Use substitute servers on the local network.

* guix/scripts/discover.scm: New file.
* Makefile.am (MODULES): Add it.
* nix/nix-daemon/guix-daemon.cc (options): Add "discover" option,
(parse-opt): parse it,
(main): start "guix discover" process when the option is set.
* guix/scripts/substitute.scm (%local-substitute-urls): New variable,
(substitute-urls): add it.
* gnu/services/base.scm (<guix-configuration>): Add "discover?"
field,
(guix-shepherd-service): honor it.
* doc/guix.texi (Invoking guix-daemon): Document "discover" option,
(Base Services): ditto.

2 files changed, 189 insertions, 1 deletions

diff --git a/guix/scripts/discover.scm b/guix/scripts/discover.scm
new file mode 100644
index 0000000000..b17dbdcb3c
--- /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-substitute-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-substitute-urls #:key (file (%publish-file)))
+  "Read substitute 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 internal)
+  (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..8e5953b877 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 +1079,38 @@ found."
      ;; daemon.
      '("http://ci.guix.gnu.org"))))
 
+;; In order to prevent using large number of discovered local substitute
+;; servers, limit the local substitute urls list size.
+(define %max-substitute-urls 50)
+
+(define* (randomize-substitute-urls urls
+                                    #:key
+                                    (max %max-substitute-urls))
+  "Return a list containing MAX urls from URLS, picked randomly. If URLS list
+is shorter than MAX elements, then it is directly returned."
+  (define (random-item list)
+    (list-ref list (random (length list))))
+
+  (if (<= (length urls) max)
+      urls
+      (let loop ((res '())
+                 (urls urls))
+        (if (eq? (length res) max)
+            res
+            (let ((url (random-item urls)))
+              (loop (cons url res) (delete url urls)))))))
+
+(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 "discover")
+      (randomize-substitute-urls (read-substitute-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