class Connection

1. 9.x drupal/core/lib/Drupal/Core/Database/Connection.php \Drupal\Core\Database\Connection
2. 9.x drupal/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php \Drupal\Core\Database\Driver\mysql\Connection
3. 9.x drupal/core/lib/Drupal/Core/Database/Driver/sqlite/Connection.php \Drupal\Core\Database\Driver\sqlite\Connection
4. 9.x drupal/core/lib/Drupal/Core/Database/Driver/pgsql/Connection.php \Drupal\Core\Database\Driver\pgsql\Connection

Tasks.php in drupal/core/lib/Drupal/Core/Database/Driver/sqlite/Install/Tasks.php

Definition of Drupal\Core\Database\Driver\sqlite\Install\Tasks

Response::getConnection in drupal/core/vendor/guzzle/http/Guzzle/Http/Message/Response.php

Get the Connection HTTP header

class Connection extends DatabaseConnection {

  /**
   * Whether this database connection supports savepoints.
   *
   * Version of sqlite lower then 3.6.8 can't use savepoints.
   * See http://www.sqlite.org/releaselog/3_6_8.html
   *
   * @var boolean
   */
  protected $savepointSupport = FALSE;

  /**
   * Error code for "Unable to open database file" error.
   */
  const DATABASE_NOT_FOUND = 14;

  /**
   * Whether or not the active transaction (if any) will be rolled back.
   *
   * @var boolean
   */
  protected $willRollback;

  /**
   * All databases attached to the current database. This is used to allow
   * prefixes to be safely handled without locking the table
   *
   * @var array
   */
  protected $attachedDatabases = array();

  /**
   * Whether or not a table has been dropped this request: the destructor will
   * only try to get rid of unnecessary databases if there is potential of them
   * being empty.
   *
   * This variable is set to public because Schema needs to
   * access it. However, it should not be manually set.
   *
   * @var boolean
   */
  var $tableDropped = FALSE;
  public function __construct(array $connection_options = array()) {

    // We don't need a specific PDOStatement class here, we simulate it below.
    $this->statementClass = NULL;

    // This driver defaults to transaction support, except if explicitly passed FALSE.
    $this->transactionSupport = $this->transactionalDDLSupport = !isset($connection_options['transactions']) || $connection_options['transactions'] !== FALSE;
    $this->connectionOptions = $connection_options;

    // Allow PDO options to be overridden.
    $connection_options += array(
      'pdo' => array(),
    );
    $connection_options['pdo'] += array(
      // Convert numeric values to strings when fetching.
      PDO::ATTR_STRINGIFY_FETCHES => TRUE,
    );
    parent::__construct('sqlite:' . $connection_options['database'], '', '', $connection_options['pdo']);

    // Attach one database for each registered prefix.
    $prefixes = $this->prefixes;
    foreach ($prefixes as $table => &$prefix) {

      // Empty prefix means query the main database -- no need to attach anything.
      if (!empty($prefix)) {

        // Only attach the database once.
        if (!isset($this->attachedDatabases[$prefix])) {
          $this->attachedDatabases[$prefix] = $prefix;
          $this
            ->query('ATTACH DATABASE :database AS :prefix', array(
            ':database' => $connection_options['database'] . '-' . $prefix,
            ':prefix' => $prefix,
          ));
        }

        // Add a ., so queries become prefix.table, which is proper syntax for
        // querying an attached database.
        $prefix .= '.';
      }
    }

    // Regenerate the prefixes replacement table.
    $this
      ->setPrefix($prefixes);

    // Detect support for SAVEPOINT.
    $version = $this
      ->query('SELECT sqlite_version()')
      ->fetchField();
    $this->savepointSupport = version_compare($version, '3.6.8') >= 0;

    // Create functions needed by SQLite.
    $this
      ->sqliteCreateFunction('if', array(
      $this,
      'sqlFunctionIf',
    ));
    $this
      ->sqliteCreateFunction('greatest', array(
      $this,
      'sqlFunctionGreatest',
    ));
    $this
      ->sqliteCreateFunction('pow', 'pow', 2);
    $this
      ->sqliteCreateFunction('length', 'strlen', 1);
    $this
      ->sqliteCreateFunction('md5', 'md5', 1);
    $this
      ->sqliteCreateFunction('concat', array(
      $this,
      'sqlFunctionConcat',
    ));
    $this
      ->sqliteCreateFunction('substring', array(
      $this,
      'sqlFunctionSubstring',
    ), 3);
    $this
      ->sqliteCreateFunction('substring_index', array(
      $this,
      'sqlFunctionSubstringIndex',
    ), 3);
    $this
      ->sqliteCreateFunction('rand', array(
      $this,
      'sqlFunctionRand',
    ));

