demux: add a on-disk cache

Somewhat similar to the old --cache-file, except for the demuxer cache.
Instead of keeping packet data in memory, it's written to disk and read
back when needed.

The idea is to reduce main memory usage, while allowing fast seeking in
large cached network streams (especially live streams). Keeping the
packet metadata on disk would be rather hard (would use mmap or so, or
rewrite the entire demux.c packet queue handling), and since it's
relatively small, just keep it in memory.

Also for simplicity, the disk cache is append-only. If you're watching
really long livestreams, and need pruning, you're probably out of luck.
This still could be improved by trying to free unused blocks with
fallocate(), but since we're writing multiple streams in an interleaved
manner, this is slightly hard.

Some rather gross ugliness in packet.h: we want to store the file
position of the cached data somewhere, but on 32 bit architectures, we
don't have any usable 64 bit members for this, just the buf/len fields,
which add up to 64 bit - so the shitty union aliases this memory.

Error paths untested. Side data (the complicated part of trying to
serialize ffmpeg packets) untested.

Stream recording had to be adjusted. Some minor details change due to
this, but probably nothing important.

The change in attempt_range_joining() is because packets in cache
have no valid len field. It was a useful check (heuristically
finding broken cases), but not a necessary one.

Various other approaches were tried. It would be interesting to list
them and to mention the pros and cons, but I don't feel like it.

1 files changed, 5 insertions, 0 deletions

diff --git a/DOCS/man/input.rst b/DOCS/man/input.rst
index 814d7a6825..9049bf8c9d 100644
--- a/DOCS/man/input.rst
+++ b/DOCS/man/input.rst
@@ -1462,6 +1462,10 @@ Property list
     (may not account correctly for various overhead), and stops at the
     demuxer position (it ignores seek ranges after it).
 
+    ``file-cache-bytes`` is the number of bytes stored in the file cache. This
+    includes all overhead, and possibly unused data (like pruned data). This
+    member is missing if the file cache is not active.
+
     When querying the property with the client API using ``MPV_FORMAT_NODE``,
     or with Lua ``mp.get_property_native``, this will return a mpv_node with
     the following contents:
@@ -1476,6 +1480,7 @@ Property list
             "bof-cached"        MPV_FORMAT_FLAG
             "eof-cached"        MPV_FORMAT_FLAG
             "fw-bytes"          MPV_FORMAT_INT64
+            "file-cache-bytes"  MPV_FORMAT_INT64
 
     Other fields (might be changed or removed in the future):