[bug#44800,v2,3/3] Use substitute servers on the local network.

diff --git a/Makefile.am b/Makefile.am
index 7049da9594..41b366eb75 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -257,6 +257,7 @@  MODULES =					\
   guix/import/texlive.scm   			\
   guix/import/utils.scm				\
   guix/scripts.scm				\
+  guix/scripts/discover.scm			\
   guix/scripts/download.scm			\
   guix/scripts/perform-download.scm		\
   guix/scripts/build.scm			\
diff --git a/doc/guix.texi b/doc/guix.texi
index f8efc34310..72531533ff 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -1584,6 +1584,10 @@  Unless @option{--lose-logs} is used, all the build logs are kept in the
 @var{localstatedir}.  To save space, the daemon automatically compresses
 them with Bzip2 by default.
 
+@item --use-local-publish[=yes|no]
+Whether to use publish servers discovered a the local network, using
+Avahi, for substitutution.
+
 @item --disable-deduplication
 @cindex deduplication
 Disable automatic file ``deduplication'' in the store.
@@ -14999,6 +15003,10 @@  disables the timeout.
 The type of compression used for build logs---one of @code{gzip},
 @code{bzip2}, or @code{none}.
 
+@item @code{use-local-publish?} (default: @code{#f})
+Whether to use publish servers discovered a the local network, using
+Avahi, for substitutution.
+
 @item @code{extra-options} (default: @code{'()})
 List of extra command-line options for @command{guix-daemon}.
 
diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index 87c247bdf1..718fa4096a 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -1529,6 +1529,8 @@  archive' public keys, with GUIX."
                     (default 0))
   (log-compression  guix-configuration-log-compression
                     (default 'bzip2))
+  (use-local-publish? guix-configuration-use-local-publish?
+                      (default #f))
   (extra-options    guix-configuration-extra-options ;list of strings
                     (default '()))
   (log-file         guix-configuration-log-file   ;string
@@ -1570,8 +1572,8 @@  proxy of 'guix-daemon'...~%")
   (match-record config <guix-configuration>
     (guix build-group build-accounts authorize-key? authorized-keys
           use-substitutes? substitute-urls max-silent-time timeout
-          log-compression extra-options log-file http-proxy tmpdir
-          chroot-directories)
+          log-compression use-local-publish? extra-options log-file
+          http-proxy tmpdir chroot-directories)
     (list (shepherd-service
            (documentation "Run the Guix daemon.")
            (provision '(guix-daemon))
@@ -1605,6 +1607,9 @@  proxy of 'guix-daemon'...~%")
                           #$@(if use-substitutes?
                                  '()
                                  '("--no-substitutes"))
+                          #$@(if use-local-publish?
+                                 '("--use-local-publish=yes")
+                                 '())
                           "--substitute-urls" #$(string-join substitute-urls)
                           #$@extra-options
 
