[php-src] Issue #9151: MySQL connections are not released synchronously

Issue: https://github.com/php/php-src/issues/9151 Author: mvorisek ### Description The following code: ```php <?php $dbDsn = 'mysql:host=127.0.0.1;dbname=testdb'; $dbUser = 'user'; $dbPassword = 'user_password'; $dbRootPassword = 'root_password'; $rootPdo = new \PDO($dbDsn, 'root', $dbRootPassword); $rootPdo->exec('ALTER USER \'' . $dbUser . '\'@\'%\' WITH MAX_USER_CONNECTIONS 1'); $rootPdo = null; foreach (range(0, 50_000) as $i) { try { $pdo = new \PDO($dbDsn, $dbUser, $dbPassword); $res = $pdo->query('SELECT \'test\' as v')->fetchAll(\PDO::FETCH_ASSOC); if ($res !== [['v' => 'test']]) { throw new \Error('Unexpected result: ' . serialize($res)); } } catch (\Exception $e) { throw new \Exception('Failed after ' . $i . ' iterations', 0, $e); } finally { $pdo = null; } } ``` Resulted in this output: ``` Exception: Failed after 3945 iterations Caused by PDOException: SQLSTATE[HY000] [1226] User 'user' has exceeded the 'max_user_connections' resource (current value: 1) ``` but I expected no output/error instead. This issue is present with both MySQL mysqli and pdo_mysql drivers. I tested also pdo_pgsql driver (ALTER ROLE user CONNECTION LIMIT 1) and PostgreSQL is unaffected. It seems the native mysql driver does release the MySQL connection too early and does not synchronously wait until the connection is fully released from the database. ### PHP Version 7.4 - master ### Operating System linux