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[HttpFoundation] fix PDO session handler under high concurrency #10652

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merged 3 commits into from Apr 11, 2014
Merged

[HttpFoundation] fix PDO session handler under high concurrency #10652

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e58d7cf
[HttpFoundation] fix PDO session handler under high concurrency
Tobion Apr 8, 2014
05ea19a
[HttpFoundation] use MERGE SQL for MS SQL Server session storage
Tobion Apr 8, 2014
5c08e29
[HttpFoundation] use insert or replace for sqlite session handler
Tobion Apr 9, 2014
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184 changes: 92 additions & 92 deletions src/Symfony/Component/HttpFoundation/Session/Storage/Handler/PdoSessionHandler.php
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Expand Up @@ -16,18 +16,34 @@
*
* @author Fabien Potencier <fabien@symfony.com>
* @author Michael Williams <michael.williams@funsational.com>
* @author Tobias Schultze <http://tobion.de>
*/
class PdoSessionHandler implements \SessionHandlerInterface
{
/**
* @var \PDO PDO instance.
* @var \PDO PDO instance
*/
private $pdo;

/**
* @var array Database options.
* @var string Table name
*/
private $dbOptions;
private $table;

/**
* @var string Column for session id
*/
private $idCol;

/**
* @var string Column for session data
*/
private $dataCol;

/**
* @var string Column for timestamp
*/
private $timeCol;

/**
* Constructor.
Expand All @@ -52,11 +68,16 @@ public function __construct(\PDO $pdo, array $dbOptions = array())
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 = $pdo;
$this->dbOptions = array_merge(array(
$dbOptions = array_merge(array(
'db_id_col' => 'sess_id',
'db_data_col' => 'sess_data',
'db_time_col' => 'sess_time',
), $dbOptions);

$this->table = $dbOptions['db_table'];
$this->idCol = $dbOptions['db_id_col'];
$this->dataCol = $dbOptions['db_data_col'];
$this->timeCol = $dbOptions['db_time_col'];
}

/**
Expand All @@ -80,19 +101,15 @@ public function close()
*/
public function destroy($id)
{
// get table/column
$dbTable = $this->dbOptions['db_table'];
$dbIdCol = $this->dbOptions['db_id_col'];

// delete the record associated with this id
$sql = "DELETE FROM $dbTable WHERE $dbIdCol = :id";
$sql = "DELETE FROM $this->table WHERE $this->idCol = :id";

try {
$stmt = $this->pdo->prepare($sql);
$stmt->bindParam(':id', $id, \PDO::PARAM_STR);
$stmt->execute();
} catch (\PDOException $e) {
throw new \RuntimeException(sprintf('PDOException was thrown when trying to manipulate session data: %s', $e->getMessage()), 0, $e);
throw new \RuntimeException(sprintf('PDOException was thrown when trying to delete a session: %s', $e->getMessage()), 0, $e);
}

return true;
Expand All @@ -103,19 +120,15 @@ public function destroy($id)
*/
public function gc($lifetime)
{
// get table/column
$dbTable = $this->dbOptions['db_table'];
$dbTimeCol = $this->dbOptions['db_time_col'];

// delete the session records that have expired
$sql = "DELETE FROM $dbTable WHERE $dbTimeCol < :time";
$sql = "DELETE FROM $this->table WHERE $this->timeCol < :time";

try {
$stmt = $this->pdo->prepare($sql);
$stmt->bindValue(':time', time() - $lifetime, \PDO::PARAM_INT);
$stmt->execute();
} catch (\PDOException $e) {
throw new \RuntimeException(sprintf('PDOException was thrown when trying to manipulate session data: %s', $e->getMessage()), 0, $e);
throw new \RuntimeException(sprintf('PDOException was thrown when trying to delete expired sessions: %s', $e->getMessage()), 0, $e);
}

return true;
Expand All @@ -126,29 +139,20 @@ public function gc($lifetime)
*/
public function read($id)
{
// get table/columns
$dbTable = $this->dbOptions['db_table'];
$dbDataCol = $this->dbOptions['db_data_col'];
$dbIdCol = $this->dbOptions['db_id_col'];
$sql = "SELECT $this->dataCol FROM $this->table WHERE $this->idCol = :id";

try {
$sql = "SELECT $dbDataCol FROM $dbTable WHERE $dbIdCol = :id";

