[gnome-control-center] user-accounts: Fix definition of MAXNAMELEN

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 09b94a9b539056bedb1d19b7f99591122143b689
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Wed Jan 31 14:31:02 2018 -0600

    user-accounts: Fix definition of MAXNAMELEN
    
    Currently we get MAXNAMELEN from glibc's LOGIN_NAME_MAX, if available,
    and sysconf otherwise. But the maximum username length supported by
    glibc and the kernel is actually way larger than the maximum length that
    actually works in practice. On Linux, anything larger than 32 characters
    is not going to fit into utmp, and will therefore be rejected by
    useradd. Then gnome-control-center will spit out a confusing error
    message dialog.
    
    Let's spare our users from that.
    
    useradd (in shadow-utils) gets its max name size from the following
    magic:
    
     /* Maximum length of usernames */
     #ifdef HAVE_UTMPX_H
     # include <utmpx.h>
     # define USER_NAME_MAX_LENGTH (sizeof (((struct utmpx *)NULL)->ut_user))
     #else
     # include <utmp.h>
     # ifdef HAVE_STRUCT_UTMP_UT_USER
     #  define USER_NAME_MAX_LENGTH (sizeof (((struct utmp *)NULL)->ut_user))
     # else
     #  ifdef HAVE_STRUCT_UTMP_UT_NAME
     #   define USER_NAME_MAX_LENGTH (sizeof (((struct utmp *)NULL)->ut_name))
     #  else
     #   define USER_NAME_MAX_LENGTH 32
     #  endif
     # endif
     #endif
    
    It's more work than necessary. utmpx is standardized by POSIX (it's
    actually an XSI extension), whereas utmp is not, so let's just use
    utmpx. This ought to work on at least FreeBSD as well. And if any free
    operating systems that care about GNOME don't have utmpx yet, no doubt
    they'll send patches.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=724193

 panels/user-accounts/um-utils.c |   25 +++++++------------------
 1 files changed, 7 insertions(+), 18 deletions(-)
---
diff --git a/panels/user-accounts/um-utils.c b/panels/user-accounts/um-utils.c
index f0de95c..c01bc1f 100644
--- a/panels/user-accounts/um-utils.c
+++ b/panels/user-accounts/um-utils.c
@@ -25,6 +25,7 @@
 #include <sys/types.h>
 #include <limits.h>
 #include <unistd.h>
+#include <utmpx.h>
 #include <pwd.h>
 
 #include <gio/gio.h>
@@ -404,24 +405,12 @@ rounded_rectangle (cairo_t *cr,
         cairo_close_path (cr);
 }
 
-static guint
-get_login_name_max (void)
-{
-#ifdef LOGIN_NAME_MAX
-        return LOGIN_NAME_MAX;
-#else
-        static gint length;
-
-        if (!length) {
-                length = sysconf (_SC_LOGIN_NAME_MAX);
-                g_assert_cmpint (length, >=, 0);
-        }
-
-        return length;
-#endif
-}
-
-#define MAXNAMELEN  get_login_name_max ()
+/* Taken from defines.h in shadow-utils. On Linux, this value is much smaller
+ * than the sysconf limit LOGIN_NAME_MAX, and values larger than this will
+ * result in failure when running useradd. We could check UT_NAMESIZE instead,
+ * but that is nonstandard. Better to use POSIX utmpx.
+ */
+#define MAXNAMELEN  (sizeof (((struct utmpx *)NULL)->ut_user))
 
 static gboolean
 is_username_used (const gchar *username)