DbLog::log

public DbLog::log($level, $message, array $context = array())

public function log($level, $message, array $context = array()) {
  // Remove any backtraces since they may contain an unserializable variable.
  unset($context['backtrace']);

  // Convert PSR3-style messages to SafeMarkup::format() style, so they can be
  // translated too in runtime.
  $message_placeholders = $this->parser->parseMessagePlaceholders($message, $context);

  try {
    $this->connection
      ->insert('watchdog')
      ->fields(array(
        'uid' => $context['uid'],
        'type' => Unicode::substr($context['channel'], 0, 64),
        'message' => $message,
        'variables' => serialize($message_placeholders),
        'severity' => $level,
        'link' => $context['link'],
        'location' => $context['request_uri'],
        'referer' => $context['referer'],
        'hostname' => Unicode::substr($context['ip'], 0, 128),
        'timestamp' => $context['timestamp'],
      ))
      ->execute();
  }
  catch (\Exception $e) {
    // When running Drupal on MySQL or MariaDB you can run into several errors
    // that corrupt the database connection. Some examples for these kind of
    // errors on the database layer are "1100 - Table 'xyz' was not locked
    // with LOCK TABLES" and "1153 - Got a packet bigger than
    // 'max_allowed_packet' bytes". If such an error happens, the MySQL server
    // invalidates the connection and answers all further requests in this
    // connection with "2006 - MySQL server had gone away". In that case the
    // insert statement above results in a database exception. To ensure that
    // the causal error is written to the log we try once to open a dedicated
    // connection and write again.
    if (
    // Only handle database related exceptions.
    ($e instanceof DatabaseException || $e instanceof \PDOException) && 
      // Avoid an endless loop of re-write attempts.
      $this->connection->getTarget() != self::DEDICATED_DBLOG_CONNECTION_TARGET
      ) {
      // Open a dedicated connection for logging.
      $key = $this->connection->getKey();
      $info = Database::getConnectionInfo($key);
      Database::addConnectionInfo($key, self::DEDICATED_DBLOG_CONNECTION_TARGET, $info['default']);
      $this->connection = Database::getConnection(self::DEDICATED_DBLOG_CONNECTION_TARGET, $key);
      // Now try once to log the error again.
      $this->log($level, $message, $context);
    }
    else {
      throw $e;
    }
  }
}