sub: make preloading more robust

Subtitles can be preloaded, which means they're fully read and copied
into ASS_Track. This in turn is mainly for the sake of being able to do
subtitle seeking (when it comes down to it, subtitle seeking is the
cause for most trouble here).

Commit a714f8e92 broke preloaded subtitles which have events with
unknown duration, such as some MicroDVD samples. The event list gets
cleared on every seek, so the property of being preloaded obviously gets
lost.

Fix this by moving most of the preloading logic to dec_sub.c. If the
subtitle list gets cleared, they are not considered preloaded anymore,
and the logic for demuxed subtitles is used.

As another minor thing, preloadeding subtitles did neither disable the
demux stream, nor did it discard packets. Thus you could get queue
overflows in theory (harmless, but annoying). Fix this by explicitly
discarding packets in preloaded mode.

In summary, now the only difference between preloaded and normal
demuxing are:
1. a seek is issued, and all packets are read on start
2. during playback, discard the packets instead of feeding them to the
   subtitle decoder

This is still petty annoying. It would be nice if maintaining the
subtitle index (and maybe a subtitle packet cache for instant subtitle
presentation when seeking back) could be maintained in the demuxer
instead. Half of all file formats with interleaved subtitles have
this anyway (mp4, mkv muxed with newer mkvmerge).

player/core.h

@@ -147,10 +147,6 @@ struct track {
     struct vo_chain *vo_c;
     struct ao_chain *ao_c;
     struct lavfi_pad *sink;
-
-    // For external subtitles, which are read fully on init. Do not attempt
-    // to read packets from them.
-    bool preloaded;
 };
 
 // Summarizes video filtering and output.

player/sub.c

@@ -99,18 +99,15 @@ static bool update_subtitle(struct MPContext *mpctx, double video_pts,
 
     video_pts -= opts->sub_delay;
 
-    if (!track->preloaded && track->demuxer->fully_read && !opts->sub_clear_on_seek)
-    {
+    if (track->demuxer->fully_read && sub_can_preload(dec_sub)) {
         // Assume fully_read implies no interleaved audio/video streams.
         // (Reading packets will change the demuxer position.)
         demux_seek(track->demuxer, 0, 0);
-        track->preloaded = sub_read_all_packets(track->d_sub);
+        sub_preload(dec_sub);
     }
 
-    if (!track->preloaded) {
-        if (!sub_read_packets(dec_sub, video_pts))
-            return false;
-    }
+    if (!sub_read_packets(dec_sub, video_pts))
+        return false;
 
     // Handle displaying subtitles on terminal; never done for secondary subs
     if (mpctx->current_track[0][STREAM_SUB] == track && !mpctx->video_out)

sub/dec_sub.c

@@ -53,6 +53,7 @@ struct dec_sub {
 
     struct sh_stream *sh;
     double last_pkt_pts;
+    bool preload_attempted;
 
     struct mp_codec_params *codec;
     double start, end;
@@ -96,6 +97,7 @@ static struct sd *init_decoder(struct dec_sub *sub)
             .driver = driver,
             .attachments = sub->attachments,
             .codec = sub->codec,
+            .preload_ok = true,
         };
 
         if (sd->driver->init(sd) >= 0)
@@ -167,16 +169,20 @@ static void update_segment(struct dec_sub *sub)
     }
 }
 
-// Read all packets from the demuxer and decode/add them. Returns false if
-// there are circumstances which makes this not possible.
-bool sub_read_all_packets(struct dec_sub *sub)
+bool sub_can_preload(struct dec_sub *sub)
+{
+    bool r;
+    pthread_mutex_lock(&sub->lock);
+    r = sub->sd->driver->accept_packets_in_advance && !sub->preload_attempted;
+    pthread_mutex_unlock(&sub->lock);
+    return r;
+}
+
+void sub_preload(struct dec_sub *sub)
 {
     pthread_mutex_lock(&sub->lock);
 
-    if (!sub->sd->driver->accept_packets_in_advance) {
-        pthread_mutex_unlock(&sub->lock);
-        return false;
-    }
+    sub->preload_attempted = true;
 
     for (;;) {
         struct demux_packet *pkt = demux_read_packet(sub->sh);
@@ -187,7 +193,6 @@ bool sub_read_all_packets(struct dec_sub *sub)
     }
 
     pthread_mutex_unlock(&sub->lock);
-    return true;
 }
 
 // Read packets from the demuxer stream passed to sub_create(). Return true if
@@ -230,7 +235,9 @@ bool sub_read_packets(struct dec_sub *sub, double video_pts)
             break;
         }
 
-        sub->sd->driver->decode(sub->sd, pkt);
+        if (!(sub->preload_attempted && sub->sd->preload_ok))
+            sub->sd->driver->decode(sub->sd, pkt);
+
         talloc_free(pkt);
     }
     pthread_mutex_unlock(&sub->lock);

sub/dec_sub.h

@@ -32,7 +32,8 @@ void sub_destroy(struct dec_sub *sub);
 void sub_lock(struct dec_sub *sub);
 void sub_unlock(struct dec_sub *sub);
 
-bool sub_read_all_packets(struct dec_sub *sub);
+bool sub_can_preload(struct dec_sub *sub);
+void sub_preload(struct dec_sub *sub);
 bool sub_read_packets(struct dec_sub *sub, double video_pts);
 void sub_get_bitmaps(struct dec_sub *sub, struct mp_osd_res dim, double pts,
                      struct sub_bitmaps *res);

sub/sd.h

@@ -19,6 +19,10 @@ struct sd {
 
     struct attachment_list *attachments;
     struct mp_codec_params *codec;
+
+    // Set to false as soon as the decoder discards old subtitle events.
+    // (only needed if sd_functions.accept_packets_in_advance == false)
+    bool preload_ok;
 };
 
 struct sd_functions {

sub/sd_ass.c

@@ -452,6 +452,7 @@ static void get_bitmaps(struct sd *sd, struct mp_osd_res dim, double pts,
     if (ctx->duration_unknown && pts != MP_NOPTS_VALUE) {
         mp_ass_flush_old_events(track, ts);
         ctx->num_seen_packets = 0;
+        sd->preload_ok = false;
     }
     if (no_ass)
         fill_plaintext(sd, pts);
@@ -612,6 +613,7 @@ static void reset(struct sd *sd)
     if (sd->opts->sub_clear_on_seek || ctx->duration_unknown) {
         ass_flush_events(ctx->ass_track);
         ctx->num_seen_packets = 0;
+        sd->preload_ok = false;
     }
     if (ctx->converter)
         lavc_conv_reset(ctx->converter);