sub: add option to workaround broken mkv files

See additions to options.rst.

DOCS/man/options.rst

@@ -1322,6 +1322,13 @@ Subtitles
     Can be used to disable display of subtitles, but still select and decode
     them.
 
+``--sub-clear-on-seek``
+    (Obscure, rarely useful.) Can be used to play broken mkv files with
+    duplicate ReadOrder fields. ReadOrder is the first field in a
+    Matroska-style ASS subtitle packets. It should be unique, and libass
+    uses it for fast elimination of duplicates. This option disables caching
+    of subtitles across seeks, so after a seek libass can't eliminate subtitle
+    packets with the same ReadOrder as earlier packets.
 
 Window
 ------

options/options.c

@@ -337,6 +337,7 @@ const m_option_t mp_opts[] = {
     OPT_FLOATRANGE("osd-bar-h", osd_bar_h, 0, 0.1, 50),
     OPT_SUBSTRUCT("osd", osd_style, osd_style_conf, 0),
     OPT_SUBSTRUCT("sub-text", sub_text_style, osd_style_conf, 0),
+    OPT_FLAG("sub-clear-on-seek", sub_clear_on_seek, 0),
 
 //---------------------- libao/libvo options ------------------------
     OPT_SETTINGSLIST("vo", vo.video_driver_list, 0, &vo_obj_list),

options/options.h

@@ -230,6 +230,7 @@ typedef struct MPOpts {
     int ass_hinting;
     int ass_shaper;
     int sub_scale_with_window;
+    int sub_clear_on_seek;
 
     int hwdec_api;
     char *hwdec_codecs;

player/sub.c

@@ -172,6 +172,8 @@ void update_subtitles(struct MPContext *mpctx)
 static void reinit_subdec(struct MPContext *mpctx, struct track *track,
                           struct dec_sub *dec_sub)
 {
+    struct MPOpts *opts = mpctx->opts;
+
     if (sub_is_initialized(dec_sub))
         return;
 
@@ -189,7 +191,7 @@ static void reinit_subdec(struct MPContext *mpctx, struct track *track,
     // Don't do this if the file has video/audio streams. Don't do it even
     // if it has only sub streams, because reading packets will change the
     // demuxer position.
-    if (!track->preloaded && track->is_external) {
+    if (!track->preloaded && track->is_external && !opts->sub_clear_on_seek) {
         demux_seek(track->demuxer, 0, SEEK_ABSOLUTE);
         track->preloaded = sub_read_all_packets(dec_sub, track->stream);
     }

sub/sd_ass.c

@@ -270,7 +270,7 @@ static void fix_events(struct sd *sd)
 static void reset(struct sd *sd)
 {
     struct sd_ass_priv *ctx = sd->priv;
-    if (ctx->flush_on_seek)
+    if (ctx->flush_on_seek || sd->opts->sub_clear_on_seek)
         ass_flush_events(ctx->ass_track);
     ctx->flush_on_seek = false;
 }