install: Discover local substitute servers.

* gnu/installer/substitutes.scm: New file.
* gnu/installer/newt/substitutes.scm: New file.
* gnu/local.mk (INSTALLER_MODULES): Add them.
* po/guix/POTFILES.in: Add gnu/installer/newt/substitutes.scm.
* gnu/installer/proxy.scm (with-silent-shepherd): Move to ...
* gnu/installer/utils.scm: ... here.
* gnu/installer/record.scm (<installer>)[substitutes-page]: New field.
* gnu/installer/newt.scm (substitutes-page): New procedure,
(newt-installer): register it.
* gnu/installer.scm (installer-steps): Add "substitutes-page" step.
* gnu/system/install.scm (%installation-services): Add avahi-service-type and
enable substitute server discover in guix-service-type.
[<name-service-switch>]: Set it to %mdns-host-lookup-nss.

gnu/installer.scm

@@ -266,6 +266,13 @@ selected keymap."
           (compute (lambda _
                      ((installer-network-page current-installer)))))
 
+         ;; Ask whether to enable substitute server discovery.
+         (installer-step
+          (id 'substitutes)
+          (description (G_ "Substitute server discovery"))
+          (compute (lambda _
+                     ((installer-substitutes-page current-installer)))))
+
          ;; Prompt for users (name, group and home directory).
          (installer-step
           (id 'user)

gnu/installer/newt.scm

@@ -30,6 +30,7 @@
   #:use-module (gnu installer newt page)
   #:use-module (gnu installer newt partition)
   #:use-module (gnu installer newt services)
+  #:use-module (gnu installer newt substitutes)
   #:use-module (gnu installer newt timezone)
   #:use-module (gnu installer newt user)
   #:use-module (gnu installer newt utils)
@@ -101,6 +102,9 @@ problem. The backtrace is displayed below. Please report it by email to \
 (define (network-page)
   (run-network-page))
 
+(define (substitutes-page)
+  (run-substitutes-page))
+
 (define (hostname-page)
   (run-hostname-page))
 
@@ -130,6 +134,7 @@ problem. The backtrace is displayed below. Please report it by email to \
    (locale-page locale-page)
    (menu-page menu-page)
    (network-page network-page)
+   (substitutes-page substitutes-page)
    (timezone-page timezone-page)
    (hostname-page hostname-page)
    (user-page user-page)

gnu/installer/newt/substitutes.scm

@@ -0,0 +1,43 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2020 Mathieu Othacehe <othacehe@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu installer newt substitutes)
+  #:use-module (gnu installer substitutes)
+  #:use-module (gnu installer utils)
+  #:use-module (guix i18n)
+  #:use-module (newt)
+  #:use-module (ice-9 match)
+  #:export (run-substitutes-page))
+
+(define* (run-substitutes-page)
+  (match (current-clients)
+    (()
+     (case (choice-window
+            (G_ "Substitute server discovery.")
+            (G_ "Enable") (G_ "Disable")
+            (G_ " By turning this option on, you allow Guix to fetch \
+substitutes (pre-built binaries) during installation from servers \
+discovered on your local area network (LAN) in addition to the official \
+server.  This can increase download throughput.
+
+ There are no security risks: only genuine substitutes may be retrieved from \
+those servers.  However, eavesdroppers on your LAN may be able to see what \
+software you are installing."))
+       ((1) (enable-discovery))
+       ((2) (disable-discovery))))
+    (_ #f)))

gnu/installer/proxy.scm

@@ -17,15 +17,11 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (gnu installer proxy)
+  #:use-module (gnu installer utils)
   #:use-module (gnu services herd)
   #:export (set-http-proxy
             clear-http-proxy))
 
