bug#59784: [version 1.4.0rc1] Retrying a failed install fails

[Ludovic Courtès]

Grrr, I’m really silly: we have the same problem (copying the database
before the daemon has been stopped) just a few lines above.

How about this:

diff --git a/gnu/installer/final.scm b/gnu/installer/final.scm
index 044f79372b..360b34d8cb 100644
--- a/gnu/installer/final.scm
+++ b/gnu/installer/final.scm
@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2018, 2020 Mathieu Othacehe <m.othacehe@gmail.com>
-;;; Copyright © 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2019, 2020, 2022 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -196,14 +196,15 @@ (define (assert-exit x)
              ;; the loaded cow-store locale files will prevent umounting.
              (install-locale locale)
 
-             ;; Save the database, so that it can be restored once the
-             ;; cow-store is umounted.
+             ;; Stop the daemon and save the database, so that it can be
+             ;; restored once the cow-store is umounted.
+             (stop-service 'guix-daemon)
              (copy-file database-file saved-database)
+
              (mount-cow-store (%installer-target-dir) backing-directory))
            (lambda ()
              ;; We need to drag the guix-daemon to the container MNT
              ;; namespace, so that it can operate on the cow-store.
-             (stop-service 'guix-daemon)
              (start-service 'guix-daemon (list (number->string (getpid))))
 
              (setvbuf (current-output-port) 'none)
@@ -213,10 +214,13 @@ (define (assert-exit x)
 
              (set! ret (run-command install-command #:tty? #t)))
            (lambda ()
-             ;; Restart guix-daemon so that it does no keep the MNT namespace
+             ;; Stop guix-daemon so that it does no keep the MNT namespace
              ;; alive.
-             (restart-service 'guix-daemon)
+             (stop-service 'guix-daemon)
+
+             ;; Restore the database and restart it.
              (copy-file saved-database database-file)
+             (start-service 'guix-daemon)
 
              ;; Finally umount the cow-store and exit the container.
              (unmount-cow-store (%installer-target-dir) backing-directory)

?

This time, I believe we only ever copy the database when we’re sure no
guix-daemon process is accessing it.

Ludo’.