mov: Compare frag times in correct time base when seeking a stream without a corresponding sidx

Some muxers, such as GPAC, create files with only one sidx, but two streams
muxed into the same fragments pointed to by this sidx.

Prevously, in such a case, when we seeked in such files, we fell back
to, for example, using the sidx associated with the video stream, to
seek the audio stream, leaving the seekhead in the wrong place.

We can still do this, but we need to take care to compare timestamps
in the same time base.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/mov.c

@@ -1271,15 +1271,18 @@ static int64_t get_stream_info_time(MOVFragmentStreamInfo * frag_stream_info)
     return frag_stream_info->tfdt_dts;
 }
 
-static int64_t get_frag_time(MOVFragmentIndex *frag_index,
-                             int index, int track_id)
+static int64_t get_frag_time(AVFormatContext *s, AVStream *dst_st,
+                             MOVFragmentIndex *frag_index, int index)
 {
     MOVFragmentStreamInfo * frag_stream_info;
+    MOVStreamContext *sc = dst_st->priv_data;
     int64_t timestamp;
-    int i;
+    int i, j;
 
-    if (track_id >= 0) {
-        frag_stream_info = get_frag_stream_info(frag_index, index, track_id);
+    // If the stream is referenced by any sidx, limit the search
+    // to fragments that referenced this stream in the sidx
+    if (sc->has_sidx) {
+        frag_stream_info = get_frag_stream_info(frag_index, index, dst_st->id);
         if (frag_stream_info->sidx_pts != AV_NOPTS_VALUE)
             return frag_stream_info->sidx_pts;
         if (frag_stream_info->first_tfra_pts != AV_NOPTS_VALUE)
@@ -1288,28 +1291,27 @@ static int64_t get_frag_time(MOVFragmentIndex *frag_index,
     }
 
     for (i = 0; i < frag_index->item[index].nb_stream_info; i++) {
+        AVStream *frag_stream = NULL;
         frag_stream_info = &frag_index->item[index].stream_info[i];
+        for (j = 0; j < s->nb_streams; j++)
+            if (s->streams[j]->id == frag_stream_info->id)
+                frag_stream = s->streams[j];
+        if (!frag_stream) {
+            av_log(s, AV_LOG_WARNING, "No stream matching sidx ID found.\n");
+            continue;
+        }
         timestamp = get_stream_info_time(frag_stream_info);
         if (timestamp != AV_NOPTS_VALUE)
-            return timestamp;
+            return av_rescale_q(timestamp, frag_stream->time_base, dst_st->time_base);
     }
     return AV_NOPTS_VALUE;
 }
 
-static int search_frag_timestamp(MOVFragmentIndex *frag_index,
+static int search_frag_timestamp(AVFormatContext *s, MOVFragmentIndex *frag_index,
                                  AVStream *st, int64_t timestamp)
 {
     int a, b, m, m0;
     int64_t frag_time;
-    int id = -1;
-
-    if (st) {
-        // If the stream is referenced by any sidx, limit the search
-        // to fragments that referenced this stream in the sidx
-        MOVStreamContext *sc = st->priv_data;
-        if (sc->has_sidx)
-            id = st->id;
-    }
 
     a = -1;
     b = frag_index->nb_items;
@@ -1318,7 +1320,7 @@ static int search_frag_timestamp(MOVFragmentIndex *frag_index,
         m0 = m = (a + b) >> 1;
 
         while (m < b &&
-               (frag_time = get_frag_time(frag_index, m, id)) == AV_NOPTS_VALUE)
+               (frag_time = get_frag_time(s, st, frag_index, m)) == AV_NOPTS_VALUE)
             m++;
 
         if (m < b && frag_time <= timestamp)
@@ -8862,7 +8864,7 @@ static int mov_seek_fragment(AVFormatContext *s, AVStream *st, int64_t timestamp
     if (!mov->frag_index.complete)
         return 0;
 
-    index = search_frag_timestamp(&mov->frag_index, st, timestamp);
+    index = search_frag_timestamp(s, &mov->frag_index, st, timestamp);
     if (index < 0)
         index = 0;
     if (!mov->frag_index.item[index].headers_read)