comparison vendor/symfony/http-foundation/Session/Storage/Handler/PdoSessionHandler.php @ 0:4c8ae668cc8c

Initial import (non-working)

+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <[e-mail]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\HttpFoundation\Session\Storage\Handler;
+
+/**
+ * Session handler using a PDO connection to read and write data.
+ *
+ * It works with MySQL, PostgreSQL, Oracle, SQL Server and SQLite and implements
+ * different locking strategies to handle concurrent access to the same session.
+ * Locking is necessary to prevent loss of data due to race conditions and to keep
+ * the session data consistent between read() and write(). With locking, requests
+ * for the same session will wait until the other one finished writing. For this
+ * reason it's best practice to close a session as early as possible to improve
+ * concurrency. PHPs internal files session handler also implements locking.
+ *
+ * Attention: Since SQLite does not support row level locks but locks the whole database,
+ * it means only one session can be accessed at a time. Even different sessions would wait
+ * for another to finish. So saving session in SQLite should only be considered for
+ * development or prototypes.
+ *
+ * Session data is a binary string that can contain non-printable characters like the null byte.
+ * For this reason it must be saved in a binary column in the database like BLOB in MySQL.
+ * Saving it in a character column could corrupt the data. You can use createTable()
+ * to initialize a correctly defined table.
+ *
+ * @see http://php.net/sessionhandlerinterface
+ *
+ * @author Fabien Potencier <[e-mail]>
+ * @author Michael Williams <[e-mail]>
+ * @author Tobias Schultze <http://tobion.de>
+ */
+class PdoSessionHandler implements \SessionHandlerInterface
+{
+    /**
+     * No locking is done. This means sessions are prone to loss of data due to
+     * race conditions of concurrent requests to the same session. The last session
+     * write will win in this case. It might be useful when you implement your own
+     * logic to deal with this like an optimistic approach.
+     */
+    const LOCK_NONE = 0;
+
+    /**
+     * Creates an application-level lock on a session. The disadvantage is that the
+     * lock is not enforced by the database and thus other, unaware parts of the
+     * application could still concurrently modify the session. The advantage is it
+     * does not require a transaction.
+     * This mode is not available for SQLite and not yet implemented for oci and sqlsrv.
+     */
+    const LOCK_ADVISORY = 1;
+
+    /**
+     * Issues a real row lock. Since it uses a transaction between opening and
+     * closing a session, you have to be careful when you use same database connection
+     * that you also use for your application logic. This mode is the default because
+     * it's the only reliable solution across DBMSs.
+     */
+    const LOCK_TRANSACTIONAL = 2;
+
+    /**
+     * @var \PDO|null PDO instance or null when not connected yet
+     */
+    private $pdo;
+
+    /**
+     * @var string|null|false DSN string or null for session.save_path or false when lazy connection disabled
+     */
+    private $dsn = false;
+
+    /**
+     * @var string Database driver
+     */
+    private $driver;
+
+    /**
+     * @var string Table name
+     */
+    private $table = 'sessions';
+
+    /**
+     * @var string Column for session id
+     */
+    private $idCol = 'sess_id';
+
+    /**
+     * @var string Column for session data
+     */
+    private $dataCol = 'sess_data';
+
+    /**
+     * @var string Column for lifetime
+     */
+    private $lifetimeCol = 'sess_lifetime';
+
+    /**
+     * @var string Column for timestamp
+     */
+    private $timeCol = 'sess_time';
+
+    /**
+     * @var string Username when lazy-connect
+     */
+    private $username = '';
+
+    /**
+     * @var string Password when lazy-connect
+     */
+    private $password = '';
+
+    /**
+     * @var array Connection options when lazy-connect
+     */
+    private $connectionOptions = array();
+
+    /**
+     * @var int The strategy for locking, see constants
+     */
+    private $lockMode = self::LOCK_TRANSACTIONAL;
+
+    /**
+     * It's an array to support multiple reads before closing which is manual, non-standard usage.
