Hi all,

I'm sharing infos in the hope to save some time to people interested
in supporting unmerged layout on Devuan.

First thing: at the end of this usrmerge transition, the usrmerge package
is expected to go away and the essential package "base-files" is expected
to ship /bin /sbin (and so on) as symlinks.
So, I think, the first step to support a unmerged layout is to fork
"base-files"
into something like "base-files-unmerged" that Breaks and Replaces the
official package.
While the forked package installs /bin /sbin and so as directories the
system
becomes broken, so the package need to do some symlink farming.
I think this can be done by having a postinstall script that uses perl as
interpreter.
This symlink list should guarantee that essential functionality of the
system
works, including boot and shutdown.

at this point there are at least two options:

A) continue with symlink farming approach:
    create a symlink for each program that is expected to be available in
/bin.
    for example /bin/foo would be a symlink to /usr/bin/foo
I'm not sure people that feel strong against usrmerge will see a value in
this
setup ( /bin and /sbin filled with symlinks) but there are advantages:
* only one package is forked
* deb packages no longer need to obey to restrictions listed as mitigations
in
    https://subdivi.de/~helmut/dep17.html
   (while Debian will need to obey the list forever)
* you can have extra packages that install stuff in /bin /sbin
Still separate /usr layouts need to boot via initramfs and a list of
non-essential
programs that needs symlinks has to be maintained.

B) on top of "base-files-unmerged", fork other packages:
I don't have the exact number, but I recall Helmut writing about 500
packages
involved (because of systemd units) that are about half (or maybe 2/3) of
the
problem, so I think the number rages from 750 to 1000 sources.
However, 500 are involved because of systemd unit (and we don't care about
these) so the number can be as low as ~250.
Anyway, if one focus on the essential set the number shrinks to something
like ~40 packages, and another ~40 with priority important.
Not necessary all of those 80 packages need to be rebuilded.

regardless that A or B is chosen, there is another problem: with the merged
layout it doesn't matter if one calls /bin/foo or /usr/bin/foo, but with
the unmerged one
it does; so B) may need to create symlink for essential+important binaries
under /usr.
And both A and B will need to create and maintain a certain number of
symlinks for
non essential/required programs that are invoked with the "wrong" path;
the number of such program is unknown and may increase in future.

Please keep in mind that the above is just a draft, I have not done any
test to ensure
that it really works. I don't have the time and energy to continue this
work right now, but
I may have in the future so if anyone starts working on this, please keep
me in the loop!

Cheers,
Lorenzo