core: improve seeking in external files

This affects streams loaded with -subfile and -audiofile. They could get
out of sync when they were deselected, and the main file was seeked. Add
code to seek external files when they are selected (see
init_demux_stream()).

Use avformat_seek_file() under certain circumstances. Both av_seek_frame()
("old" API) and avformat_seek_file() ("new" API) seem to be broken with
some formats. At least the vobsub demuxer doesn't implement the old API
(and the old API doesn't fallback to the new API), while the fallback
from new API to old API gives bad results. For example, seeking forward
with small step sizes seems to fail with the new API (tested with
Matroska by trying to seek 1 second forward relative to priv->last_pts).
Since only subtitle demuxers implement the new API anyway, checking
whether iformat->read_seek2 is set to test whether the old API is not
supported gives best results. This is a hack at best, but makes things
work.

Remove backwards seeking on seek failure. This was annoying, and only
was there to compensate for obscure corner cases (see 1ad332). In
particular, files with completely broken seeking that used to skip back
to the start on every seek request may now terminate playback.

3 files changed, 41 insertions, 13 deletions

diff --git a/core/mplayer.c b/core/mplayer.c
index 4eb4ff0e23..81302f284f 100644
--- a/core/mplayer.c
+++ b/core/mplayer.c
@@ -439,6 +439,23 @@ static void print_file_properties(struct MPContext *mpctx, const char *filename)
 /// step size of mixer changes
 int volstep = 3;
 
+// Time used to seek external tracks to.
+static double get_main_demux_pts(struct MPContext *mpctx)
+{
+    double main_new_pos = MP_NOPTS_VALUE;
+    if (mpctx->demuxer) {
+        for (int type = 0; type < STREAM_TYPE_COUNT; type++) {
+            struct demux_stream *ds = mpctx->demuxer->ds[type];
+            if (ds->sh && main_new_pos == MP_NOPTS_VALUE) {
+                demux_fill_buffer(mpctx->demuxer, ds);
+                if (ds->first)
+                    main_new_pos = ds->first->pts;
+            }
+        }
+    }
+    return main_new_pos;
+}
+
 static void set_demux_field(struct MPContext *mpctx, enum stream_type type,
                             struct sh_stream *s)
 {
@@ -456,8 +473,13 @@ static void init_demux_stream(struct MPContext *mpctx, enum stream_type type)
     struct track *track = mpctx->current_track[type];
     set_demux_field(mpctx, type, track ? track->stream : NULL);
     struct sh_stream *stream = mpctx->sh[type];
-    if (stream)
+    if (stream) {
         demuxer_switch_track(stream->demuxer, type, stream);
+        if (track->is_external) {
+            double pts = get_main_demux_pts(mpctx);
+            demux_seek(stream->demuxer, pts, audio_delay, SEEK_ABSOLUTE);
+        }
+    }
 }
 
 static void cleanup_demux_stream(struct MPContext *mpctx, enum stream_type type)
@@ -2863,18 +2885,12 @@ static int seek(MPContext *mpctx, struct seek_params seek,
         have_external_tracks |= track && track->is_external && track->demuxer;
     }
     if (have_external_tracks) {
-        double main_new_pos = MP_NOPTS_VALUE;
-        if (seek.type == MPSEEK_ABSOLUTE)
+        double main_new_pos;
+        if (seek.type == MPSEEK_ABSOLUTE) {
             main_new_pos = seek.amount - mpctx->video_offset;
-        for (int type = 0; type < STREAM_TYPE_COUNT; type++) {
-            struct demux_stream *ds = mpctx->demuxer->ds[type];
-            if (ds->sh && main_new_pos == MP_NOPTS_VALUE) {
-                demux_fill_buffer(mpctx->demuxer, ds);
-                if (ds->first)
-                    main_new_pos = ds->first->pts;
-            }
+        } else {
+            main_new_pos = get_main_demux_pts(mpctx);
         }
-        assert(main_new_pos != MP_NOPTS_VALUE);
         for (int type = 0; type < STREAM_TYPE_COUNT; type++) {
             struct track *track = mpctx->current_track[type];
             if (track && track->is_external && track->demuxer)
diff --git a/demux/demux.c b/demux/demux.c
index 82159057d8..f765b2ae87 100644
--- a/demux/demux.c
+++ b/demux/demux.c
@@ -1077,6 +1077,10 @@ int demux_seek(demuxer_t *demuxer, float rel_seek_secs, float audio_delay,
             mp_tmsg(MSGT_SEEK, MSGL_WARN, "Cannot seek in this file.\n");
         return 0;
     }
+
+    if (rel_seek_secs == MP_NOPTS_VALUE && (flags & SEEK_ABSOLUTE))
+        return 0;
+
     // clear demux buffers:
     demux_flush(demuxer);
     demuxer->video->eof = 0;
diff --git a/demux/demux_lavf.c b/demux/demux_lavf.c
index c1c15f345a..42b687dc20 100644
--- a/demux/demux_lavf.c
+++ b/demux/demux_lavf.c
@@ -886,9 +886,17 @@ static void demux_seek_lavf(demuxer_t *demuxer, float rel_seek_secs,
         priv->last_pts += rel_seek_secs * priv->avfc->duration;
     } else
         priv->last_pts += rel_seek_secs * AV_TIME_BASE;
-    if (av_seek_frame(priv->avfc, -1, priv->last_pts, avsflags) < 0) {
-        avsflags ^= AVSEEK_FLAG_BACKWARD;
+
+    if (!priv->avfc->iformat->read_seek2) {
+        // Normal seeking.
         av_seek_frame(priv->avfc, -1, priv->last_pts, avsflags);
+    } else {
+        // av_seek_frame() won't work. Use "new" seeking API. We don't use this
+        // API by default, because there are some major issues.
+        // Set max_ts==ts, so that demuxing starts from an earlier position in
+        // the worst case.
+        avformat_seek_file(priv->avfc, -1, INT64_MIN,
+                           priv->last_pts, priv->last_pts, avsflags);
     }
 }