services: mcron: Validate jobs at build time.

That way, run-time errors in the job specs are caught at build time.

* gnu/services/mcron.scm (job-file): Remove.
(job-files): New procedure.
(mcron-shepherd-services): Adjust accordingly.

1 files changed, 21 insertions, 3 deletions

diff --git a/gnu/services/mcron.scm b/gnu/services/mcron.scm
index d9627c6bd0..045d6e2fe8 100644
--- a/gnu/services/mcron.scm
+++ b/gnu/services/mcron.scm
@@ -57,8 +57,26 @@
   (jobs              mcron-configuration-jobs     ;list of <mcron-job>
                      (default '())))
 
-(define (job-file job)
-  (scheme-file "mcron-job" job))
+(define (job-files mcron jobs)
+  "Return a list of file-like object for JOBS, a list of gexps."
+  (define (validated-file job)
+    ;; This procedure behaves like 'scheme-file' but it runs 'mcron
+    ;; --schedule' to detect any error in JOB.
+    (computed-file "mcron-job"
+                   (with-imported-modules '((guix build utils))
+                     #~(begin
+                         (use-modules (guix build utils))
+
+                         (call-with-output-file "job"
+                           (lambda (port)
+                             (write '#$job port)))
+
+                         (invoke #+(file-append mcron "/bin/mcron")
+                                 "--schedule=20" "job")
+                         (copy-file "job" #$output)))
+                   #:options '(#:env-vars (("COLUMNS" . "150")))))
+
+  (map validated-file jobs))
 
 (define (shepherd-schedule-action mcron files)
   "Return a Shepherd action that runs MCRON with '--schedule' for the given
@@ -101,7 +119,7 @@ files."
     (($ <mcron-configuration> mcron ())           ;nothing to do!
      '())
     (($ <mcron-configuration> mcron jobs)
-     (let ((files (map job-file jobs)))
+     (let ((files (job-files mcron jobs)))
        (list (shepherd-service
               (provision '(mcron))
               (requirement '(user-processes))