installer: Check if ci.guix.gnu.org can be reached.

* gnu/installer.scm (installer-program): Add gnutls extension.
* gnu/installer/newt/network.scm (wait-service-online): Check if the CI server
can be reached.

gnu/installer.scm

@@ -44,6 +44,7 @@ (define-module (gnu installer)
   #:use-module (gnu packages linux)
   #:use-module (gnu packages ncurses)
   #:use-module (gnu packages package-management)
+  #:use-module (gnu packages tls)
   #:use-module (gnu packages xorg)
   #:use-module (gnu system locale)
   #:use-module (ice-9 match)
@@ -351,7 +352,7 @@ (define installer-builder
     ;; packages …), etc. modules.
     (with-extensions (list guile-gcrypt guile-newt
                            guile-parted guile-bytestructures
-                           guile-json-3 guile-git guix)
+                           guile-json-3 guile-git guix gnutls)
       (with-imported-modules `(,@(source-module-closure
                                   `(,@modules
                                     (gnu services herd)

gnu/installer/newt/network.scm

@@ -30,6 +30,8 @@ (define-module (gnu installer newt network)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
   #:use-module (ice-9 match)
+  #:use-module (web client)
+  #:use-module (web response)
   #:use-module (newt)
   #:export (run-network-page))
 
@@ -119,8 +121,23 @@ (define (wait-technology-powered technology)
 (define (wait-service-online)
   "Display a newt scale until connman detects an Internet access. Do
 FULL-VALUE tentatives, spaced by 1 second."
+  (define (ci-available?)
+    (dynamic-wind
+      (lambda ()
+        (sigaction SIGALRM
+          (lambda _ #f))
+        (alarm 3))
+      (lambda ()
+        (false-if-exception
+         (= (response-code
+             (http-request "https://ci.guix.gnu.org"))
+            200)))
+      (lambda ()
+        (alarm 0))))
+
   (define (online?)
-    (or (connman-online?)
+    (or (and (connman-online?)
+             (ci-available?))
         (file-exists? "/tmp/installer-assume-online")))
 
   (let* ((full-value 5))
@@ -137,7 +154,7 @@ (define (online?)
     (unless (online?)
       (run-error-page
        (G_ "The selected network does not provide access to the \
-Internet, please try again.")
+Internet and the Guix substitute server, please try again.")
        (G_ "Connection error"))
       (raise
        (condition