code: func (p *purgeMgr) PrepareForExpiringKeys(expiringAtBlk uint64) {
  	p.waitGrp.Add(1)
  	go func() {
  		p.lock.Lock()
  		p.waitGrp.Done()
  		defer p.lock.Unlock()
  		p.workingset = p.prepareWorkingsetFor(expiringAtBlk)
  	}()
  	p.waitGrp.Wait()
  }
docs: // PrepareForExpiringKeys implements function in the interface 'PurgeMgr'