-(define-syntax-rule (with-silent-shepherd exp ...)
-  (parameterize ((shepherd-message-port
-                  (%make-void-port "w")))
-    exp ...))
-
 (define (set-http-proxy proxy)
   (with-silent-shepherd
     (with-shepherd-action 'guix-daemon

gnu/installer/record.scm

@@ -33,6 +33,7 @@
             installer-locale-page
             installer-menu-page
             installer-network-page
+            installer-substitutes-page
             installer-timezone-page
             installer-hostname-page
             installer-user-page
@@ -73,6 +74,8 @@
   (menu-page installer-menu-page)
   ;; procedure void -> void
   (network-page installer-network-page)
+  ;; procedure void -> void
+  (substitutes-page installer-substitutes-page)
   ;; procedure (zonetab) -> posix-timezone
   (timezone-page installer-timezone-page)
   ;; procedure void -> void

gnu/installer/substitutes.scm

@@ -0,0 +1,41 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2020 Mathieu Othacehe <m.othacehe@gmail.com>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu installer substitutes)
+  #:use-module (gnu installer utils)
+  #:use-module (gnu services herd)
+  #:export (enable-discovery
+            disable-discovery))
+
+(define (enable-discovery)
+  (with-silent-shepherd
+    (with-shepherd-action 'guix-daemon
+        ('discover "on")
+        result
+      result)))
+
+(define (disable-discovery)
+  (with-silent-shepherd
+    (with-shepherd-action 'guix-daemon
+        ('discover "off")
+        result
+      result)))
+
+;; Local Variables:
+;; eval: (put 'with-silent-shepherd 'scheme-indent-function 0)
+;; End:

gnu/installer/utils.scm

@@ -18,6 +18,7 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (gnu installer utils)
+  #:use-module (gnu services herd)
   #:use-module (guix utils)
   #:use-module (guix build utils)
   #:use-module (guix i18n)
@@ -43,7 +44,9 @@
             with-server-socket
             current-server-socket
             current-clients
-            send-to-clients))
+            send-to-clients
+
+            with-silent-shepherd))
 
 (define* (read-lines #:optional (port (current-input-port)))
   "Read lines from PORT and return them as a list."
@@ -233,3 +236,9 @@ accepting socket."
 
   (current-clients (reverse remainder))
   exp)
+
+(define-syntax-rule (with-silent-shepherd exp ...)
+  "Evaluate EXP while discarding shepherd messages."
+  (parameterize ((shepherd-message-port
+                  (%make-void-port "w")))
+    exp ...))

gnu/local.mk

@@ -717,6 +717,7 @@ INSTALLER_MODULES =                             \
   %D%/installer/record.scm		        \
   %D%/installer/services.scm			\
   %D%/installer/steps.scm			\
+  %D%/installer/substitutes.scm			\
   %D%/installer/tests.scm			\
   %D%/installer/timezone.scm			\
   %D%/installer/user.scm			\
@@ -733,6 +734,7 @@ INSTALLER_MODULES =                             \
   %D%/installer/newt/page.scm			\
   %D%/installer/newt/partition.scm		\
   %D%/installer/newt/services.scm		\
+  %D%/installer/newt/substitutes.scm		\
   %D%/installer/newt/timezone.scm		\
   %D%/installer/newt/user.scm			\
   %D%/installer/newt/utils.scm			\

gnu/system/install.scm

@@ -34,6 +34,7 @@
   #:use-module ((guix store) #:select (%store-prefix))
   #:use-module (gnu installer)
   #:use-module (gnu system locale)
+  #:use-module (gnu services avahi)
   #:use-module (gnu services dbus)
   #:use-module (gnu services networking)
   #:use-module (gnu services shepherd)
@@ -335,6 +336,10 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
           ;; The usual services.
           (syslog-service)
 
+          ;; Use the Avahi daemon to discover substitute servers on the local
+          ;; network.  It can be faster than fetching from remote servers.
+          (service avahi-service-type)
+
           ;; The build daemon.  Register the default substitute server key(s)
           ;; as trusted to allow the installation process to use substitutes by
           ;; default.
@@ -435,6 +440,7 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
     (host-name "gnu")
     (timezone "Europe/Paris")
     (locale "en_US.utf8")
+    (name-service-switch %mdns-host-lookup-nss)
     (bootloader (bootloader-configuration
                  (bootloader grub-bootloader)
                  (target "/dev/sda")))

po/guix/POTFILES.in

@@ -25,6 +25,7 @@ gnu/installer/newt/network.scm
 gnu/installer/newt/page.scm
 gnu/installer/newt/partition.scm
 gnu/installer/newt/services.scm
+gnu/installer/newt/substitutes.scm
 gnu/installer/newt/timezone.scm
 gnu/installer/newt/user.scm
 gnu/installer/newt/utils.scm