demux: redo timed metadata

The old implementation didn't work for the OGG case. Discard the old
shit code (instead of fixing it), and write new shit code. The old code
was already over a year old, so it's about time to rewrite it for no
reason anyway.

While it's true that the old code appears to be broken, the main reason
to rewrite this is to make it simpler. While the amount of code seems to
be about the same, both the concept and the actual tag handling are
simpler. The result is probably a bit more correct.

The packet struct shrinks by 8 byte. That fact that it wasted 8 bytes
per packet for a rather obscure use case was the reason I started this
at all (and when I found that OGG updates didn't work). While these 8
bytes aren't going to hurt, the packet struct was getting too bloated.
If you buffer a lot of data, these extra fields will add up. Still quite
some effort for 8 bytes. Fortunately, it's not like there are any
managers that need to be convinced whether it's worth doing. The freedom
to waste time on dumb shit.

The old implementation attached the current metadata to each packet.
When the decoder read the packet, the packet's metadata was made
current. The new implementation stores metadata as separate list, and
requires that the player frontend tells it the current playback time,
which will be used to find the currently valid metadata. In both cases,
the objective was to correctly update metadata even if a lot of data is
buffered ahead (and to update them correctly when seeking within the
demuxer cache).

The new implementation is actually slightly more correct, because it
uses the playback time for the metadata lookup. Consider if you have an
audio filter which buffers 15 seconds (unfortunately such a filter
exists), then the old code would update the current title 15 seconds too
early, while the new one does it correctly.

The new code also simplifies mixing the 3 metadata sources (global, per
stream, ICY). We assume these aren't mixed in a meaningful way. The old
code tried to be a bit more "exact". I didn't bother to look how the old
code did this, but the new code simply always "merges" with the previous
metadata, so if a newer tag removes a field, it's going to stick around
anyway.

I tried to keep it simple. Other approaches include making metadata a
special sh_stream with metadata packets. This would have been
conceptually clean, but the implementation would probably have been
unnatural (and doesn't match well with libavformat's API anyway). It
would have been nice to make the metadata updates chapter points (makes
a lot of sense for the intended use case, web radio current song
information), but I don't think it would have been a good idea to make
chapters suddenly so dynamic. (Still an idea to keep in mind; the new
code actually makes it easier to work towards this.)

You could mention how subtitles are timed metadata, and actually are
implemented as sparse packet streams in some formats. mp4 implements
chapters as special subtitle stream, AFAIK. (Ironically, this is very
not-ideal for files. It would be useful for streaming like web radio,
but mp4 is extremely bad for streaming by design for other reasons.)

bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
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bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla

1 files changed, 3 insertions, 6 deletions

diff --git a/demux/demux.h b/demux/demux.h
index 4cbb33b588..99acc1f015 100644
--- a/demux/demux.h
+++ b/demux/demux.h
@@ -284,12 +284,12 @@ int demuxer_add_attachment(struct demuxer *demuxer, char *name,
                            char *type, void *data, size_t data_size);
 int demuxer_add_chapter(demuxer_t *demuxer, char *name,
                         double pts, uint64_t demuxer_id);
-void demux_set_stream_tags(struct demuxer *demuxer, struct sh_stream *sh,
-                           struct mp_tags *tags);
+void demux_stream_tags_changed(struct demuxer *demuxer, struct sh_stream *sh,
+                               struct mp_tags *tags, double pts);
 void demux_close_stream(struct demuxer *demuxer);
 
 void demux_metadata_changed(demuxer_t *demuxer);
-void demux_update(demuxer_t *demuxer);
+void demux_update(demuxer_t *demuxer, double playback_pts);
 
 bool demux_is_network_cached(demuxer_t *demuxer);
 
@@ -306,7 +306,4 @@ bool demux_matroska_uid_cmp(struct matroska_segment_uid *a,
 
 const char *stream_type_name(enum stream_type type);
 
-void mp_packet_tags_unref(struct mp_packet_tags *tags);
-void mp_packet_tags_setref(struct mp_packet_tags **dst, struct mp_packet_tags *src);
-
 #endif /* MPLAYER_DEMUXER_H */