General: Check that user login is stored faithfully by the database.

This checks that if user logins etc. cannot be stored faithfully in the
database, the write operation will fail rather than corrupt user data (and
potentially lock out users).

Props dmsnell.

Author: arnt

tests/phpunit/tests/user.php

@@ -732,6 +732,34 @@ public function test_user_update_email_error() {
 		}
 	}
 
+	/**
+	 * Our goal here is to test whether user data can be corrupted
+	 * while being stored in the database. The user data can be almost
+	 * any unicode, while the database might theoretically use e.g.
+	 * ISO-8859-1. ISO-8859-1 can handle a user with login Noël but
+	 * will mishandle a user with login Łukasz.
+	 *
+	 * The database tests provide coverage for this kind of thing
+	 * in general; this test exists to provide additional coverage
+	 * against the risk of locking a user out.
+	 *
+	 * Since the database in the unit test harness uses UTF-8, this test
+	 * needs to set a user login that UTF-8 cannot handle. 0xC0 is that
+	 * (0xC0 never occurs in a valid UTF-8 string). If wpdb refuses to
+	 * store that, we trust that the same logic will also refuse to store
+	 * other impossible strings, such as Łukasz in a database that cannot
+	 * store Ł.
+	 *
+	 * Based on this review comment: https://github.com/WordPress/wordpress-develop/pull/5237#issuecomment-3504963005
+	 *
+	 * @ticket 31992
+	 */
+	public function test_user_corrupted_login() {
+		global $wpdb;
+		$rows = $wpdb->update( $wpdb->users, array( 'user_login' => hex2bin("c0") ), array( 'ID' => $this->author->ID ) );
+		$this->assertFalse($rows);
+	}
+
 	/**
 	 * @ticket 27317
 	 * @dataProvider data_illegal_user_logins