diff --git a/guix/scripts/discover.scm b/guix/scripts/discover.scm
new file mode 100644
index 0000000000..d17b2bcfe4
--- /dev/null
+++ b/guix/scripts/discover.scm
@@ -0,0 +1,158 @@ 
+;;; 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 (guix scripts discover)
+  #:use-module (guix avahi)
+  #:use-module (guix config)
+  #:use-module (guix scripts)
+  #:use-module (guix ui)
+  #:use-module (guix build syscalls)
+  #:use-module (guix build utils)
+  #:use-module (guix scripts publish)
+  #:use-module (ice-9 rdelim)
+  #:use-module (srfi srfi-37)
+  #:export (read-publish-urls
+
+            guix-discover))
+
+(define (show-help)
+  (format #t (G_ "Usage: guix discover [OPTION]...
+Discover Guix related services using Avahi.\n"))
+  (display (G_ "
+  -c, --cache=DIRECTORY     cache discovery results in DIRECTORY"))
+  (display (G_ "
+  -h, --help                display this help and exit"))
+  (display (G_ "
+  -V, --version             display version information and exit"))
+  (newline)
+  (show-bug-report-information))
+
+(define %options
+  (list (option '(#\c "cache") #t #f
+                (lambda (opt name arg result)
+                  (alist-cons 'cache arg result)))
+        (option '(#\h "help") #f #f
+                (lambda _
+                  (show-help)
+                  (exit 0)))
+        (option '(#\V "version") #f #f
+                (lambda _
+                  (show-version-and-exit "guix discover")))))
+
+(define %default-options
+  `((cache . ,%state-directory)))
+
+
+;;;
+;;; Publish servers.
+;;;
+
+(define %publish-services
+  ;; Set of discovered publish services.
+  (make-hash-table))
+
+(define (publish-file cache-directory)
+  "Return the name of the file storing the discovered publish services inside
+CACHE-DIRECTORY."
+  (let ((directory (string-append cache-directory "/discover")))
+    (string-append directory "/publish")))
+
+(define %publish-file
+  (make-parameter (publish-file %state-directory)))
+
+(define* (write-publish-file #:key (file (%publish-file)))
+  "Dump the content of %PUBLISH-SERVICES hash table into FILE.  Use a write
+lock on FILE to synchronize with any potential readers."
+  (with-file-lock file
+    (call-with-output-file file
+      (lambda (port)
+        (hash-for-each
+         (lambda (name service)
+           (format port "http://~a:~a~%"
+                   (avahi-service-address service)
+                   (avahi-service-port service)))
+         %publish-services)))
+        (chmod file #o644)))
+
+(define (call-with-read-file-lock file thunk)
+  "Call THUNK with a read lock on FILE."
+  (let ((port #f))
+    (dynamic-wind
+      (lambda ()
+        (set! port
+              (let ((port (open-file file "r0")))
+                (fcntl-flock port 'read-lock)
+                port)))
+      thunk
+      (lambda ()
+        (when port
+          (unlock-file port))))))
+
+(define-syntax-rule (with-read-file-lock file exp ...)
+  "Wait to acquire a read lock on FILE and evaluate EXP in that context."
+  (call-with-read-file-lock file (lambda () exp ...)))
+
+(define* (read-publish-urls #:key (file (%publish-file)))
+  "Read publish urls list from FILE and return it.  Use a read lock on FILE to
+synchronize with the writer."
+  (with-read-file-lock file
+    (call-with-input-file file
+      (lambda (port)
+        (let loop ((url (read-line port))
+                   (urls '()))
+          (if (eof-object? url)
+              urls
+              (loop (read-line port) (cons url urls))))))))
+
+
+;;;
+;;; Entry point.
+;;;
+
+(define %services
+  ;; List of services we want to discover.
+  (list publish-service-type))
+
+(define (service-proc action service)
+  (let ((name (avahi-service-name service))
+        (type (avahi-service-type service)))
+    (when (string=? type publish-service-type)
+      (case action
+        ((new-service)
+         (hash-set! %publish-services name service))
+        ((remove-service)
+         (hash-remove! %publish-services name)))
+      (write-publish-file))))
+
+(define-command (guix-discover . args)
+  (category plumbing)
+  (synopsis "discover Guix related services using Avahi")
+
+  (with-error-handling
+    (let* ((opts (args-fold* args %options
+                             (lambda (opt name arg result)
+                               (leave (G_ "~A: unrecognized option~%") name))
+                             (lambda (arg result)
+                               (leave (G_ "~A: extraneous argument~%") arg))
+                             %default-options))
+           (cache (assoc-ref opts 'cache))
+           (publish-file (publish-file cache)))
+      (parameterize ((%publish-file publish-file))
+        (mkdir-p (dirname publish-file))
+        (avahi-browse-service-thread service-proc
+                                     #:types %services)))))
diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index ddb885d344..16e8fe6106 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -27,6 +27,7 @@ 
   #:use-module (guix config)
   #:use-module (guix records)
   #:use-module ((guix serialization) #:select (restore-file))
+  #:use-module (guix scripts discover)
   #:use-module (gcrypt hash)
   #:use-module (guix base32)
   #:use-module (guix base64)
@@ -1078,9 +1079,17 @@  found."
      ;; daemon.
      '("http://ci.guix.gnu.org"))))
 
