Suggested by Florian Pelz <pelzflorian@pelzflorian.de>
in <http://bugs.gnu.org/35996>.
* gnu/build/accounts.scm (%password-lock-file): New variable.
* gnu/build/activation.scm (activate-users+groups): Wrap calls to
'user+group-databases', 'write-group', etc. into 'with-file-lock'.
Starting from commit 0ae735bcc8, Guix
System would preserve the user shell across reconfigure and reboot.
This was done so as to allow for the use of 'chsh'.
This proved to be a misguided decision. This commit goes back to
considering user shells as config and not "state."
* gnu/build/accounts.scm (allocate-passwd): Do not use shell from
PREVIOUS.
Ludovic Courtès