+     *
+     * @var \PDOStatement[] An array of statements to release advisory locks
+     */
+    private $unlockStatements = array();
+
+    /**
+     * @var bool True when the current session exists but expired according to session.gc_maxlifetime
+     */
+    private $sessionExpired = false;
+
+    /**
+     * @var bool Whether a transaction is active
+     */
+    private $inTransaction = false;
+
+    /**
+     * @var bool Whether gc() has been called
+     */
+    private $gcCalled = false;
+
+    /**
+     * Constructor.
+     *
+     * You can either pass an existing database connection as PDO instance or
+     * pass a DSN string that will be used to lazy-connect to the database
+     * when the session is actually used. Furthermore it's possible to pass null
+     * which will then use the session.save_path ini setting as PDO DSN parameter.
+     *
+     * List of available options:
+     *  * db_table: The name of the table [default: sessions]
+     *  * db_id_col: The column where to store the session id [default: sess_id]
+     *  * db_data_col: The column where to store the session data [default: sess_data]
+     *  * db_lifetime_col: The column where to store the lifetime [default: sess_lifetime]
+     *  * db_time_col: The column where to store the timestamp [default: sess_time]
+     *  * db_username: The username when lazy-connect [default: '']
+     *  * db_password: The password when lazy-connect [default: '']
+     *  * db_connection_options: An array of driver-specific connection options [default: array()]
+     *  * lock_mode: The strategy for locking, see constants [default: LOCK_TRANSACTIONAL]
+     *
+     * @param \PDO|string|null $pdoOrDsn A \PDO instance or DSN string or null
+     * @param array            $options  An associative array of options
+     *
+     * @throws \InvalidArgumentException When PDO error mode is not PDO::ERRMODE_EXCEPTION
+     */
+    public function __construct($pdoOrDsn = null, array $options = array())
+    {
+        if ($pdoOrDsn instanceof \PDO) {
+            if (\PDO::ERRMODE_EXCEPTION !== $pdoOrDsn->getAttribute(\PDO::ATTR_ERRMODE)) {
+                throw new \InvalidArgumentException(sprintf('"%s" requires PDO error mode attribute be set to throw Exceptions (i.e. $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION))', __CLASS__));
+            }
+
+            $this->pdo = $pdoOrDsn;
+            $this->driver = $this->pdo->getAttribute(\PDO::ATTR_DRIVER_NAME);
+        } else {
+            $this->dsn = $pdoOrDsn;
+        }
+
+        $this->table = isset($options['db_table']) ? $options['db_table'] : $this->table;
+        $this->idCol = isset($options['db_id_col']) ? $options['db_id_col'] : $this->idCol;
+        $this->dataCol = isset($options['db_data_col']) ? $options['db_data_col'] : $this->dataCol;
+        $this->lifetimeCol = isset($options['db_lifetime_col']) ? $options['db_lifetime_col'] : $this->lifetimeCol;
+        $this->timeCol = isset($options['db_time_col']) ? $options['db_time_col'] : $this->timeCol;
+        $this->username = isset($options['db_username']) ? $options['db_username'] : $this->username;
+        $this->password = isset($options['db_password']) ? $options['db_password'] : $this->password;
+        $this->connectionOptions = isset($options['db_connection_options']) ? $options['db_connection_options'] : $this->connectionOptions;
+        $this->lockMode = isset($options['lock_mode']) ? $options['lock_mode'] : $this->lockMode;
+    }
+
+    /**
+     * Creates the table to store sessions which can be called once for setup.
+     *
+     * Session ID is saved in a column of maximum length 128 because that is enough even
+     * for a 512 bit configured session.hash_function like Whirlpool. Session data is
+     * saved in a BLOB. One could also use a shorter inlined varbinary column
+     * if one was sure the data fits into it.
