ui: Modify ‘spawn’ only when it is defined.

This fixes a regression introduced in a6094158aa4fb8ec217e41c000264ba6e2095a3b
whereby upgrading from an old Guix revision, running on Guile version earlier
than 3.0.9, would break while building ‘module-import-compiled.drv’.

* guix/ui.scm <top level>: Set ‘spawn’ only when it’s defined.

Reported-by: Yelninei <yelninei@tutamail.com>
Reported-by: Danny Milosavljevic <dannym@friendly-machines.com>
Fixes: guix/guix#9852
Fixes: guix/guix#10809
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Merges: #10830

1 files changed, 5 insertions, 2 deletions

diff --git a/guix/ui.scm b/guix/ui.scm
index dafc04b20f7..e2c9a568eb9 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -750,8 +750,11 @@ FILE."
 (set! mkdir
   (error-reporting-wrapper mkdir (directory . args) directory))
 
-(set! spawn
-  (error-reporting-wrapper spawn (filename . args) filename))
+;; 'spawn' appeared in Guile 3.0.9.  Address the situation where updating from
+;; an old Guix running on an older version of Guile lacking 'spawn'.
+(when (module-defined? the-scm-module 'spawn)
+  (set! spawn
+    (error-reporting-wrapper spawn (filename . args) filename)))
 
 (define (make-regexp* regexp . flags)
   "Like 'make-regexp' but error out if REGEXP is invalid, reporting the error