MDL-63174 user: user_create_user to throw exception on empty username

The core API function user_create_user() did not check the case when the
given username was empty.

Also adding a missing string 'usernamelowercase' for the existing lower
case check and unit tests.

lang/en/error.php

@@ -364,6 +364,7 @@ $string['invaliduserid'] = 'Invalid user id';
 $string['invaliduserfield'] = 'Invalid user field: {$a}';
 $string['invaliduserdata'] = 'Invalid user data: {$a}';
 $string['invalidusername'] = 'The given username contains invalid characters';
+$string['invalidusernameblank'] = 'The username cannot be blank';
 $string['invalidxmlfile'] = '"{$a}" is not a valid XML file';
 $string['iplookupfailed'] = 'Cannot find geo information about this IP address {$a}';
 $string['iplookupprivate'] = 'Cannot display lookup of private IP address';
@@ -565,6 +566,7 @@ $string['userautherror'] = 'Unknown auth plugin';
 $string['userauthunsupported'] = 'Auth plugin not supported here';
 $string['useremailduplicate'] = 'Duplicate address';
 $string['usermustbemnet'] = 'Users in the MNET access control list must be remote MNET users';
+$string['usernamelowercase'] = 'The username must be in lower case';
 $string['usernotaddederror'] = 'User not added - error';
 $string['usernotaddedregistered'] = 'User not added - already registered';
 $string['usernotavailable'] = 'The details of this user are not available to you';

user/lib.php

@@ -48,12 +48,16 @@ function user_create_user($user, $updatepassword = true, $triggerevent = true) {
     }
 
     // Check username.
+    if (trim($user->username) === '') {
+        throw new moodle_exception('invalidusernameblank');
+    }
+
     if ($user->username !== core_text::strtolower($user->username)) {
         throw new moodle_exception('usernamelowercase');
-    } else {
-        if ($user->username !== core_user::clean_field($user->username, 'username')) {
-            throw new moodle_exception('invalidusername');
-        }
+    }
+
+    if ($user->username !== core_user::clean_field($user->username, 'username')) {
+        throw new moodle_exception('invalidusername');
     }
 
     // Save the password in a temp value for later.

user/tests/userlib_test.php

@@ -241,6 +241,55 @@ class core_userliblib_testcase extends advanced_testcase {
         $this->assertDebuggingNotCalled();
     }
 
+    /**
+     * Test that {@link user_create_user()} throws exception when invalid username is provided.
+     *
+     * @dataProvider data_create_user_invalid_username
+     * @param string $username Invalid username
+     * @param string $expectmessage Expected exception message
+     */
+    public function test_create_user_invalid_username($username, $expectmessage) {
+        global $CFG;
+
+        $this->resetAfterTest();
+        $CFG->extendedusernamechars = false;
+
+        $user = [
+            'username' => $username,
+        ];
+
+        $this->expectException('moodle_exception');
+        $this->expectExceptionMessage($expectmessage);
+
+        user_create_user($user);
+    }
+
+    /**
+     * Data provider for {@link self::test_create_user_invalid_username()}.
+     *
+     * @return array
+     */
+    public function data_create_user_invalid_username() {
+        return [
+            'empty_string' => [
+                '',
+                'The username cannot be blank',
+            ],
+            'only_whitespace' => [
+                "\t\t  \t\n ",
+                'The username cannot be blank',
+            ],
+            'lower_case' => [
+                'Mudrd8mz',
+                'The username must be in lower case',
+            ],
+            'extended_chars' => [
+                'dmudrák',
+                'The given username contains invalid characters',
+            ],
+        ];
+    }
+
     /**
      * Test function user_count_login_failures().
      */