code: func (db *DB) compTriggerRange(compC chan<- cCmd, level int, min, max []byte) (err error) {
  	ch := make(chan error)
  	defer close(ch)
  	// Send cmd.
  	select {
  	case compC <- cRange{level, min, max, ch}:
  	case err := <-db.compErrC:
  		return err
  	case <-db.closeC:
  		return ErrClosed
  	}
  	// Wait cmd.
  	select {
  	case err = <-ch:
  	case err = <-db.compErrC:
  	case <-db.closeC:
  		return ErrClosed
  	}
  	return err
  }
docs: // Send range compaction request.