+     *
+     * @throws \PDOException    When the table already exists
+     * @throws \DomainException When an unsupported PDO driver is used
+     */
+    public function createTable()
+    {
+        // connect if we are not yet
+        $this->getConnection();
+
+        switch ($this->driver) {
+            case 'mysql':
+                // We use varbinary for the ID column because it prevents unwanted conversions:
+                // - character set conversions between server and client
+                // - trailing space removal
+                // - case-insensitivity
+                // - language processing like é == e
+                $sql = "CREATE TABLE $this->table ($this->idCol VARBINARY(128) NOT NULL PRIMARY KEY, $this->dataCol BLOB NOT NULL, $this->lifetimeCol MEDIUMINT NOT NULL, $this->timeCol INTEGER UNSIGNED NOT NULL) COLLATE utf8_bin, ENGINE = InnoDB";
+                break;
+            case 'sqlite':
+                $sql = "CREATE TABLE $this->table ($this->idCol TEXT NOT NULL PRIMARY KEY, $this->dataCol BLOB NOT NULL, $this->lifetimeCol INTEGER NOT NULL, $this->timeCol INTEGER NOT NULL)";
+                break;
+            case 'pgsql':
+                $sql = "CREATE TABLE $this->table ($this->idCol VARCHAR(128) NOT NULL PRIMARY KEY, $this->dataCol BYTEA NOT NULL, $this->lifetimeCol INTEGER NOT NULL, $this->timeCol INTEGER NOT NULL)";
+                break;
+            case 'oci':
+                $sql = "CREATE TABLE $this->table ($this->idCol VARCHAR2(128) NOT NULL PRIMARY KEY, $this->dataCol BLOB NOT NULL, $this->lifetimeCol INTEGER NOT NULL, $this->timeCol INTEGER NOT NULL)";
+                break;
+            case 'sqlsrv':
+                $sql = "CREATE TABLE $this->table ($this->idCol VARCHAR(128) NOT NULL PRIMARY KEY, $this->dataCol VARBINARY(MAX) NOT NULL, $this->lifetimeCol INTEGER NOT NULL, $this->timeCol INTEGER NOT NULL)";
+                break;
+            default:
+                throw new \DomainException(sprintf('Creating the session table is currently not implemented for PDO driver "%s".', $this->driver));
+        }
+
+        try {
+            $this->pdo->exec($sql);
+        } catch (\PDOException $e) {
+            $this->rollback();
+
+            throw $e;
+        }
+    }
+
+    /**
+     * Returns true when the current session exists but expired according to session.gc_maxlifetime.
+     *
+     * Can be used to distinguish between a new session and one that expired due to inactivity.
+     *
+     * @return bool Whether current session expired
+     */
+    public function isSessionExpired()
+    {
+        return $this->sessionExpired;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function open($savePath, $sessionName)
+    {
+        if (null === $this->pdo) {
+            $this->connect($this->dsn ?: $savePath);
+        }
+
+        return true;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function read($sessionId)
+    {
+        try {
+            return $this->doRead($sessionId);
+        } catch (\PDOException $e) {
+            $this->rollback();
+
+            throw $e;
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function gc($maxlifetime)
+    {
+        // We delay gc() to close() so that it is executed outside the transactional and blocking read-write process.
+        // This way, pruning expired sessions does not block them from being started while the current session is used.
+        $this->gcCalled = true;
+
+        return true;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function destroy($sessionId)
+    {
+        // delete the record associated with this id
+        $sql = "DELETE FROM $this->table WHERE $this->idCol = :id";
+
+        try {
+            $stmt = $this->pdo->prepare($sql);
+            $stmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
+            $stmt->execute();
+        } catch (\PDOException $e) {
+            $this->rollback();
+
+            throw $e;
+        }
+
+        return true;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function write($sessionId, $data)
+    {
+        $maxlifetime = (int) ini_get('session.gc_maxlifetime');
+
+        try {
+            // We use a single MERGE SQL query when supported by the database.
+            $mergeStmt = $this->getMergeStatement($sessionId, $data, $maxlifetime);
+            if (null !== $mergeStmt) {
+                $mergeStmt->execute();
+
+                return true;
+            }
+
+            $updateStmt = $this->pdo->prepare(
+                "UPDATE $this->table SET $this->dataCol = :data, $this->lifetimeCol = :lifetime, $this->timeCol = :time WHERE $this->idCol = :id"
+            );
+            $updateStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
+            $updateStmt->bindParam(':data', $data, \PDO::PARAM_LOB);
+            $updateStmt->bindParam(':lifetime', $maxlifetime, \PDO::PARAM_INT);
+            $updateStmt->bindValue(':time', time(), \PDO::PARAM_INT);
+            $updateStmt->execute();
+
+            // When MERGE is not supported, like in Postgres < 9.5, we have to use this approach that can result in
+            // duplicate key errors when the same session is written simultaneously (given the LOCK_NONE behavior).
