This commit introduces logic to read other volumes from the same source
as the primary archive. Both .rar formats as well as 7z are supported for now.
It also changes the libarchive callback structure to be per-volume
consistent with the libarchive intenal client data array constructed
with archive_read_append_callback_data.
Added open, close and switch callbacks. Only the latter is strictly
required to make sure that the streams always start at position 0, but
leaving all volumes open can eat a lot of memory for archives with many
parts.
As expected, probing with libarchive is a disaster. Both libavformat and
libarchive are too eager to misdetect file formats just because files
"might" be of a specific type. In this case, it's mp3 vs. tar. To be
fair, neither file format has an actual header. I'm not sure why we'd
need tar support, but since libarchive provides it, and idiots on the
internet apparently pack media files in tar sometimes (really, idiots),
keep it for now, and probe tar last.
libarchive uses a quite confusing ifdeffery mess for some of the types
used in callbacks. Currently, archive_read_set_seek_callback() causes a
warning at least on Windows due to mismatching return type. The header
file uses __LA_INT64_T as return type, so I think the user is intended
to use int64_t.
(The ssize_t return type for the read_cb seems correct, on the other
hand.)
Things like .gz etc., which have no real file header. A mixed bag,
because it e.g. tends to misdetect mp3 files as compressed files or
something (of course it has no mp3 support - I don't know as what it
detects them). But requested by someone (or maybe not, I'm not sure
how to interpret that).
This works similar to the existing .rar support, but uses libarchive.
libarchive supports a number of formats, including zip and (most of)
rar.
Unfortunately, seeking does not work too well. Most libarchive readers
do not support seeking, so it's emulated by skipping data until the
target position. On backwards seek, the file is reopened. This works
fine on a local machine (and if the file is not too large), but will
perform not so well over network connection.
This is disabled by default for now. One reason is that we try
libarchive on every file we open, before trying libavformat, and I'm not
sure if I trust libarchive that much yet. Another reason is that this
breaks multivolume rar support. While libarchive supports seeking in
rar, and (probably) supports multivolume archive, our support of
libarchive (probably) does not. I don't care about multivolume rar, but
vocal users do.
Kevin Mitchell
wm4