demux: let a demuxer mark a stream as absent

Media can stop providing a stream mid-playback, for example a disc
program that does not carry the Dolby Vision enhancement layer the
previous one had. Since mpv always try to keep at least 1 packet
buffered in all tracks, it can keep reading it till EOF.

demux/demux.c

@@ -900,7 +900,7 @@ static void update_stream_eager_state(struct demux_internal *in)
         struct demux_stream *s = in->streams[n]->ds;
 
         s->still_image = s->sh->still_image;
-        s->eager = s->selected && !s->sh->attached_picture;
+        s->eager = s->selected && !s->sh->absent && !s->sh->attached_picture;
         if (s->eager && !s->still_image)
             any_av_streams |= s->type != STREAM_SUB;
         any_streams |= s->selected;
@@ -933,6 +933,23 @@ void demux_set_stream_still_image(demuxer_t *demuxer, struct sh_stream *sh,
     mp_mutex_unlock(&in->lock);
 }
 
+// Set whether the media provides data for this stream at all.
+void demux_set_stream_absent(demuxer_t *demuxer, struct sh_stream *sh,
+                             bool absent)
+{
+    struct demux_internal *in = demuxer->in;
+    mp_assert(demuxer == in->d_thread);
+
+    mp_mutex_lock(&in->lock);
+    if (sh->absent != absent) {
+        MP_VERBOSE(in, "stream %d is %s\n", sh->index,
+                   absent ? "absent" : "present");
+        sh->absent = absent;
+        update_stream_eager_state(in);
+    }
+    mp_mutex_unlock(&in->lock);
+}
+
 static void update_stream_selection_state(struct demux_internal *in,
                                           struct demux_stream *ds)
 {

demux/demux.h

@@ -331,6 +331,8 @@ void demux_drive_nav(struct demuxer *demuxer);
 void demux_nav_refresh(struct demuxer *demuxer);
 void demux_set_stream_still_image(struct demuxer *demuxer,
                                   struct sh_stream *sh, bool still_image);
+void demux_set_stream_absent(struct demuxer *demuxer, struct sh_stream *sh,
+                             bool absent);
 
 bool demux_cancel_test(struct demuxer *demuxer);
 bool demux_read_interrupted(struct demuxer *demuxer);

demux/stheader.h

@@ -68,6 +68,7 @@ struct sh_stream {
     bool commentary_track;      // container flag
     bool image;                 // video stream is an image
     bool still_image;           // video consists of multiple sparse still images
+    bool absent;                // the media provides no data for this stream
     int hls_bitrate;
     int *program_ids;
     int num_program_ids;