$stmt = $this->pdo->prepare($sql);
$stmt->bindParam(':id', $id, \PDO::PARAM_STR);

$stmt->execute();
// it is recommended to use fetchAll so that PDO can close the DB cursor
// we anyway expect either no rows, or one row with one column. fetchColumn, seems to be buggy #4777

// We use fetchAll instead of fetchColumn to make sure the DB cursor gets closed
$sessionRows = $stmt->fetchAll(\PDO::FETCH_NUM);
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This was one main problem: It creates duplicate keys when session created meanwhile (between select and insert). There is no need to create an entry in read() at all.


if (count($sessionRows) == 1) {
if ($sessionRows) {
return base64_decode($sessionRows[0][0]);
}

// session does not exist, create it
$this->createNewSession($id);

return '';
} catch (\PDOException $e) {
throw new \RuntimeException(sprintf('PDOException was thrown when trying to read the session data: %s', $e->getMessage()), 0, $e);
Expand All @@ -160,84 +164,80 @@ public function read($id)
*/
public function write($id, $data)
{
// get table/column
$dbTable = $this->dbOptions['db_table'];
$dbDataCol = $this->dbOptions['db_data_col'];
$dbIdCol = $this->dbOptions['db_id_col'];
$dbTimeCol = $this->dbOptions['db_time_col'];

//session data can contain non binary safe characters so we need to encode it
// Session data can contain non binary safe characters so we need to encode it.
$encoded = base64_encode($data);

// We use a MERGE SQL query when supported by the database.
// Otherwise we have to use a transactional DELETE followed by INSERT to prevent duplicate entries under high concurrency.

try {
$driver = $this->pdo->getAttribute(\PDO::ATTR_DRIVER_NAME);

if ('mysql' === $driver) {
// MySQL would report $stmt->rowCount() = 0 on UPDATE when the data is left unchanged
// it could result in calling createNewSession() whereas the session already exists in
// the DB which would fail as the id is unique
$stmt = $this->pdo->prepare(
"INSERT INTO $dbTable ($dbIdCol, $dbDataCol, $dbTimeCol) VALUES (:id, :data, :time) " .
"ON DUPLICATE KEY UPDATE $dbDataCol = VALUES($dbDataCol), $dbTimeCol = VALUES($dbTimeCol)"
$mergeSql = $this->getMergeSql();

if (null !== $mergeSql) {
$mergeStmt = $this->pdo->prepare($mergeSql);
$mergeStmt->bindParam(':id', $id, \PDO::PARAM_STR);
$mergeStmt->bindParam(':data', $encoded, \PDO::PARAM_STR);
$mergeStmt->bindValue(':time', time(), \PDO::PARAM_INT);
$mergeStmt->execute();

return true;
}

$this->pdo->beginTransaction();

try {
$deleteStmt = $this->pdo->prepare(
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For oracle sysdate was used which is of type DATE. But we expect an INTEGER column, otherwise gc() would not work. So this didn't make sense.

"DELETE FROM $this->table WHERE $this->idCol = :id"
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WHERE $dbIdCol = :id part is not needed and also non-standard SQL MERGE syntax. Tested with http://sqlfiddle.com/#!4/a91d6/11

);
$deleteStmt->bindParam(':id', $id, \PDO::PARAM_STR);
$deleteStmt->execute();

$insertStmt = $this->pdo->prepare(
"INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time)"
);
$stmt->bindParam(':id', $id, \PDO::PARAM_STR);
$stmt->bindParam(':data', $encoded, \PDO::PARAM_STR);
$stmt->bindValue(':time', time(), \PDO::PARAM_INT);
$stmt->execute();
} elseif ('oci' === $driver) {
$stmt = $this->pdo->prepare("MERGE INTO $dbTable USING DUAL ON($dbIdCol = :id) ".
"WHEN NOT MATCHED THEN INSERT ($dbIdCol, $dbDataCol, $dbTimeCol) VALUES (:id, :data, sysdate) " .
"WHEN MATCHED THEN UPDATE SET $dbDataCol = :data WHERE $dbIdCol = :id");

