on creation only test for existing users if the backend supports user creation

this solves the issue where no users can be created any more if backends are active which always return true on userExists() like WebDAV Auth

lib/user.php

@@ -172,7 +172,7 @@ class OC_User {
 		}
 
 		// Check if user already exists
-		if( self::userExists($uid) ) {
+		if( self::userExistsForCreation($uid) ) {
 			throw new Exception('The username is already being used');
 		}
 
@@ -551,6 +551,19 @@ class OC_User {
 		return false;
 	}
 
+    public static function userExistsForCreation($uid) {
+        foreach(self::$_usedBackends as $backend) {
+            if(!$backend->implementsActions(OC_USER_BACKEND_CREATE_USER))
+                continue;
+
+            $result=$backend->userExists($uid);
+            if($result===true) {
+                return true;
+            }
+        }
+        return false;
+    }
+
 	/**
 	 * disables a user
 	 * @param string $userid the user to disable

settings/ajax/createuser.php

@@ -26,12 +26,6 @@ if(OC_User::isAdminUser(OC_User::getUser())) {
 $username = $_POST["username"];
 $password = $_POST["password"];
 
-// Does the group exist?
-if(OC_User::userExists($username)) {
-	OC_JSON::error(array("data" => array( "message" => "User already exists" )));
-	exit();
-}
-
 // Return Success story
 try {
 	if (!OC_User::createUser($username, $password)) {

settings/js/users.js

@@ -400,12 +400,6 @@ $(document).ready(function () {
         event.preventDefault();
         var username = $('#newusername').val();
         var password = $('#newuserpassword').val();
-        if ($('#content table tbody tr').filterAttr('data-uid', username).length > 0) {
-            OC.dialogs.alert(
-                t('settings', 'The username is already being used'),
-                t('settings', 'Error creating user'));
-            return;
-        }
         if ($.trim(username) == '') {
             OC.dialogs.alert(
                 t('settings', 'A valid username must be provided'),