system: Add a 'system?' field to user accounts.

* gnu/system/shadow.scm (<user-account>)[system?]: New field.
* gnu/system.scm (user-account->gexp): Add it.
* guix/build/activation.scm (add-user): Add #:system? parameter and
  honor it.
  (activate-users+groups): Handle the 'system?' part of user tuples.
  Pass it to 'add-user'.  Don't create PROFILE-DIR when SYSTEM? is
  true.
* gnu/services/dbus.scm (dbus-service): Add 'system?' field for
  "messagebus" account.
* gnu/services/base.scm (guix-build-accounts): Likewise.
* gnu/services/avahi.scm (avahi-service): Likewise.

gnu/services/avahi.scm

@@ -100,6 +100,7 @@ sockets."
       (user-accounts (list (user-account
                             (name "avahi")
                             (group "avahi")
+                            (system? #t)
                             (comment "Avahi daemon user")
                             (home-directory "/var/empty")
                             (shell

gnu/services/base.scm

@@ -327,6 +327,7 @@ starting at FIRST-UID, and under GID."
                     (lambda (n)
                       (user-account
                        (name (format #f "guixbuilder~2,'0d" n))
+                       (system? #t)
                        (uid (+ first-uid n -1))
                        (group group)
 

gnu/services/dbus.scm

@@ -90,6 +90,7 @@ and policy files.  For example, to allow avahi-daemon to use the system bus,
       (user-accounts (list (user-account
                             (name "messagebus")
                             (group "messagebus")
+                            (system? #t)
                             (comment "D-Bus system bus user")
                             (home-directory "/var/run/dbus")
                             (shell

gnu/system.scm

@@ -369,7 +369,8 @@ alias ll='ls -l'
       #$(user-account-comment account)
       #$(user-account-home-directory account)
       ,#$(user-account-shell account)             ; this one is a gexp
-      #$(user-account-password account)))
+      #$(user-account-password account)
+      #$(user-account-system? account)))
 
 (define (operating-system-activation-script os)
   "Return the activation script for OS---i.e., the code that \"activates\" the

gnu/system/shadow.scm

@@ -34,6 +34,7 @@
             user-account-comment
             user-account-home-directory
             user-account-shell
+            user-account-system?
 
             user-group
             user-group?
@@ -63,7 +64,9 @@
   (comment        user-account-comment (default ""))
   (home-directory user-account-home-directory)
   (shell          user-account-shell              ; gexp
-                  (default #~(string-append #$bash "/bin/bash"))))
+                  (default #~(string-append #$bash "/bin/bash")))
+  (system?        user-account-system?            ; Boolean
+                  (default #f)))
 
 (define-record-type* <user-group>
   user-group make-user-group

guix/build/activation.scm

@@ -47,7 +47,7 @@
     (zero? (apply system* "groupadd" args))))
 
 (define* (add-user name group
-                   #:key uid comment home shell password
+                   #:key uid comment home shell password system?
                    (supplementary-groups '())
                    (log-port (current-error-port)))
   "Create an account for user NAME part of GROUP, with the specified
@@ -82,6 +82,7 @@ properties.  Return #t on success."
                           '())
                     ,@(if shell `("-s" ,shell) '())
                     ,@(if password `("-p" ,password) '())
+                    ,@(if system? '("--system") '())
                     ,name)))
         (zero? (apply system* "useradd" args)))))
 
@@ -97,22 +98,24 @@ numeric gid or #f."
 
   (define activate-user
     (match-lambda
-     ((name uid group supplementary-groups comment home shell password)
+     ((name uid group supplementary-groups comment home shell password system?)
       (unless (false-if-exception (getpwnam name))
         (let ((profile-dir (string-append "/var/guix/profiles/per-user/"
                                           name)))
           (add-user name group
                     #:uid uid
+                    #:system? system?
                     #:supplementary-groups supplementary-groups
                     #:comment comment
                     #:home home
                     #:shell shell
                     #:password password)
 
-          ;; Create the profile directory for the new account.
-          (let ((pw (getpwnam name)))
-            (mkdir-p profile-dir)
-            (chown profile-dir (passwd:uid pw) (passwd:gid pw))))))))
+          (unless system?
+            ;; Create the profile directory for the new account.
+            (let ((pw (getpwnam name)))
+              (mkdir-p profile-dir)
+              (chown profile-dir (passwd:uid pw) (passwd:gid pw)))))))))
 
   ;; 'groupadd' aborts if the file doesn't already exist.
   (touch "/etc/group")