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+;;; 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)))))