download: Disarchive mirrors can be URL-returning procedures.

As discussed at <https://issues.guix.gnu.org/47336#16>.

* guix/build/download.scm (url-fetch)[disarchive-uris]: Accept MIRROR as
a procedure.
* guix/download.scm (%disarchive-mirrors): Add comment.  This change can
only be made once a 'guix perform-download' that understands procedures
is widely deployed.

guix/build/download.scm

@@ -747,14 +747,19 @@ otherwise simply ignore them."
                 content-addressed-mirrors))
 
   (define disarchive-uris
-    (append-map (match-lambda
-                  ((? string? mirror)
+    (append-map (lambda (mirror)
+                  (let ((make-url (match mirror
+                                    ((? string?)
+                                     (lambda (hash-algo hash)
+                                       (string-append
+                                        mirror
+                                        (symbol->string hash-algo) "/"
+                                        (bytevector->base16-string hash))))
+                                    ((? procedure?)
+                                     mirror))))
                     (map (match-lambda
                            ((hash-algo . hash)
-                           (string->uri
-                            (string-append mirror
-                                           (symbol->string hash-algo) "/"
-                                           (bytevector->base16-string hash)))))
+                            (string->uri (make-url hash-algo hash))))
                          hashes)))
                 disarchive-mirrors))
 

guix/download.scm

@@ -400,6 +400,8 @@
               (object->string %content-addressed-mirrors)))
 
 (define %disarchive-mirrors
+  ;; TODO: Eventually turn into a procedure that takes a hash algorithm
+  ;; (symbol) and hash (bytevector).
   '("https://disarchive.ngyro.com/"))
 
 (define %disarchive-mirror-file