[gnome-initial-setup] account: fix username validation

[Will Thompson <wjt src gnome org>]

commit 3a1030d8a4f07b63c78ce76185dbe676a7feea7e
Author: Xiang Fan <sfanxiang gmail com>
Date:   Sat Dec 29 22:00:19 2018 +0800

    account: fix username validation
    
    Current username validation does not conform to the requirement for
    useradd. For example, upper case letters are allowed in the validation
    but no actual account is created afterwards.
    
    Ideally, Initial Setup would ask the system what the rules are.
    gnome-control-center tries to do this by invoking:
    
        usermod --login USERNAME -- USERNAME
    
    and checking the exit code to determine whether USERNAME is legal.
    However, at least on Debian-like systems, this method accepts names
    'adduser' will reject, such as 'æ' and 'a.e'. (Debian's accountsservice
    is patched to call 'adduser'.)
    
    https://gitlab.gnome.org/GNOME/gnome-initial-setup/-/issues/117

 gnome-initial-setup/pages/account/um-utils.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)
---
diff --git a/gnome-initial-setup/pages/account/um-utils.c b/gnome-initial-setup/pages/account/um-utils.c
index 39887498..2877d94f 100644
--- a/gnome-initial-setup/pages/account/um-utils.c
+++ b/gnome-initial-setup/pages/account/um-utils.c
@@ -128,16 +128,19 @@ is_valid_username (const gchar *username, gboolean parental_controls_enabled, gc
         valid = TRUE;
 
         if (!in_use && !empty && !too_long) {
-                /* First char must be a letter, and it must only composed
-                 * of ASCII letters, digits, and a '.', '-', '_'
+                /* First char must be a lower case letter, and it must only be
+                 * composed of lower case letters, digits, '-', and '_'.
                  */
                 for (c = username; *c; c++) {
-                        if (! ((*c >= 'a' && *c <= 'z') ||
-                               (*c >= 'A' && *c <= 'Z') ||
-                               (*c >= '0' && *c <= '9') ||
-                               (*c == '_') || (*c == '.') ||
-                               (*c == '-' && c != username)))
-                           valid = FALSE;
+                        if (c == username) {
+                            if (! (*c >= 'a' && *c <= 'z'))
+                               valid = FALSE;
+                        } else {
+                            if (! ((*c >= 'a' && *c <= 'z') ||
+                                   (*c >= '0' && *c <= '9') ||
+                                   (*c == '_') || (*c == '-')))
+                               valid = FALSE;
+                        }
                 }
         }
 
@@ -152,14 +155,14 @@ is_valid_username (const gchar *username, gboolean parental_controls_enabled, gc
                 else if (too_long) {
                         *tip = g_strdup_printf (_("The username is too long."));
                 }
-                else if (username[0] == '-') {
-                        *tip = g_strdup (_("The username cannot start with a “-”."));
+                else if (!(username[0] >= 'a' && username[0] <= 'z')) {
+                        *tip = g_strdup (_("The username must start with a lower case letter from a-z."));
                 }
                 else if (parental_controls_conflict) {
                         *tip = g_strdup (_("That username isn’t available. Please try another."));
                 }
                 else {
-                        *tip = g_strdup (_("The username should only consist of upper and lower case letters 
from a-z, digits and the following characters: . - _"));
+                        *tip = g_strdup (_("The username should only consist of lower case letters from a-z, 
digits, and the following characters: - _"));
                 }
         }
         else {