+            // We can just catch such an error and re-execute the update. This is similar to a serializable
+            // transaction with retry logic on serialization failures but without the overhead and without possible
+            // false positives due to longer gap locking.
+            if (!$updateStmt->rowCount()) {
+                try {
+                    $insertStmt = $this->pdo->prepare(
+                        "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->lifetimeCol, $this->timeCol) VALUES (:id, :data, :lifetime, :time)"
+                    );
+                    $insertStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
+                    $insertStmt->bindParam(':data', $data, \PDO::PARAM_LOB);
+                    $insertStmt->bindParam(':lifetime', $maxlifetime, \PDO::PARAM_INT);
+                    $insertStmt->bindValue(':time', time(), \PDO::PARAM_INT);
+                    $insertStmt->execute();
+                } catch (\PDOException $e) {
+                    // Handle integrity violation SQLSTATE 23000 (or a subclass like 23505 in Postgres) for duplicate keys
+                    if (0 === strpos($e->getCode(), '23')) {
+                        $updateStmt->execute();
+                    } else {
+                        throw $e;
+                    }
+                }
+            }
+        } catch (\PDOException $e) {
+            $this->rollback();
+
+            throw $e;
+        }
+
+        return true;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function close()
+    {
+        $this->commit();
+
+        while ($unlockStmt = array_shift($this->unlockStatements)) {
+            $unlockStmt->execute();
+        }
+
+        if ($this->gcCalled) {
+            $this->gcCalled = false;
+
+            // delete the session records that have expired
+            $sql = "DELETE FROM $this->table WHERE $this->lifetimeCol + $this->timeCol < :time";
+
+            $stmt = $this->pdo->prepare($sql);
+            $stmt->bindValue(':time', time(), \PDO::PARAM_INT);
+            $stmt->execute();
+        }
+
+        if (false !== $this->dsn) {
+            $this->pdo = null; // only close lazy-connection
+        }
+
+        return true;
+    }
+
+    /**
+     * Lazy-connects to the database.
+     *
+     * @param string $dsn DSN string
+     */
+    private function connect($dsn)
+    {
+        $this->pdo = new \PDO($dsn, $this->username, $this->password, $this->connectionOptions);
+        $this->pdo->setAttribute(\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
+        $this->driver = $this->pdo->getAttribute(\PDO::ATTR_DRIVER_NAME);
+    }
+
+    /**
+     * Helper method to begin a transaction.
+     *
+     * Since SQLite does not support row level locks, we have to acquire a reserved lock
+     * on the database immediately. Because of https://bugs.php.net/42766 we have to create
+     * such a transaction manually which also means we cannot use PDO::commit or
+     * PDO::rollback or PDO::inTransaction for SQLite.
+     *
+     * Also MySQLs default isolation, REPEATABLE READ, causes deadlock for different sessions
+     * due to http://www.mysqlperformanceblog.com/2013/12/12/one-more-innodb-gap-lock-to-avoid/ .
+     * So we change it to READ COMMITTED.
+     */
+    private function beginTransaction()
+    {
+        if (!$this->inTransaction) {
+            if ('sqlite' === $this->driver) {
+                $this->pdo->exec('BEGIN IMMEDIATE TRANSACTION');
+            } else {
+                if ('mysql' === $this->driver) {
+                    $this->pdo->exec('SET TRANSACTION ISOLATION LEVEL READ COMMITTED');
+                }
+                $this->pdo->beginTransaction();
+            }
+            $this->inTransaction = true;
+        }
+    }
+
+    /**
+     * Helper method to commit a transaction.
+     */
+    private function commit()
+    {
+        if ($this->inTransaction) {
+            try {
+                // commit read-write transaction which also releases the lock
+                if ('sqlite' === $this->driver) {
+                    $this->pdo->exec('COMMIT');
+                } else {
+                    $this->pdo->commit();
+                }
+                $this->inTransaction = false;
+            } catch (\PDOException $e) {
+                $this->rollback();
+
+                throw $e;
+            }
+        }
+    }
+
+    /**
+     * Helper method to rollback a transaction.
