services: guix: Add 'set-http-proxy' action.

Fixes <https://bugs.gnu.org/25569>.
Reported by Divan Santana <divan@santanas.co.za>.

* gnu/services/base.scm (shepherd-set-http-proxy-action): New procedure.
(guix-shepherd-service): Add 'actions' field.  Change 'start' to a
lambda; check the value of the "http_proxy" environment variable and
add "http_proxy" and "https_proxy" to #:environment-variables as a
function of that.
* gnu/tests/base.scm (run-basic-test)["guix-daemon set-http-proxy
action", "guix-daemon set-http-proxy action, clear"]: New tests.
* doc/guix.texi (Base Services): Document it.

doc/guix.texi

@@ -12779,9 +12779,24 @@ List of extra command-line options for @command{guix-daemon}.
 File where @command{guix-daemon}'s standard output and standard error
 are written.
 
+@cindex HTTP proxy, for @code{guix-daemon}
+@cindex proxy, for @code{guix-daemon} HTTP access
 @item @code{http-proxy} (default: @code{#f})
-The HTTP proxy used for downloading fixed-output derivations and
+The URL of the HTTP and HTTPS proxy used for downloading fixed-output
-substitutes.
+derivations and substitutes.
+
+It is also possible to change the daemon's proxy at run time through the
+@code{set-http-proxy} action, which restarts it:
+
+@example
+herd set-http-proxy guix-daemon http://localhost:8118
+@end example
+
+To clear the proxy settings, run:
+
+@example
+herd set-http-proxy guix-daemon
+@end example
 
 @item @code{tmpdir} (default: @code{#f})
 A directory path where the @command{guix-daemon} will perform builds.

gnu/services/base.scm

@@ -1640,6 +1640,30 @@ archive' public keys, with GUIX."
 (define %default-guix-configuration
   (guix-configuration))
 
+(define shepherd-set-http-proxy-action
+  ;; Shepherd action to change the HTTP(S) proxy.
+  (shepherd-action
+   (name 'set-http-proxy)
+   (documentation
+    "Change the HTTP(S) proxy used by 'guix-daemon' and restart it.")
+   (procedure #~(lambda* (_ #:optional proxy)
+                  (let ((environment (environ)))
+                    ;; A bit of a hack: communicate PROXY to the 'start'
+                    ;; method via environment variables.
+                    (if proxy
+                        (begin
+                          (format #t "changing HTTP/HTTPS \
+proxy of 'guix-daemon' to ~s...~%"
+                                  proxy)
+                          (setenv "http_proxy" proxy))
+                        (begin
+                          (format #t "clearing HTTP/HTTPS \
+proxy of 'guix-daemon'...~%")
+                          (unsetenv "http_proxy")))
+                    (action 'guix-daemon 'restart)
+                    (environ environment)
+                    #t)))))
+
 (define (guix-shepherd-service config)
   "Return a <shepherd-service> for the Guix daemon service with CONFIG."
   (match-record config <guix-configuration>
@@ -1651,9 +1675,16 @@ archive' public keys, with GUIX."
            (documentation "Run the Guix daemon.")
            (provision '(guix-daemon))
            (requirement '(user-processes))
+           (actions (list shepherd-set-http-proxy-action))
            (modules '((srfi srfi-1)))
            (start
-            #~(make-forkexec-constructor
+            #~(lambda _
+                (define proxy
+                  ;; HTTP/HTTPS proxy.  The 'http_proxy' variable is set by
+                  ;; the 'set-http-proxy' action.
+                  (or (getenv "http_proxy") #$http-proxy))
+
+                (fork+exec-command
                  (cons* #$(file-append guix "/bin/guix-daemon")
                         "--build-users-group" #$build-group
                         "--max-silent-time" #$(number->string max-silent-time)
@@ -1665,33 +1696,37 @@ archive' public keys, with GUIX."
                         "--substitute-urls" #$(string-join substitute-urls)
                         #$@extra-options
 
-                      ;; Add CHROOT-DIRECTORIES and all their dependencies (if
+                        ;; Add CHROOT-DIRECTORIES and all their dependencies
-                      ;; these are store items) to the chroot.
+                        ;; (if these are store items) to the chroot.
                         (append-map (lambda (file)
                                       (append-map (lambda (directory)
                                                     (list "--chroot-directory"
                                                           directory))
                                                   (call-with-input-file file
                                                     read)))
-                                  '#$(map references-file chroot-directories)))
+                                    '#$(map references-file
+                                            chroot-directories)))
 
                  #:environment-variables
-               (list #$@(if http-proxy
+                 (append (list #$@(if tmpdir
-                            (list (string-append "http_proxy=" http-proxy))
-                            '())
-                     #$@(if tmpdir
                                       (list (string-append "TMPDIR=" tmpdir))
                                       '())
 
-                     ;; Make sure we run in a UTF-8 locale so that 'guix
+                               ;; Make sure we run in a UTF-8 locale so that
-                     ;; offload' correctly restores nars that contain UTF-8
+                               ;; 'guix offload' correctly restores nars that
-                     ;; file names such as 'nss-certs'.  See
+                               ;; contain UTF-8 file names such as
+                               ;; 'nss-certs'.  See
                                ;; <https://bugs.gnu.org/32942>.
                                (string-append "GUIX_LOCPATH="
-                                    #$glibc-utf8-locales "/lib/locale")
+                                              #$glibc-utf8-locales
+                                              "/lib/locale")
                                "LC_ALL=en_US.utf8")
+                         (if proxy
+                             (list (string-append "http_proxy=" proxy)
+                                   (string-append "https_proxy=" proxy))
+                             '()))
 
-               #:log-file #$log-file))
+                 #:log-file #$log-file)))
            (stop #~(make-kill-destructor))))))
 
 (define (guix-accounts config)

gnu/tests/base.scm

@@ -459,6 +459,21 @@ info --version")
             (marionette-eval '(readlink "/var/guix/gcroots/profiles")
                              marionette))
 
+          (test-equal "guix-daemon set-http-proxy action"
+            '(#t)                                 ;one value, #t
+            (marionette-eval '(with-shepherd-action 'guix-daemon
+                                  ('set-http-proxy "http://localhost:8118")
+                                  result
+                                result)
+                             marionette))
+
+          (test-equal "guix-daemon set-http-proxy action, clear"
+            '(#t)                                 ;one value, #t
+            (marionette-eval '(with-shepherd-action 'guix-daemon
+                                  ('set-http-proxy)
+                                  result
+                                result)
+                             marionette))
 
           (test-assert "screendump"
             (begin