demux: disable stream cache if no tracks are selected

Slightly helps with timeline stuff, like EDL. There is no need to keep
network (or even just disk I/O) busy for all segments at the same time,
because 1. the data won't be needed any time soon, and 2. will probably
be discarded anyway if the stream is seeked when segment is resumed.

Partially fixes #2692.

demux/demux.c

@@ -558,11 +558,18 @@ static void execute_trackswitch(struct demux_internal *in)
 {
     in->tracks_switched = false;
 
+    bool any_selected = false;
+    for (int n = 0; n < in->num_streams; n++)
+        any_selected |= in->streams[n]->ds->selected;
+
     pthread_mutex_unlock(&in->lock);
 
     if (in->d_thread->desc->control)
         in->d_thread->desc->control(in->d_thread, DEMUXER_CTRL_SWITCHED_TRACKS, 0);
 
+    stream_control(in->d_thread->stream, STREAM_CTRL_SET_READAHEAD,
+                   &(int){any_selected});
+
     pthread_mutex_lock(&in->lock);
 
     if (in->start_refresh_seek)
@@ -1077,6 +1084,7 @@ static struct demuxer *open_given_type(struct mpv_global *global,
         demux_init_cache(demuxer);
         demux_changed(in->d_thread, DEMUX_EVENT_ALL);
         demux_update(demuxer);
+        stream_control(demuxer->stream, STREAM_CTRL_SET_READAHEAD, &(int){false});
         return demuxer;
     }
 

player/loadfile.c

@@ -312,7 +312,10 @@ bool timeline_switch_to_time(struct MPContext *mpctx, double pts)
 
     if (mpctx->demuxer) {
         demux_stop_thread(mpctx->demuxer);
-        demux_flush(mpctx->demuxer);
+        for (int i = 0; i < demux_get_num_stream(mpctx->demuxer); i++) {
+            struct sh_stream *sh = demux_get_stream(mpctx->demuxer, i);
+            demuxer_select_track(mpctx->demuxer, sh, false);
+        }
     }
 
     mpctx->demuxer = n->source;