    // Execute sqlite init_commands.
    if (isset($connection_options['init_commands'])) {
      $this
        ->exec(implode('; ', $connection_options['init_commands']));
    }
  }

  /**
   * Destructor for the SQLite connection.
   *
   * We prune empty databases on destruct, but only if tables have been
   * dropped. This is especially needed when running the test suite, which
   * creates and destroy databases several times in a row.
   */
  public function __destruct() {
    if ($this->tableDropped && !empty($this->attachedDatabases)) {
      foreach ($this->attachedDatabases as $prefix) {

        // Check if the database is now empty, ignore the internal SQLite tables.
        try {
          $count = $this
            ->query('SELECT COUNT(*) FROM ' . $prefix . '.sqlite_master WHERE type = :type AND name NOT LIKE :pattern', array(
            ':type' => 'table',
            ':pattern' => 'sqlite_%',
          ))
            ->fetchField();

          // We can prune the database file if it doesn't have any tables.
          if ($count == 0) {

            // Detach the database.
            $this
              ->query('DETACH DATABASE :schema', array(
              ':schema' => $prefix,
            ));

            // Destroy the database file.
            unlink($this->connectionOptions['database'] . '-' . $prefix);
          }
        } catch (Exception $e) {

          // Ignore the exception and continue. There is nothing we can do here
          // to report the error or fail safe.
        }
      }
    }
  }

  /**
   * SQLite compatibility implementation for the IF() SQL function.
   */
  public function sqlFunctionIf($condition, $expr1, $expr2 = NULL) {
    return $condition ? $expr1 : $expr2;
  }

  /**
   * SQLite compatibility implementation for the GREATEST() SQL function.
   */
  public function sqlFunctionGreatest() {
    $args = func_get_args();
    foreach ($args as $k => $v) {
      if (!isset($v)) {
        unset($args);
      }
    }
    if (count($args)) {
      return max($args);
    }
    else {
      return NULL;
    }
  }

  /**
   * SQLite compatibility implementation for the CONCAT() SQL function.
   */
  public function sqlFunctionConcat() {
    $args = func_get_args();
    return implode('', $args);
  }

  /**
   * SQLite compatibility implementation for the SUBSTRING() SQL function.
   */
  public function sqlFunctionSubstring($string, $from, $length) {
    return substr($string, $from - 1, $length);
  }

  /**
   * SQLite compatibility implementation for the SUBSTRING_INDEX() SQL function.
   */
  public function sqlFunctionSubstringIndex($string, $delimiter, $count) {

    // If string is empty, simply return an empty string.
    if (empty($string)) {
      return '';
    }
    $end = 0;
    for ($i = 0; $i < $count; $i++) {
      $end = strpos($string, $delimiter, $end + 1);
      if ($end === FALSE) {
        $end = strlen($string);
      }
    }
    return substr($string, 0, $end);
  }

  /**
   * SQLite compatibility implementation for the RAND() SQL function.
   */
  public function sqlFunctionRand($seed = NULL) {
    if (isset($seed)) {
      mt_srand($seed);
    }
    return mt_rand() / mt_getrandmax();
  }

  /**
   * SQLite-specific implementation of DatabaseConnection::prepare().
   *
   * We don't use prepared statements at all at this stage. We just create
   * a Statement object, that will create a PDOStatement
   * using the semi-private PDOPrepare() method below.
   */
  public function prepare($query, $options = array()) {
    return new Statement($this, $query, $options);
  }

  /**
   * NEVER CALL THIS FUNCTION: YOU MIGHT DEADLOCK YOUR PHP PROCESS.
   *
   * This is a wrapper around the parent PDO::prepare method. However, as
   * the PDO SQLite driver only closes SELECT statements when the PDOStatement
   * destructor is called and SQLite does not allow data change (INSERT,
   * UPDATE etc) on a table which has open SELECT statements, you should never
   * call this function and keep a PDOStatement object alive as that can lead
   * to a deadlock. This really, really should be private, but as Statement
   * needs to call it, we have no other choice but to expose this function to
   * the world.
   */
  public function PDOPrepare($query, array $options = array()) {
    return parent::prepare($query, $options);
  }
  public function queryRange($query, $from, $count, array $args = array(), array $options = array()) {
    return $this
      ->query($query . ' LIMIT ' . (int) $from . ', ' . (int) $count, $args, $options);
  }
  public function queryTemporary($query, array $args = array(), array $options = array()) {

