dashenc: Heuristically fill in the duration of packets that need it

This avoids that the mp4 muxer does a similar heuristic, adjusting
the timestamps in a way that the dash muxer doesn't know the actual
timestamps written to the file in the end. By making sure that the
mp4 muxer internal heuristic isn't applied, we know the exact
timestamps written to file, so that the timestamps in manifest match
the files.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/dashenc.c

@@ -69,6 +69,7 @@ typedef struct OutputStream {
     int nb_segments, segments_size, segment_index;
     Segment **segments;
     int64_t first_pts, start_pts, max_pts;
+    int64_t last_dts;
     int bit_rate;
     char bandwidth_str[64];
 
@@ -648,6 +649,7 @@ static int dash_write_header(AVFormatContext *s)
         set_codec_str(s, st->codec, os->codec_str, sizeof(os->codec_str));
         os->first_pts = AV_NOPTS_VALUE;
         os->max_pts = AV_NOPTS_VALUE;
+        os->last_dts = AV_NOPTS_VALUE;
         os->segment_index = 1;
     }
 
@@ -860,6 +862,16 @@ static int dash_write_packet(AVFormatContext *s, AVPacket *pkt)
     if (ret < 0)
         return ret;
 
+    // Fill in a heuristic guess of the packet duration, if none is available.
+    // The mp4 muxer will do something similar (for the last packet in a fragment)
+    // if nothing is set (setting it for the other packets doesn't hurt).
+    // By setting a nonzero duration here, we can be sure that the mp4 muxer won't
+    // invoke its heuristic (this doesn't have to be identical to that algorithm),
+    // so that we know the exact timestamps of fragments.
+    if (!pkt->duration && os->last_dts != AV_NOPTS_VALUE)
+        pkt->duration = pkt->dts - os->last_dts;
+    os->last_dts = pkt->dts;
+
     // If forcing the stream to start at 0, the mp4 muxer will set the start
     // timestamps to 0. Do the same here, to avoid mismatches in duration/timestamps.
     if (os->first_pts == AV_NOPTS_VALUE &&