
> or
> To change the default "not set" value of cluster_pos to something else
> than 0 and change all the checks as well.
>

Yes you are correct on both counts. Thanks for pointing me in the right
direction. The new patch below fixes both problems.

ffserver appears to use a new dyn_packet_buf each time it calls
av_write_frame() and was not setting the pos member variable to reflect the
current stream position. I've added a one line change to fix that.

I also changed the code in matroskaenc.c to use -1 as the "I'm not
currently working on a cluster" signal instead of 0. This avoids problems
where avio_tell() returns 0. I've updated all the checks against
cluster_pos and added a minor fix to mkv_add_cuepoint() to prevent invalid
cue entries from getting created with invalid cluster_pos values.

Aaron

---
 ffserver.c                |    1 +
 libavformat/matroskaenc.c |   11 ++++++-----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/ffserver.c b/ffserver.c
index bddcb7d..2b819d0 100644
--- a/ffserver.c
+++ b/ffserver.c
@@ -2380,6 +2380,7 @@ static int http_prepare_data(HTTPContext *c)
                         ret = ffio_open_dyn_packet_buf(&ctx->pb,
max_packet_size);
                     } else {
                         ret = avio_open_dyn_buf(&ctx->pb);
+                        ctx->pb->pos = c->data_count;
                     }
                     if (ret < 0) {
                         /* XXX: potential leak */
diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 0b36725..400ac97 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -372,7 +372,7 @@ static int mkv_add_cuepoint(mkv_cues *cues, int stream,
int64_t ts, int64_t clus
     if (entries == NULL)
         return AVERROR(ENOMEM);

-    if (ts < 0)
+    if (ts < 0 || cluster_pos < cues->segment_offset)
         return 0;

     entries[cues->num_entries  ].pts = ts;
@@ -978,6 +978,7 @@ static int mkv_write_header(AVFormatContext *s)
     av_init_packet(&mkv->cur_audio_pkt);
     mkv->cur_audio_pkt.size = 0;
     mkv->audio_buffer_size  = 0;
+    mkv->cluster_pos = -1;

     avio_flush(pb);
     return 0;
@@ -1150,7 +1151,7 @@ static int mkv_write_packet_internal(AVFormatContext
*s, AVPacket *pkt)
         pb = mkv->dyn_bc;
     }

-    if (!mkv->cluster_pos) {
+    if (mkv->cluster_pos == -1) {
         mkv->cluster_pos = avio_tell(s->pb);
         mkv->cluster = start_ebml_master(pb, MATROSKA_ID_CLUSTER, 0);
         put_ebml_uint(pb, MATROSKA_ID_CLUSTERTIMECODE, FFMAX(0, ts));
@@ -1204,14 +1205,14 @@ static int mkv_write_packet(AVFormatContext *s,
AVPacket *pkt)

     // start a new cluster every 5 MB or 5 sec, or 32k / 1 sec for
streaming or
     // after 4k and on a keyframe
-    if (mkv->cluster_pos &&
+    if (mkv->cluster_pos != -1 &&
         ((!s->pb->seekable && (cluster_size > 32*1024 || ts >
mkv->cluster_pts + 1000))
          ||                      cluster_size > 5*1024*1024 || ts >
mkv->cluster_pts + 5000
          || (codec->codec_type == AVMEDIA_TYPE_VIDEO && keyframe &&
cluster_size > 4*1024))) {
         av_log(s, AV_LOG_DEBUG, "Starting new cluster at offset %" PRIu64
                " bytes, pts %" PRIu64 "\n", avio_tell(pb), ts);
         end_ebml_master(pb, mkv->cluster);
-        mkv->cluster_pos = 0;
+        mkv->cluster_pos = -1;
         if (mkv->dyn_bc)
             mkv_flush_dynbuf(s);
     }
@@ -1255,7 +1256,7 @@ static int mkv_write_trailer(AVFormatContext *s)
     if (mkv->dyn_bc) {
         end_ebml_master(mkv->dyn_bc, mkv->cluster);
         mkv_flush_dynbuf(s);
-    } else if (mkv->cluster_pos) {
+    } else if (mkv->cluster_pos != -1) {
         end_ebml_master(pb, mkv->cluster);
     }

-- 
1.7.7.3