services: Add 'guix-publish-service'.

* gnu/services/base.scm (<guix-publish-configuration>): New record
  type.
  (guix-publish-dmd-service): New procedure.
  (%guix-publish-accounts, guix-publish-service-type): New variables.
  (guix-publish-service): New procedure.
* doc/guix.texi (Invoking guix publish): Add xref to
  'guix-publish-service' anchor.
  (Base Services): Document 'guix-publish-service'.

doc/guix.texi

@@ -4918,6 +4918,11 @@ Reference Manual}) on @var{port} (37146 by default).  This is used
 primarily for debugging a running @command{guix publish} server.
 @end table
 
+Enabling @command{guix publish} on a GuixSD system is a one-liner: just
+add a call to @code{guix-publish-service} in the @code{services} field
+of the @code{operating-system} declaration (@pxref{guix-publish-service,
+@code{guix-publish-service}}).
+
 
 @node Invoking guix challenge
 @section Invoking @command{guix challenge}
@@ -6428,6 +6433,17 @@ Return a service to load console keymap from @var{file} using
 @command{loadkeys} command.
 @end deffn
 
+@anchor{guix-publish-service}
+@deffn {Scheme Procedure} guix-publish-service [#:guix @var{guix}] @
+           [#:port 80] [#:host "localhost"]
+Return a service that runs @command{guix publish} listening on @var{host}
+and @var{port} (@pxref{Invoking guix publish}).
+
+This assumes that @file{/etc/guix} already contains a signing key pair as
+created by @command{guix archive --generate-key} (@pxref{Invoking guix
+archive}).  If that is not the case, the service will fail to start.
+@end deffn
+
 
 @node Networking Services
 @subsubsection Networking Services

gnu/services/base.scm

@@ -76,6 +76,10 @@
             guix-configuration?
             guix-service
             guix-service-type
+            guix-publish-configuration
+            guix-publish-configuration?
+            guix-publish-service
+            guix-publish-service-type
 
             %base-services))
 
@@ -705,6 +709,11 @@ If configuration file name @var{config-file} is not specified, use some
 reasonable default settings."
   (service syslog-service-type config-file))
 
+
+;;;
+;;; Guix services.
+;;;
+
 (define* (guix-build-accounts count #:key
                               (group "guixbuild")
                               (first-uid 30001)
@@ -842,6 +851,58 @@ failed to register hydra.gnu.org public key: ~a~%" status))))))))
 @var{config}."
   (service guix-service-type config))
 
+
+(define-record-type* <guix-publish-configuration>
+  guix-publish-configuration make-guix-publish-configuration
+  guix-publish-configuration?
+  (guix    guix-publish-configuration-guix        ;package
+           (default guix))
+  (port    guix-publish-configuration-port        ;number
+           (default 80))
+  (host    guix-publish-configuration-host        ;string
+           (default "localhost")))
+
+(define guix-publish-dmd-service
+  (match-lambda
+    (($ <guix-publish-configuration> guix port host)
+     (list (dmd-service
+            (provision '(guix-publish))
+            (requirement '(guix-daemon))
+            (start #~(make-forkexec-constructor
+                      (list (string-append #$guix "/bin/guix")
+                            "publish" "-u" "guix-publish"
+                            "-p" #$(number->string port)
+                            (string-append "--listen=" #$host))))
+            (stop #~(make-kill-destructor)))))))
+
+(define %guix-publish-accounts
+  (list (user-group (name "guix-publish") (system? #t))
+        (user-account
+         (name "guix-publish")
+         (group "guix-publish")
+         (system? #t)
+         (comment "guix publish user")
+         (home-directory "/var/empty")
+         (shell #~(string-append #$shadow "/sbin/nologin")))))
+
+(define guix-publish-service-type
+  (service-type (name 'guix-publish)
+                (extensions
+                 (list (service-extension dmd-root-service-type
+                                          guix-publish-dmd-service)
+                       (service-extension account-service-type
+                                          (const %guix-publish-accounts))))))
+
+(define* (guix-publish-service #:key (guix guix) (port 80) (host "localhost"))
+  "Return a service that runs @command{guix publish} listening on @var{host}
+and @var{port} (@pxref{Invoking guix publish}).
+
+This assumes that @file{/etc/guix} already contains a signing key pair as
+created by @command{guix archive --generate-key} (@pxref{Invoking guix
+archive}).  If that is not the case, the service will fail to start."
+  (service guix-publish-service-type
+           (guix-publish-configuration (guix guix) (port port) (host host))))
+
 
 ;;;
 ;;; Udev.