code: func (db *DB) compTriggerWait(compC chan<- cCmd) (err error) {
  	ch := make(chan error)
  	defer close(ch)
  	// Send cmd.
  	select {
  	case compC <- cAuto{ch}:
  	case err = <-db.compErrC:
  		return
  	case <-db.closeC:
  		return ErrClosed
  	}
  	// Wait cmd.
  	select {
  	case err = <-ch:
  	case err = <-db.compErrC:
  	case <-db.closeC:
  		return ErrClosed
  	}
  	return err
  }
docs: // This will trigger auto compaction and/or wait for all compaction to be done.