sysusers: Provide meson argument to set gid for 'users' group (#7533)

To allow better integration with distributions requiring an explicitly
set gid for the `users` group, provide the new `-Dusers-gid` option to
set to a new numeric value.

In the absence of a specified gid, we'll fallback to the default existing
behaviour of `-` as the gid value, to automatically assign the next available
gid on the system.

meson.build

@@ -633,6 +633,14 @@ tty_gid = get_option('tty-gid')
 conf.set('TTY_GID', tty_gid)
 substs.set('TTY_GID', tty_gid)
 
+# Ensure provided GID argument is numeric, otherwise fallback to default assignment
+if get_option('users-gid') != ''
+    users_gid = get_option('users-gid').to_int()
+else
+    users_gid = '-'
+endif
+substs.set('USERS_GID', users_gid)
+
 if get_option('adm-group')
         m4_defines += ['-DENABLE_ADM_GROUP']
 endif
@@ -2495,6 +2503,7 @@ status = [
         'debug shell:                       @0@ @ @1@'.format(get_option('debug-shell'),
                                                               get_option('debug-tty')),
         'TTY GID:                           @0@'.format(tty_gid),
+        'users GID:                         @0@'.format(users_gid),
         'maximum system UID:                @0@'.format(system_uid_max),
         'maximum system GID:                @0@'.format(system_gid_max),
         '/dev/kvm access mode:              @0@'.format(get_option('dev-kvm-mode')),

meson_options.txt

@@ -150,6 +150,8 @@ option('system-gid-max', type : 'string',
 option('tty-gid', type : 'string',
        description : 'the numeric GID of the "tty" group',
        value : '5')
+option('users-gid', type : 'string',
+       description : 'the numeric GID of the "users" group')
 option('adm-group', type : 'boolean',
        description : 'the ACL for adm group should be added')
 option('wheel-group', type : 'boolean',

sysusers.d/basic.conf.in

@@ -35,4 +35,4 @@ g tape    -     -            -
 g video   -     -            -
 
 # Default group for normal users
-g users   -     -            -
+g users   @USERS_GID@     -            -