demux: support multiple seekable cached ranges

Until now, the demuxer cache was limited to a single range. Extend this
to multiple range. Should be useful for slow network streams.

This commit changes a lot in the internal demuxer cache logic, so
there's a lot of room for bugs and regressions. The logic without
demuxer cache is mostly untouched, but also involved with the code
changes. Or in other words, this commit probably fucks up shit.

There are two things which makes multiple cached ranges rather hard:

1. the need to resume the demuxer at the end of a cached range when
   seeking to it
2. joining two adjacent ranges when the lowe range "grows" into it (and
   resuming the demuxer at the end of the new joined range)

"Resuming" the demuxer means that we perform a low level seek to the end
of a cached range, and properly append new packets to it, without adding
packets multiple times or creating holes due to missing packets.

Since audio and video never line up exactly, there is no clean "cut"
possible, at which you could resume the demuxer cleanly (for 1.) or
which you could use to detect that two ranges are perfectly adjacent
(for 2.). The way how the demuxer interleaves multiple streams is also
unpredictable. Typically you will have to expect that it randomly allows
one of the streams to be ahead by a bit, and so on.

To deal with this, we have heuristics in place to detect when one packet
equals or is "behind" a packet that was demuxed earlier. We reuse the
refresh seek logic (used to "reread" packets into the demuxer cache when
enabling a track), which checks for certain packet invariants.
Currently, it observes whether either the raw packet position, or the
packet DTS is strictly monotonically increasing. If none of them are
true, we discard old ranges when creating a new one.

This heavily depends on the file format and the demuxer behavior. For
example, not all file formats have DTS, and the packet position can be
unset due to libavformat not always setting it (e.g. when parsers are
used).

At the same time, we must deal with all the complicated state used to
track prefetching and seek ranges. In some complicated corner cases, we
just give up and discard other seek ranges, even if the previously
mentioned packet invariants are fulfilled.

To handle joining, we're being particularly dumb, and require a small
overlap to be confident that two ranges join perfectly. (This could be
done incrementally with as little overlap as 1 packet, but corner cases
would eat us: each stream needs to be joined separately, and the cache
pruning logic could remove overlapping packets for other streams again.)

Another restriction is that switching the cached range will always
trigger an asynchronous low level seek to resume demuxing at the new
range. Some users might find this annoying.

Dealing with interleaved subtitles is not fully handled yet. It will
clamp the seekable range to where subtitle packets are.

1 files changed, 1 insertions, 1 deletions

diff --git a/demux/demux.h b/demux/demux.h
index 81670fac6f..95e0f77c7c 100644
--- a/demux/demux.h
+++ b/demux/demux.h
@@ -40,7 +40,7 @@ enum demux_ctrl {
     DEMUXER_CTRL_REPLACE_STREAM,
 };
 
-#define MAX_SEEK_RANGES 1
+#define MAX_SEEK_RANGES 10
 
 struct demux_seek_range {
     double start, end;