+     */
+    private function rollback()
+    {
+        // We only need to rollback if we are in a transaction. Otherwise the resulting
+        // error would hide the real problem why rollback was called. We might not be
+        // in a transaction when not using the transactional locking behavior or when
+        // two callbacks (e.g. destroy and write) are invoked that both fail.
+        if ($this->inTransaction) {
+            if ('sqlite' === $this->driver) {
+                $this->pdo->exec('ROLLBACK');
+            } else {
+                $this->pdo->rollBack();
+            }
+            $this->inTransaction = false;
+        }
+    }
+
+    /**
+     * Reads the session data in respect to the different locking strategies.
+     *
+     * We need to make sure we do not return session data that is already considered garbage according
+     * to the session.gc_maxlifetime setting because gc() is called after read() and only sometimes.
+     *
+     * @param string $sessionId Session ID
+     *
+     * @return string The session data
+     */
+    private function doRead($sessionId)
+    {
+        $this->sessionExpired = false;
+
+        if (self::LOCK_ADVISORY === $this->lockMode) {
+            $this->unlockStatements[] = $this->doAdvisoryLock($sessionId);
+        }
+
+        $selectSql = $this->getSelectSql();
+        $selectStmt = $this->pdo->prepare($selectSql);
+        $selectStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
+
+        do {
+            $selectStmt->execute();
+            $sessionRows = $selectStmt->fetchAll(\PDO::FETCH_NUM);
+
+            if ($sessionRows) {
+                if ($sessionRows[0][1] + $sessionRows[0][2] < time()) {
+                    $this->sessionExpired = true;
+
+                    return '';
+                }
+
+                return is_resource($sessionRows[0][0]) ? stream_get_contents($sessionRows[0][0]) : $sessionRows[0][0];
+            }
+
+            if (self::LOCK_TRANSACTIONAL === $this->lockMode && 'sqlite' !== $this->driver) {
+                // Exclusive-reading of non-existent rows does not block, so we need to do an insert to block
+                // until other connections to the session are committed.
+                try {
+                    $insertStmt = $this->pdo->prepare(
+                        "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->lifetimeCol, $this->timeCol) VALUES (:id, :data, :lifetime, :time)"
+                    );
+                    $insertStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
+                    $insertStmt->bindValue(':data', '', \PDO::PARAM_LOB);
+                    $insertStmt->bindValue(':lifetime', 0, \PDO::PARAM_INT);
+                    $insertStmt->bindValue(':time', time(), \PDO::PARAM_INT);
+                    $insertStmt->execute();
+                } catch (\PDOException $e) {
+                    // Catch duplicate key error because other connection created the session already.
+                    // It would only not be the case when the other connection destroyed the session.
+                    if (0 === strpos($e->getCode(), '23')) {
+                        // Retrieve finished session data written by concurrent connection by restarting the loop.
+                        // We have to start a new transaction as a failed query will mark the current transaction as
+                        // aborted in PostgreSQL and disallow further queries within it.
+                        $this->rollback();
+                        $this->beginTransaction();
+                        continue;
+                    }
+
+                    throw $e;
+                }
+            }
+
+            return '';
+        } while (true);
+    }
+
+    /**
+     * Executes an application-level lock on the database.
+     *
+     * @param string $sessionId Session ID
+     *
+     * @return \PDOStatement The statement that needs to be executed later to release the lock
+     *
+     * @throws \DomainException When an unsupported PDO driver is used
+     *
+     * @todo implement missing advisory locks
+     *       - for oci using DBMS_LOCK.REQUEST
+     *       - for sqlsrv using sp_getapplock with LockOwner = Session
+     */
+    private function doAdvisoryLock($sessionId)
+    {
+        switch ($this->driver) {
+            case 'mysql':
+                // should we handle the return value? 0 on timeout, null on error
+                // we use a timeout of 50 seconds which is also the default for innodb_lock_wait_timeout
+                $stmt = $this->pdo->prepare('SELECT GET_LOCK(:key, 50)');
+                $stmt->bindValue(':key', $sessionId, \PDO::PARAM_STR);
+                $stmt->execute();
+
+                $releaseStmt = $this->pdo->prepare('DO RELEASE_LOCK(:key)');
+                $releaseStmt->bindValue(':key', $sessionId, \PDO::PARAM_STR);
+
+                return $releaseStmt;
+            case 'pgsql':
+                // Obtaining an exclusive session level advisory lock requires an integer key.
