Hi,

The attached patch maintains a historical mapping from user names to 
UIDs/GIDs (even when those users are removed from the user accounts), to 
solve <https://issues.guix.gnu.org/36508>, as proposed by Mark H Weaver 
in <https://issues.guix.gnu.org/36508#10>.

(The proposed garbage collector is not included, however.)

As I proposed in <https://issues.guix.gnu.org/36508#14>, the information 
of this mapping is kept in a special 'user'.  However, I didn't 
implement the same for groups (raises some questions about atomicity -- 
/etc/passwd and /etc/groups are separate files).

I didn't completely follow that proposal though, since:

 > https://issues.guix.gnu.org/36508#16:
 > Problem is that things like GDM would still propose those old accounts
 > (unless maybe their password is uninitialized, I’m not sure; but it’s
 > still hacky.)

, though that could perhaps be solved by adjusting GDM appropriately.

"make check TESTS=tests/accounts.scm" passes, but otherwise the patch is 
rather untested (hence, 'DRAFT') -- the new functionality will need a 
few additional tests in tests/account.scm, to avoid introducing bugs in 
the handling of /etc/passwd.

On second thought, a separate /etc/previous-uids (as proposed in
<https://issues.guix.gnu.org/36508#16> by Ludovic Courtès)
might be better (there are atomicity issues anyway, due to /etc/passwd 
and /etc/groups being separate files, and losing the historical mapping
is relatively harmless and seems unlikely to happen in practice).

Greetings,
Maxime