19/21: home-services: run-on-change: Handle first generation case.

[guix-commits]

wigust pushed a commit to branch wip-guix-home
in repository guix.

commit 5d4f39e64ab446f11d22721464c2dc418c0d9b98
Author: Andrew Tropin <andrew@trop.in>
AuthorDate: Tue Aug 31 15:24:25 2021 +0300

    home-services: run-on-change: Handle first generation case.
    
    * gnu/home-services.scm
    (compute-on-change-gexp): Handle first generation case.
    
    Signed-off-by: Oleg Pykhalov <go.wigust@gmail.com>
---
 gnu/home-services.scm | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/gnu/home-services.scm b/gnu/home-services.scm
index 2a77349..9f1e986 100644
--- a/gnu/home-services.scm
+++ b/gnu/home-services.scm
@@ -439,7 +439,10 @@ with one gexp, but many times, and all gexps must be 
idempotent.")))
       (define expressions-to-eval
         (map
          (lambda (x)
-           (let* ((file1 (string-append (getenv "GUIX_OLD_HOME") "/" (car x)))
+           (let* ((file1 (string-append
+                          (or (getenv "GUIX_OLD_HOME")
+                              "/gnu/store/non-existing-generation")
+                          "/" (car x)))
                   (file2 (string-append (getenv "GUIX_NEW_HOME") "/" (car x)))
                   (_ (format #t "Comparing ~a and\n~10t~a..." file1 file2))
                   (any-changes? (something-changed? file1 file2))
@@ -454,7 +457,8 @@ with one gexp, but many times, and all gexps must be 
idempotent.")))
             (for-each primitive-eval expressions-to-eval)
             (display "On-change gexps evaluation finished.\n\n"))
           (display "\
-On-change gexps won't evaluated, disabled by service configuration.\n"))))
+On-change gexps won't be evaluated, disabled by service
+configuration.\n"))))
 
 (define home-run-on-change-service-type
   (service-type (name 'home-run-on-change)