+                // So we convert the HEX representation of the session id to an integer.
+                // Since integers are signed, we have to skip one hex char to fit in the range.
+                if (4 === PHP_INT_SIZE) {
+                    $sessionInt1 = hexdec(substr($sessionId, 0, 7));
+                    $sessionInt2 = hexdec(substr($sessionId, 7, 7));
+
+                    $stmt = $this->pdo->prepare('SELECT pg_advisory_lock(:key1, :key2)');
+                    $stmt->bindValue(':key1', $sessionInt1, \PDO::PARAM_INT);
+                    $stmt->bindValue(':key2', $sessionInt2, \PDO::PARAM_INT);
+                    $stmt->execute();
+
+                    $releaseStmt = $this->pdo->prepare('SELECT pg_advisory_unlock(:key1, :key2)');
+                    $releaseStmt->bindValue(':key1', $sessionInt1, \PDO::PARAM_INT);
+                    $releaseStmt->bindValue(':key2', $sessionInt2, \PDO::PARAM_INT);
+                } else {
+                    $sessionBigInt = hexdec(substr($sessionId, 0, 15));
+
+                    $stmt = $this->pdo->prepare('SELECT pg_advisory_lock(:key)');
+                    $stmt->bindValue(':key', $sessionBigInt, \PDO::PARAM_INT);
+                    $stmt->execute();
+
+                    $releaseStmt = $this->pdo->prepare('SELECT pg_advisory_unlock(:key)');
+                    $releaseStmt->bindValue(':key', $sessionBigInt, \PDO::PARAM_INT);
+                }
+
+                return $releaseStmt;
+            case 'sqlite':
+                throw new \DomainException('SQLite does not support advisory locks.');
+            default:
+                throw new \DomainException(sprintf('Advisory locks are currently not implemented for PDO driver "%s".', $this->driver));
+        }
+    }
+
+    /**
+     * Return a locking or nonlocking SQL query to read session information.
+     *
+     * @return string The SQL string
+     *
+     * @throws \DomainException When an unsupported PDO driver is used
+     */
+    private function getSelectSql()
+    {
+        if (self::LOCK_TRANSACTIONAL === $this->lockMode) {
+            $this->beginTransaction();
+
+            switch ($this->driver) {
+                case 'mysql':
+                case 'oci':
+                case 'pgsql':
+                    return "SELECT $this->dataCol, $this->lifetimeCol, $this->timeCol FROM $this->table WHERE $this->idCol = :id FOR UPDATE";
+                case 'sqlsrv':
+                    return "SELECT $this->dataCol, $this->lifetimeCol, $this->timeCol FROM $this->table WITH (UPDLOCK, ROWLOCK) WHERE $this->idCol = :id";
+                case 'sqlite':
+                    // we already locked when starting transaction
+                    break;
+                default:
+                    throw new \DomainException(sprintf('Transactional locks are currently not implemented for PDO driver "%s".', $this->driver));
+            }
+        }
+
+        return "SELECT $this->dataCol, $this->lifetimeCol, $this->timeCol FROM $this->table WHERE $this->idCol = :id";
+    }
+
+    /**
+     * Returns a merge/upsert (i.e. insert or update) statement when supported by the database for writing session data.
+     *
+     * @param string $sessionId   Session ID
+     * @param string $data        Encoded session data
+     * @param int    $maxlifetime session.gc_maxlifetime
+     *
+     * @return \PDOStatement|null The merge statement or null when not supported
+     */
+    private function getMergeStatement($sessionId, $data, $maxlifetime)
+    {
+        $mergeSql = null;
+        switch (true) {
+            case 'mysql' === $this->driver:
+                $mergeSql = "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->lifetimeCol, $this->timeCol) VALUES (:id, :data, :lifetime, :time) ".