$stmt->bindParam(':id', $id, \PDO::PARAM_STR);
$stmt->bindParam(':data', $encoded, \PDO::PARAM_STR);
$stmt->execute();
} else {
$stmt = $this->pdo->prepare("UPDATE $dbTable SET $dbDataCol = :data, $dbTimeCol = :time WHERE $dbIdCol = :id");
$stmt->bindParam(':id', $id, \PDO::PARAM_STR);
$stmt->bindParam(':data', $encoded, \PDO::PARAM_STR);
$stmt->bindValue(':time', time(), \PDO::PARAM_INT);
$stmt->execute();

if (!$stmt->rowCount()) {
// No session exists in the database to update. This happens when we have called
// session_regenerate_id()
$this->createNewSession($id, $data);
}
$insertStmt->bindParam(':id', $id, \PDO::PARAM_STR);
$insertStmt->bindParam(':data', $encoded, \PDO::PARAM_STR);
$insertStmt->bindValue(':time', time(), \PDO::PARAM_INT);
$insertStmt->execute();

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This approach cannot be used as it's also not safe under high concurrency. We need to use DELETE followed by INSERT

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Another possible solution (but more a workaround) would be to catch errors for SQL STATE 23xxx and just ignore it. But it's less clean and could result in ignoring false positives because it ignores all constraint violations.

$this->pdo->commit();
} catch (\PDOException $e) {
$this->pdo->rollback();

throw $e;
}
} catch (\PDOException $e) {
throw new \RuntimeException(sprintf('PDOException was thrown when trying to write the session data: %s', $e->getMessage()), 0, $e);
throw new \RuntimeException(sprintf('PDOException was thrown when trying to write the session data: %s', $e->getMessage()), 0, $e);
}

return true;
}

/**
* Creates a new session with the given $id and $data
* Returns a merge/upsert (i.e. insert or update) SQL query when supported by the database.
*
* @param string $id
* @param string $data
*
* @return boolean True.
* @return string|null The SQL string or null when not supported
*/
private function createNewSession($id, $data = '')
private function getMergeSql()
{
// get table/column
$dbTable = $this->dbOptions['db_table'];
$dbDataCol = $this->dbOptions['db_data_col'];
$dbIdCol = $this->dbOptions['db_id_col'];
$dbTimeCol = $this->dbOptions['db_time_col'];

$sql = "INSERT INTO $dbTable ($dbIdCol, $dbDataCol, $dbTimeCol) VALUES (:id, :data, :time)";

//session data can contain non binary safe characters so we need to encode it
$encoded = base64_encode($data);
$stmt = $this->pdo->prepare($sql);
$stmt->bindParam(':id', $id, \PDO::PARAM_STR);
$stmt->bindParam(':data', $encoded, \PDO::PARAM_STR);
$stmt->bindValue(':time', time(), \PDO::PARAM_INT);
$stmt->execute();
$driver = $this->pdo->getAttribute(\PDO::ATTR_DRIVER_NAME);

switch ($driver) {
case 'mysql':
return "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time) " .
"ON DUPLICATE KEY UPDATE $this->dataCol = VALUES($this->dataCol), $this->timeCol = VALUES($this->timeCol)";
case 'oci':
// DUAL is Oracle specific dummy table
return "MERGE INTO $this->table USING DUAL ON ($this->idCol = :id) " .
"WHEN NOT MATCHED THEN INSERT ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time) " .
"WHEN MATCHED THEN UPDATE SET $this->dataCol = :data";
case 'sqlsrv':
// MS SQL Server requires MERGE be terminated by semicolon
return "MERGE INTO $this->table USING (SELECT 'x' AS dummy) AS src ON ($this->idCol = :id) " .
"WHEN NOT MATCHED THEN INSERT ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time) " .
"WHEN MATCHED THEN UPDATE SET $this->dataCol = :data;";
}

return true;
return null;
}
}