[arch-projects] [mkinitcpio] [PATCH] --rsyncable option

[Charles Duffy]

At present, mkinitcpio outputs are not particularly stable: Generating two
consecutive initramfs images from the same inputs can generate archives
which are quite entirely unlike each other from the perspective of binary
diff tools (rsync, casync, rdiff-backup, xdelta, etc). Making some
adjustments to compression and archival can greatly reduce the size of this
delta.

A ticket with a proposed patch (adding a --rsyncable option to mkinitcpio)
has been opened at https://bugs.archlinux.org/task/56128 -- however, as the
advice there has been to consult this mailing list, it may no longer be
open as of this writing. :)

The change consists of the following:
- Sort the list of files (in the C locale) before passing it to cpio.
- When --rsyncable is passed, set all datestamps to the UNIX epoch before
generating the cpio archive.
- When --rsyncable is passed, pass the --reproducible option to cpio (this
creates a dependency on GNU cpio 2.12, prevents device numbers from being
included in the archive, and renumbers inode numbers to be consistent when
file ordering is unchanged).
- When --rsyncable is passed, pass the --rsyncable and --no-name options to
gzip (the former tells it to reset the compression table on occasion,
sacrificing compression ratio to prevent the effect from changes from
cascading throughout too much of the stream; the latter prevents name and
timestamp data from being stored in the gzip header itself).

A version of the patch squashing together the two commits already present
on the ticket is attached.
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