+(define %local-substitute-urls
+  ;; If the following option is passed to the daemon, use the substitutes list
+  ;; provided by "guix discover" process.
+  (if (find-daemon-option "use-local-publish")
+      (read-publish-urls)
+      '()))
+
 (define substitute-urls
   ;; List of substitute URLs.
-  (make-parameter %default-substitute-urls))
+  (make-parameter (append %local-substitute-urls
+                          %default-substitute-urls)))
 
 (define (client-terminal-columns)
   "Return the number of columns in the client's terminal, if it is known, or a
diff --git a/nix/libstore/globals.cc b/nix/libstore/globals.cc
index 0cc001fbe4..2b621af982 100644
--- a/nix/libstore/globals.cc
+++ b/nix/libstore/globals.cc
@@ -35,6 +35,7 @@  Settings::Settings()
     maxSilentTime = 0;
     buildTimeout = 0;
     useBuildHook = true;
+    useLocalPublish = false;
     printBuildTrace = false;
     multiplexedBuildOutput = false;
     reservedSize = 8 * 1024 * 1024;
diff --git a/nix/libstore/globals.hh b/nix/libstore/globals.hh
index 27616a2283..43653aef48 100644
--- a/nix/libstore/globals.hh
+++ b/nix/libstore/globals.hh
@@ -116,6 +116,10 @@  struct Settings {
        users want to disable this from the command-line. */
     bool useBuildHook;
 
+    /* Whether to use publish servers found on the local network for
+       substitution. */
+    bool useLocalPublish;
+
     /* Whether buildDerivations() should print out lines on stderr in
        a fixed format to allow its progress to be monitored.  Each
        line starts with a "@".  The following are defined:
diff --git a/nix/nix-daemon/guix-daemon.cc b/nix/nix-daemon/guix-daemon.cc
index cd949aca67..f0ace9ce8b 100644
--- a/nix/nix-daemon/guix-daemon.cc
+++ b/nix/nix-daemon/guix-daemon.cc
@@ -89,6 +89,7 @@  builds derivations on behalf of its clients.");
 #define GUIX_OPT_TIMEOUT 18
 #define GUIX_OPT_MAX_SILENT_TIME 19
 #define GUIX_OPT_LOG_COMPRESSION 20
+#define GUIX_OPT_USE_LOCAL_PUBLISH 21
 
 static const struct argp_option options[] =
   {
@@ -129,6 +130,9 @@  static const struct argp_option options[] =
       n_("disable compression of the build logs") },
     { "log-compression", GUIX_OPT_LOG_COMPRESSION, "TYPE", 0,
       n_("use the specified compression type for build logs") },
+    { "use-local-publish", GUIX_OPT_USE_LOCAL_PUBLISH,
+      "yes/no", OPTION_ARG_OPTIONAL,
+      n_("use publish servers discovered on the local network") },
 
     /* '--disable-deduplication' was known as '--disable-store-optimization'
        up to Guix 0.7 included, so keep the alias around.  */
@@ -261,6 +265,10 @@  parse_opt (int key, char *arg, struct argp_state *state)
     case GUIX_OPT_NO_BUILD_HOOK:
       settings.useBuildHook = false;
       break;
+    case GUIX_OPT_USE_LOCAL_PUBLISH:
+      settings.useLocalPublish = string_to_bool (arg);
+      settings.set("use-local-publish", arg);
+      break;
     case GUIX_OPT_DEBUG:
       verbosity = lvlDebug;
       break;
@@ -506,6 +514,18 @@  using `--build-users-group' is highly recommended\n"));
 		    format ("extra chroot directories: '%1%'") % chroot_dirs);
 	}
 
+      if (settings.useLocalPublish)
+      {
+        Strings args;
+
+        args.push_back("guix");
+        args.push_back("discover");
+
+        startProcess([&]() {
+          execv(settings.guixProgram.c_str(), stringsToCharPtrs(args).data());
+        });
+      }
+
       printMsg (lvlDebug,
 		format ("automatic deduplication set to %1%")
 		% settings.autoOptimiseStore);