Attention Benny Pedersen,

 BP> lvm2 snapshot rollbacks ?

No.  Just plain old ext4 partitions created by fdisk (util-linux) and formatted
with mke2fs (e2fsprogs) just like mom used to do ... errrrr ... if I had a mom
that is.

 BP> but i dont know how cold it could be on the maunting top

At the moment there are still issues with m4, findutils, and gzip.  Probably
more that gentoo requires but these three are a major stumbling block and
creativity was required to overcome both the m4 and findutils issues to create
packages, even in a chrootable enviroment.  gzip is less of an issue but it is
still a deal breaker if not resolved which it was by creating the
gzip-1.9.4-9ef6-dirty source as noted in a previous post.  Same with
findutils-4.6.0.186-84e8.  With m4 the latest official m4-1.4.18 source is
being used although it required bootstrapping to the latest gnulib before it
would work with glibc-2.28.  In that case the bootstrap script in the source
package was used to create newly bootstrapped m4-1.4.18 source instead of using
git sources as in the case of gzip and findutils.

 BP>    ----- glibc-9999.ebuild begins -----

Whoa!  First off the current bootable rootfs isn't a git version but a fully
qualified GNU release.  See https://www.gnu.org/s/libc/ for the details. 
Before this release the git version(s) tagged as 2.27.9000 were tested in a
chroot partition and only once booted when gcc-8.2.0 was released about a week
prior to the official glibc-2.28 release on August first.

The bash script used here to build glibc-2.28 is exactly 1980 bytes containing
62 lines of code.

 BP>    ----- glibc-9999.ebuild ends -----
 BP>
 BP> thats all :)

Way more than enough to ascertain it's failure on either of the two machines
available here.  All build scripts here are bash based and lack the dependency
tracking gentoo prides itself on.  Your posted script requires a gentoo host as
well as the proper runtime to engage that script.  No testing required to know
that it will fail.

End transmission.

... Fudd's First Law of Opposition:
    If you push something hard enough, it will fall over.
--- GNU bash, version 4.4.23(1)-release (x86_64-silvermont-linux-gnu)