[FFmpeg-devel] [PATCH] lavf/movenc: Use a dynamic buffer when writing the mfra box

[Derek Buitenhuis]

When doing streamed output, with e.g. +dash, if the mfra box ended
up being larger than the AVIOContext write buffer, the (unchecked)
seeking back to update the box size would silently fail and produce
an invalid mfra box.

This is similar to how other boxes are written in fragmented mode.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
---
 libavformat/movenc.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index b8e45760ee..2927865cb4 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -4798,28 +4798,42 @@ static int mov_write_tfra_tag(AVIOContext *pb, MOVTrack *track)
 
 static int mov_write_mfra_tag(AVIOContext *pb, MOVMuxContext *mov)
 {
-    int64_t pos = avio_tell(pb);
-    int i;
+    AVIOContext *mfra_pb;
+    int i, ret, sz;
+    uint8_t *buf;
 
-    avio_wb32(pb, 0); /* size placeholder */
-    ffio_wfourcc(pb, "mfra");
+    ret = avio_open_dyn_buf(&mfra_pb);
+    if (ret < 0)
+        return ret;
+
+    avio_wb32(mfra_pb, 0); /* size placeholder */
+    ffio_wfourcc(mfra_pb, "mfra");
     /* An empty mfra atom is enough to indicate to the publishing point that
      * the stream has ended. */
     if (mov->flags & FF_MOV_FLAG_ISML)
-        return update_size(pb, pos);
+        goto done_mfra;
 
     for (i = 0; i < mov->nb_streams; i++) {
         MOVTrack *track = &mov->tracks[i];
         if (track->nb_frag_info)
-            mov_write_tfra_tag(pb, track);
+            mov_write_tfra_tag(mfra_pb, track);
     }
 
-    avio_wb32(pb, 16);
-    ffio_wfourcc(pb, "mfro");
-    avio_wb32(pb, 0); /* version + flags */
-    avio_wb32(pb, avio_tell(pb) + 4 - pos);
+    avio_wb32(mfra_pb, 16);
+    ffio_wfourcc(mfra_pb, "mfro");
+    avio_wb32(mfra_pb, 0); /* version + flags */
+    avio_wb32(mfra_pb, avio_tell(mfra_pb) + 4);
 
-    return update_size(pb, pos);
+done_mfra:
+
+    sz  = update_size(mfra_pb, 0);
+    ret = avio_close_dyn_buf(mfra_pb, &buf);
+    if (ret < 0)
+        return ret;
+    avio_write(pb, buf, ret);
+    av_free(buf);
+
+    return sz;
 }
 
 static int mov_write_mdat_tag(AVIOContext *pb, MOVMuxContext *mov)
@@ -6987,7 +7001,9 @@ static int mov_write_trailer(AVFormatContext *s)
         }
         if (!(mov->flags & FF_MOV_FLAG_SKIP_TRAILER)) {
             avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_TRAILER);
-            mov_write_mfra_tag(pb, mov);
+            res = mov_write_mfra_tag(pb, mov);
+            if (res < 0)
+                return res;
         }
     }
 
-- 
2.27.0.rc2