demux, command: add a third stream recording mechanism

That's right, and it's probably not the end of it. I'll just claim that
I have no idea how to create a proper user interface for this, so I'm
creating multiple partially-orthogonal, of which some may work better in
each of its special use cases.

Until now, there was --record-file. You get relatively good control
about what is muxed, and it can use the cache. But it sucks that it's
bound to playback. If you pause while it's set, muxing stops. If you
seek while it's set, the output will be sort-of trashed, and that's by
design.

Then --stream-record was added. This is a bit better (especially for
live streams), but you can't really control well when muxing stops or
ends. In particular, it can't use the cache (it just dumps whatever the
underlying demuxer returns).

Today, the idea is that the user should just be able to select a time
range to dump to a file, and it should not affected by the user seeking
around in the cache. In addition, the stream may still be running, so
there's some need to continue dumping, even if it's redundant to
--stream-record.

One notable thing is that it uses the async command shit. Not sure
whether this is a good idea. Maybe not, but whatever. Also, a user can
always use the "async" prefix to pretend it doesn't.

Much of this was barely tested (especially the reinterleaving crap),
let's just hope it mostly works. I'm sure you can tolerate the one or
other crash?

1 files changed, 2 insertions, 4 deletions

diff --git a/DOCS/man/options.rst b/DOCS/man/options.rst
index ea47fc79b0..b590526f3e 100644
--- a/DOCS/man/options.rst
+++ b/DOCS/man/options.rst
@@ -5880,10 +5880,8 @@ Miscellaneous
 
     If this is set at runtime, the old file is closed, and the new file is
     opened. Note that this will write only data that is appended at the end of
-    the cache, and the already cached data cannot be written. (A fix for that
-    would be a command that dumps the cache using a given time range, possibly
-    with the option to be open-ended, which would continue to write data
-    appended to the cache. Such a command doesn't exist yet.)
+    the cache, and the already cached data cannot be written. You can try the
+    ``dump-cache`` command as an alternative.
 
 ``--lavfi-complex=<string>``
     Set a "complex" libavfilter filter, which means a single filter graph can