[bug#55792] [PATCH 3/3] hydra: web: Make mcron job derivations deterministic.

["Ludovic Courtès" <ludo@gnu.org>]

The result of (hash config period) varies between runs.  Thus, starting
from bdb914a1242cb0c00957050c96d1e279d2eb5dec, each run of "guix system
build -d ..." would produce a different derivation.

This commit fixes that.

* hydra/modules/sysadmin/web.scm (static-web-site-mcron-jobs)[record->list]:
New procedure.
Use it.
---
 hydra/modules/sysadmin/web.scm | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/hydra/modules/sysadmin/web.scm b/hydra/modules/sysadmin/web.scm
index e04ac1b..f26ed24 100644
--- a/hydra/modules/sysadmin/web.scm
+++ b/hydra/modules/sysadmin/web.scm
@@ -179,11 +179,19 @@ that's built with Haunt or similar."
                            (basename
                             (static-web-site-configuration-directory config)))))
 
+  (define (record->list record)
+    (let ((fields (record-type-fields <static-web-site-configuration>)))
+      (map (lambda (n)
+             (struct-ref record n))
+           (iota (length fields)))))
+
   (map (lambda (config)
          ;; Add an offset to spread web site updates over the period to avoid I/O
-         ;; load peaks when there are several such jobs.
+         ;; load peaks when there are several such jobs.  Compute a hash over
+         ;; a list representation of CONFIG, rather than over CONFIG, because
+         ;; hash of a struct depends on the object identity of its vtable.
          (let* ((period (static-web-site-configuration-period config))
-                (offset (hash config period)))
+                (offset (hash (record->list config) period)))
            #~(job (lambda (now)
                     (let ((elapsed (modulo now #$period)))
                       (+ now (- #$period elapsed) #$offset)))
-- 
2.36.1