movenc: Honor flush requests with delay_moov, when some tracks lack samples

This also makes sure that a fragmented file without the empty_moov
flag (i.e. with a non-empty initial moov fragment) actually gets
written, if some of the tracks turn out to not have any samples.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -3167,7 +3167,7 @@ static int mov_flush_fragment_interleaving(AVFormatContext *s, MOVTrack *track)
     return 0;
 }
 
-static int mov_flush_fragment(AVFormatContext *s)
+static int mov_flush_fragment(AVFormatContext *s, int force)
 {
     MOVMuxContext *mov = s->priv_data;
     int i, first_track = -1;
@@ -3214,7 +3214,7 @@ static int mov_flush_fragment(AVFormatContext *s)
             if (!mov->tracks[i].entry)
                 break;
         /* Don't write the initial moov unless all tracks have data */
-        if (i < mov->nb_streams)
+        if (i < mov->nb_streams && !force)
             return 0;
 
         if ((ret = ffio_open_null_buf(&moov_buf)) < 0)
@@ -3343,17 +3343,17 @@ static int mov_flush_fragment(AVFormatContext *s)
     return 0;
 }
 
-static int mov_auto_flush_fragment(AVFormatContext *s)
+static int mov_auto_flush_fragment(AVFormatContext *s, int force)
 {
     MOVMuxContext *mov = s->priv_data;
     int had_moov = mov->moov_written;
-    int ret = mov_flush_fragment(s);
+    int ret = mov_flush_fragment(s, force);
     if (ret < 0)
         return ret;
     // If using delay_moov, the first flush only wrote the moov,
     // not the actual moof+mdat pair, thus flush once again.
     if (!had_moov && mov->flags & FF_MOV_FLAG_DELAY_MOOV)
-        ret = mov_flush_fragment(s);
+        ret = mov_flush_fragment(s, force);
     return ret;
 }
 
@@ -3568,7 +3568,7 @@ err:
 static int mov_write_packet(AVFormatContext *s, AVPacket *pkt)
 {
     if (!pkt) {
-        mov_flush_fragment(s);
+        mov_flush_fragment(s, 1);
         return 1;
     } else {
         MOVMuxContext *mov = s->priv_data;
@@ -3604,7 +3604,7 @@ static int mov_write_packet(AVFormatContext *s, AVPacket *pkt)
                 // for the other ones that are flushed at the same time.
                 trk->track_duration = pkt->dts - trk->start_dts;
                 trk->end_pts = pkt->pts;
-                mov_auto_flush_fragment(s);
+                mov_auto_flush_fragment(s, 0);
             }
         }
 
@@ -4243,7 +4243,7 @@ static int mov_write_trailer(AVFormatContext *s)
             mov_write_moov_tag(pb, mov, s);
         }
     } else {
-        mov_auto_flush_fragment(s);
+        mov_auto_flush_fragment(s, 1);
         for (i = 0; i < mov->nb_streams; i++)
            mov->tracks[i].data_offset = 0;
         if (mov->flags & FF_MOV_FLAG_GLOBAL_SIDX) {