substitute: Make '%allow-unauthenticated-substitutes?' public.

* guix/scripts/substitute.scm (warn-about-missing-authentication): New
procedure.
(%allow-unauthenticated-substitutes?): Turn into a public parameter and
use 'warn-about-missing-authentication'.
(valid-narinfo?): Adjust accordingly.
* tests/substitute.scm (call-with-narinfo): Likewise.

guix/scripts/substitute.scm

@@ -86,6 +86,8 @@ (define-module (guix scripts substitute)
             read-narinfo
             write-narinfo
 
+            %allow-unauthenticated-substitutes?
+
             substitute-urls
             guix-substitute))
 
@@ -118,15 +120,21 @@ (define %narinfo-cache-directory
           (string-append %state-directory "/substitute/cache"))
       (string-append (cache-directory #:ensure? #f) "/substitute")))
 
+(define (warn-about-missing-authentication)
+  (warning (G_ "authentication and authorization of substitutes \
+disabled!~%"))
+  #t)
+
 (define %allow-unauthenticated-substitutes?
   ;; Whether to allow unchecked substitutes.  This is useful for testing
   ;; purposes, and should be avoided otherwise.
-  (and (and=> (getenv "GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES")
-              (cut string-ci=? <> "yes"))
-       (begin
-         (warning (G_ "authentication and authorization of substitutes \
-disabled!~%"))
-         #t)))
+  (make-parameter
+   (and=> (getenv "GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES")
+          (cut string-ci=? <> "yes"))
+   (lambda (value)
+     (when value
+       (warn-about-missing-authentication))
+     value)))
 
 (define %narinfo-ttl
   ;; Number of seconds during which cached narinfo lookups are considered
@@ -370,7 +378,7 @@ (define %mandatory-fields
 (define* (valid-narinfo? narinfo #:optional (acl (current-acl))
                          #:key verbose?)
   "Return #t if NARINFO's signature is not valid."
-  (or %allow-unauthenticated-substitutes?
+  (or (%allow-unauthenticated-substitutes?)
       (let ((hash      (narinfo-sha256 narinfo))
             (signature (narinfo-signature narinfo))
             (uri       (uri->string (first (narinfo-uris narinfo)))))

tests/substitute.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2014 Nikita Karetnikov <nikita@karetnikov.org>
-;;; Copyright © 2014, 2015, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -169,9 +169,7 @@ (define* (call-with-narinfo narinfo thunk
           (cute write-file
                 (string-append narinfo-directory "/example.out") <>))
 
-        (set! (@@ (guix scripts substitute)
-                  %allow-unauthenticated-substitutes?)
-              #f))
+        (%allow-unauthenticated-substitutes? #f))
       thunk
       (lambda ()
         (when (file-exists? cache-directory)