code: protected function releaseLock()
      {
          // Again silencing the file system operation, because there might be
          // multiple possible reasons to fail and we are handling the error
          // anyways.
          if ( !@unlink( $this->lockDir . '/lock' ) )
          {
              $this->logger->log(
                  'Failure releasing lock: ' . ( isset( $php_errormsg ) ? $php_errormsg : 'Unknown error - enable the track_errors ini directive.' ),
                  Logger::ERROR
              );
              return;
          }
  
          $this->logger->log( 'Released lock.', Logger::INFO );
      }
docs: Release lock
  
  Method to release the aquired lock, after the work has been done.
  
  @return void