Refactor how channel derivations are handled

Yet again...

This makes the channel-derivations for each system accessible within the
load-new-guix-revision procedure, in preparation for storing them in the
database.

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

@@ -714,7 +714,7 @@ WHERE job_id = $1"
        (build-derivations store (list derivation))))
     (derivation->output-path derivation)))
 
-(define (channel->derivation-file-names-by-system store channel)
+(define (channel->derivation-file-names-by-system channel)
   (define use-container? (defined?
                            'open-inferior/container
                            (resolve-module '(guix inferior))))
@@ -732,12 +732,11 @@ WHERE job_id = $1"
                  ,(channel-instance-checkout channel-instance)))))
          (map
           (lambda (system)
-            (log-time
             (simple-format
-              #f "computing the derivation-file-name for ~A"
+             (current-error-port)
+             "guix-data-service: computing the derivation-file-name for ~A\n"
              system)
-             (lambda ()
+
-               (with-store store
             ((set-current-system system) store)
             (let ((manifest
                    (catch #t
@@ -777,9 +776,12 @@ WHERE job_id = $1"
                          (simple-format
                           (current-error-port)
                           "error ~A: ~A\n" key args)
-                              #f))))))))))
+                         #f)))))))
           (list ,@systems)))))
 
+  (with-store store
+    (set-build-options store #:fallback? #t)
+
     (let ((inferior
            (if use-container?
                (open-inferior/container
@@ -851,9 +853,9 @@ WHERE job_id = $1"
                              key parameters))))
         (lambda args
           (close-inferior inferior)
-        #f))))
+          #f)))))
 
-(define (channel->manifest-store-item conn store channel)
+(define (channel->derivations-by-system conn channel)
   (let* ((derivation-file-names-by-system
           (log-time
            "computing the channel derivation"
@@ -864,12 +866,7 @@ WHERE job_id = $1"
               conn
               'channel->manifest-store-item
               (lambda ()
-                (channel->derivation-file-names-by-system store channel))))))
+                (channel->derivation-file-names-by-system channel)))))))
-         (derivation
-          (read-derivation-from-file (assoc-ref
-                                      (assoc-ref derivation-file-names-by-system
-                                                 (%current-system))
-                                      'profile))))
     (for-each
      (match-lambda
        ((system . derivation-file-name)
@@ -878,26 +875,35 @@ WHERE job_id = $1"
                        system
                        derivation-file-name)))
      derivation-file-names-by-system)
+
+    derivation-file-names-by-system))
+
+(define (channel-derivations-by-system->guix-store-item
+         channel-derivations-by-system)
+
+  (define (store-item->guix-store-item filename)
+    (dirname
+     (readlink
+      (string-append filename "/bin"))))
+
+  (let ((derivation-file-name-for-current-system
+         (assoc-ref
+          (assoc-ref channel-derivations-by-system
+                     (%current-system))
+          'profile)))
+    (if derivation-file-name-for-current-system
+        (let ((derivation-for-current-system
+               (read-derivation-from-file derivation-file-name-for-current-system)))
+          (with-store store
+            (set-build-options store #:fallback? #t)
+
             (log-time
              "building the channel derivation"
              (lambda ()
-       (build-derivations store (list derivation))))
+               (build-derivations store (list derivation-for-current-system)))))
-    (derivation->output-path derivation)))
 
-(define (channel->guix-store-item conn channel)
+          (store-item->guix-store-item
-  (catch
+           (derivation->output-path derivation-for-current-system)))
-    #t
-    (lambda ()
-      (with-store store
-        (set-build-options store #:fallback? #t)
-        (dirname
-         (readlink
-          (string-append (channel->manifest-store-item conn
-                                                       store
-                                                       channel)
-                         "/bin")))))
-    (lambda args
-      (simple-format #t "guix-data-service: load-new-guix-revision: error: ~A\n" args)
         #f)))
 
 (define (glibc-locales-for-guix-store-path store store-path)
@@ -1186,8 +1192,12 @@ ORDER BY packages.name, packages.version"
                          conn
                          git-repository-id))
                    (commit commit)))
+         (channel-derivations-by-system
+          (channel->derivations-by-system conn
+                                          channel-for-commit))
          (store-item
-          (channel->guix-store-item conn channel-for-commit)))
+          (channel-derivations-by-system->guix-store-item
+           channel-derivations-by-system)))
     (if store-item
         (let ((guix-revision-id
                (insert-guix-revision conn git-repository-id