+                    "ON DUPLICATE KEY UPDATE $this->dataCol = VALUES($this->dataCol), $this->lifetimeCol = VALUES($this->lifetimeCol), $this->timeCol = VALUES($this->timeCol)";
+                break;
+            case 'oci' === $this->driver:
+                // DUAL is Oracle specific dummy table
+                $mergeSql = "MERGE INTO $this->table USING DUAL ON ($this->idCol = ?) ".
+                    "WHEN NOT MATCHED THEN INSERT ($this->idCol, $this->dataCol, $this->lifetimeCol, $this->timeCol) VALUES (?, ?, ?, ?) ".
+                    "WHEN MATCHED THEN UPDATE SET $this->dataCol = ?, $this->lifetimeCol = ?, $this->timeCol = ?";
+                break;
+            case 'sqlsrv' === $this->driver && version_compare($this->pdo->getAttribute(\PDO::ATTR_SERVER_VERSION), '10', '>='):
+                // MERGE is only available since SQL Server 2008 and must be terminated by semicolon
+                // It also requires HOLDLOCK according to http://weblogs.sqlteam.com/dang/archive/2009/01/31/UPSERT-Race-Condition-With-MERGE.aspx
+                $mergeSql = "MERGE INTO $this->table WITH (HOLDLOCK) USING (SELECT 1 AS dummy) AS src ON ($this->idCol = ?) ".
+                    "WHEN NOT MATCHED THEN INSERT ($this->idCol, $this->dataCol, $this->lifetimeCol, $this->timeCol) VALUES (?, ?, ?, ?) ".
+                    "WHEN MATCHED THEN UPDATE SET $this->dataCol = ?, $this->lifetimeCol = ?, $this->timeCol = ?;";
+                break;
+            case 'sqlite' === $this->driver:
+                $mergeSql = "INSERT OR REPLACE INTO $this->table ($this->idCol, $this->dataCol, $this->lifetimeCol, $this->timeCol) VALUES (:id, :data, :lifetime, :time)";
+                break;
+            case 'pgsql' === $this->driver && version_compare($this->pdo->getAttribute(\PDO::ATTR_SERVER_VERSION), '9.5', '>='):
+                $mergeSql = "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->lifetimeCol, $this->timeCol) VALUES (:id, :data, :lifetime, :time) ".
+                    "ON CONFLICT ($this->idCol) DO UPDATE SET ($this->dataCol, $this->lifetimeCol, $this->timeCol) = (EXCLUDED.$this->dataCol, EXCLUDED.$this->lifetimeCol, EXCLUDED.$this->timeCol)";
+                break;
+        }
+
+        if (null !== $mergeSql) {
+            $mergeStmt = $this->pdo->prepare($mergeSql);
+
+            if ('sqlsrv' === $this->driver || 'oci' === $this->driver) {
+                $mergeStmt->bindParam(1, $sessionId, \PDO::PARAM_STR);
+                $mergeStmt->bindParam(2, $sessionId, \PDO::PARAM_STR);
+                $mergeStmt->bindParam(3, $data, \PDO::PARAM_LOB);
+                $mergeStmt->bindParam(4, $maxlifetime, \PDO::PARAM_INT);
+                $mergeStmt->bindValue(5, time(), \PDO::PARAM_INT);
+                $mergeStmt->bindParam(6, $data, \PDO::PARAM_LOB);
+                $mergeStmt->bindParam(7, $maxlifetime, \PDO::PARAM_INT);
+                $mergeStmt->bindValue(8, time(), \PDO::PARAM_INT);
+            } else {
+                $mergeStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
+                $mergeStmt->bindParam(':data', $data, \PDO::PARAM_LOB);
+                $mergeStmt->bindParam(':lifetime', $maxlifetime, \PDO::PARAM_INT);
+                $mergeStmt->bindValue(':time', time(), \PDO::PARAM_INT);
+            }
+
+            return $mergeStmt;
+        }
+    }
+
+    /**
+     * Return a PDO instance.
+     *
+     * @return \PDO
+     */
+    protected function getConnection()
+    {
+        if (null === $this->pdo) {
+            $this->connect($this->dsn ?: ini_get('session.save_path'));
+        }
+
+        return $this->pdo;
+    }
+}