    // Generate a new temporary table name and protect it from prefixing.
    // SQLite requires that temporary tables to be non-qualified.
    $tablename = $this
      ->generateTemporaryTableName();
    $prefixes = $this->prefixes;
    $prefixes[$tablename] = '';
    $this
      ->setPrefix($prefixes);
    $this
      ->query(preg_replace('/^SELECT/i', 'CREATE TEMPORARY TABLE ' . $tablename . ' AS SELECT', $query), $args, $options);
    return $tablename;
  }
  public function driver() {
    return 'sqlite';
  }
  public function databaseType() {
    return 'sqlite';
  }

  /**
   * Overrides \Drupal\Core\Database\Connection::createDatabase().
   *
   * @param string $database
   *   The name of the database to create.
   *
   * @throws DatabaseNotFoundException
   */
  public function createDatabase($database) {

    // Verify the database is writable.
    $db_directory = new SplFileInfo(dirname($database));
    if (!$db_directory
      ->isDir() && !drupal_mkdir($db_directory
      ->getPathName(), 0755, TRUE)) {
      throw new DatabaseNotFoundException('Unable to create database directory ' . $db_directory
        ->getPathName());
    }
  }
  public function mapConditionOperator($operator) {

    // We don't want to override any of the defaults.
    static $specials = array(
      'LIKE' => array(
        'postfix' => " ESCAPE '\\'",
      ),
      'NOT LIKE' => array(
        'postfix' => " ESCAPE '\\'",
      ),
    );
    return isset($specials[$operator]) ? $specials[$operator] : NULL;
  }
  public function prepareQuery($query) {
    return $this
      ->prepare($this
      ->prefixTables($query));
  }
  public function nextId($existing_id = 0) {
    $transaction = $this
      ->startTransaction();

    // We can safely use literal queries here instead of the slower query
    // builder because if a given database breaks here then it can simply
    // override nextId. However, this is unlikely as we deal with short strings
    // and integers and no known databases require special handling for those
    // simple cases. If another transaction wants to write the same row, it will
    // wait until this transaction commits.
    $stmt = $this
      ->query('UPDATE {sequences} SET value = GREATEST(value, :existing_id) + 1', array(
      ':existing_id' => $existing_id,
    ));
    if (!$stmt
      ->rowCount()) {
      $this
        ->query('INSERT INTO {sequences} (value) VALUES (:existing_id + 1)', array(
        ':existing_id' => $existing_id,
      ));
    }

    // The transaction gets committed when the transaction object gets destroyed
    // because it gets out of scope.
    return $this
      ->query('SELECT value FROM {sequences}')
      ->fetchField();
  }
  public function rollback($savepoint_name = 'drupal_transaction') {
    if ($this->savepointSupport) {
      return parent::rollBack($savepoint_name);
    }
    if (!$this
      ->inTransaction()) {
      throw new TransactionNoActiveException();
    }

    // A previous rollback to an earlier savepoint may mean that the savepoint
    // in question has already been rolled back.
    if (!in_array($savepoint_name, $this->transactionLayers)) {
      return;
    }

    // We need to find the point we're rolling back to, all other savepoints
    // before are no longer needed.
    while ($savepoint = array_pop($this->transactionLayers)) {
      if ($savepoint == $savepoint_name) {

        // Mark whole stack of transactions as needed roll back.
        $this->willRollback = TRUE;

        // If it is the last the transaction in the stack, then it is not a
        // savepoint, it is the transaction itself so we will need to roll back
        // the transaction rather than a savepoint.
        if (empty($this->transactionLayers)) {
          break;
        }
        return;
      }
    }
    if ($this
      ->supportsTransactions()) {
      PDO::rollBack();
    }
  }
  public function pushTransaction($name) {
    if ($this->savepointSupport) {
      return parent::pushTransaction($name);
    }
    if (!$this
      ->supportsTransactions()) {
      return;
    }
    if (isset($this->transactionLayers[$name])) {
      throw new TransactionNameNonUniqueException($name . " is already in use.");
    }
    if (!$this
      ->inTransaction()) {
      PDO::beginTransaction();
    }
    $this->transactionLayers[$name] = $name;
  }
  public function popTransaction($name) {
    if ($this->savepointSupport) {
      return parent::popTransaction($name);
    }
    if (!$this
      ->supportsTransactions()) {
      return;
    }
    if (!$this
      ->inTransaction()) {
      throw new TransactionNoActiveException();
    }

    // Commit everything since SAVEPOINT $name.
    while ($savepoint = array_pop($this->transactionLayers)) {
      if ($savepoint != $name) {
        continue;
      }

      // If there are no more layers left then we should commit or rollback.
      if (empty($this->transactionLayers)) {

        // If there was any rollback() we should roll back whole transaction.
        if ($this->willRollback) {
          $this->willRollback = FALSE;
          PDO::rollBack();
        }
        elseif (!PDO::commit()) {
          throw new TransactionCommitFailedException();
        }
      }
      else {
        break;
      }
    }
  }

}