Catch the guix available when built

And use it for the hosting the inferiors, rather than computing the guix
package at runtime. This simplifies the behaviour when the Guix Data Service
is deployed as a Guix package.

configure.ac

@@ -35,6 +35,8 @@ guilemoduledir="${datarootdir}/guile/site/${GUILE_EFFECTIVE_VERSION}"
 AC_SUBST([guilemoduledir])
 AC_SUBST([GUILE_EFFECTIVE_VERSION])
 
+AC_PATH_PROG([guix], [guix])
+
 AC_PATH_PROG([sqitch], [sqitch])
 AC_PATH_PROG([psql], [psql])
 

guix-data-service/config.scm.in

@@ -33,6 +33,8 @@
            (host        . "localhost")
            (port        . 8765)
 
+           (guix        . "@guix@")
+
            (sqitch      . "@sqitch@")
            (sqitch-psql . "@psql@")
            (sqitch-plan . ,(let ((installed-plan

guix-data-service/jobs/load-new-guix-revision.scm

@@ -12,6 +12,7 @@
   #:use-module (guix packages)
   #:use-module (guix derivations)
   #:use-module (guix build utils)
+  #:use-module (guix-data-service config)
   #:use-module (guix-data-service model package)
   #:use-module (guix-data-service model git-repository)
   #:use-module (guix-data-service model guix-revision)
@@ -245,18 +246,27 @@
       (if (and store-path
                (file-exists? store-path))
           store-path
-          (begin
-            (invalidate-derivation-caches!)
-            (hash-clear! (@@ (guix packages) %derivation-cache))
-            (let* ((guix-package (@ (gnu packages package-management)
-                                    guix))
-                   (derivation (package-derivation store guix-package)))
-              (build-derivations store (list derivation))
+          (let ((config-guix (%config 'guix)))
+            (if (and (file-exists? config-guix)
+                     (string-prefix? "/gnu/store/" config-guix))
+                (begin
+                  (set! store-path
+                    (dirname
+                     (dirname
+                      (%config 'guix))))
+                  store-path)
+                (begin
+                  (invalidate-derivation-caches!)
+                  (hash-clear! (@@ (guix packages) %derivation-cache))
+                  (let* ((guix-package (@ (gnu packages package-management)
+                                          guix))
+                         (derivation (package-derivation store guix-package)))
+                    (build-derivations store (list derivation))
 
-              (let ((new-store-path
-                     (derivation->output-path derivation)))
-                (set! store-path new-store-path)
-                new-store-path)))))))
+                    (let ((new-store-path
+                           (derivation->output-path derivation)))
+                      (set! store-path new-store-path)
+                      new-store-path)))))))))
 
 (define (nss-certs-store-path store)
   (let* ((nss-certs-package (@ (gnu packages certs)