func replicaGCShouldQueueImpl(
	now, lastCheck, lastActivity hlc.Timestamp, isCandidate bool,
) (bool, float64) {
	timeout := ReplicaGCQueueInactivityThreshold
	var priority float64

	if isCandidate {
		// If the range is a candidate (which happens if its former replica set
		// ignores it), let it expire much earlier.
		timeout = ReplicaGCQueueCandidateTimeout
		priority++
	} else if now.Less(lastCheck.Add(ReplicaGCQueueInactivityThreshold.Nanoseconds(), 0)) {
		// Return false immediately if the previous check was less than the
		// check interval in the past. Note that we don't do this is the
		// replica is in candidate state, in which case we want to be more
		// aggressive - a failed rebalance attempt could have checked this
		// range, and candidate state suggests that a retry succeeded. See
		// #7489.
		return false, 0
	}

	shouldQ := lastActivity.Add(timeout.Nanoseconds(), 0).Less(now)

	if !shouldQ {
		return false, 0
	}

	return shouldQ, priority
}
// InitOrJoinRequest executes a RequestLease command asynchronously and returns a
// channel on which the result will be posted. If there's already a request in
// progress, we join in waiting for the results of that request.
// It is an error to call InitOrJoinRequest() while a request is in progress
// naming another replica as lease holder.
//
// replica is used to schedule and execute async work (proposing a RequestLease
// command). replica.mu is locked when delivering results, so calls from the
// replica happen either before or after a result for a pending request has
// happened.
//
// transfer needs to be set if the request represents a lease transfer (as
// opposed to an extension, or acquiring the lease when none is held).
//
// Note: Once this function gets a context to be used for cancellation, instead
// of replica.store.Stopper().ShouldQuiesce(), care will be needed for cancelling
// the Raft command, similar to replica.addWriteCmd.
func (p *pendingLeaseRequest) InitOrJoinRequest(
	replica *Replica,
	nextLeaseHolder roachpb.ReplicaDescriptor,
	timestamp hlc.Timestamp,
	startKey roachpb.Key,
	transfer bool,
) <-chan *roachpb.Error {
	if nextLease := p.RequestPending(); nextLease != nil {
		if nextLease.Replica.ReplicaID == nextLeaseHolder.ReplicaID {
			// Join a pending request asking for the same replica to become lease
			// holder.
			return p.JoinRequest()
		}
		llChan := make(chan *roachpb.Error, 1)
		// We can't join the request in progress.
		llChan <- roachpb.NewErrorf("request for different replica in progress "+
			"(requesting: %+v, in progress: %+v)",
			nextLeaseHolder.ReplicaID, nextLease.Replica.ReplicaID)
		return llChan
	}
	llChan := make(chan *roachpb.Error, 1)
	// No request in progress. Let's propose a Lease command asynchronously.
	// TODO(tschottdorf): get duration from configuration, either as a
	// config flag or, later, dynamically adjusted.
	startStasis := timestamp.Add(int64(replica.store.ctx.rangeLeaseActiveDuration), 0)
	expiration := startStasis.Add(int64(replica.store.Clock().MaxOffset()), 0)
	reqSpan := roachpb.Span{
		Key: startKey,
	}
	var leaseReq roachpb.Request
	reqLease := roachpb.Lease{
		Start:       timestamp,
		StartStasis: startStasis,
		Expiration:  expiration,
		Replica:     nextLeaseHolder,
	}
	if transfer {
		leaseReq = &roachpb.TransferLeaseRequest{
			Span:  reqSpan,
			Lease: reqLease,
		}
	} else {
		leaseReq = &roachpb.RequestLeaseRequest{
			Span:  reqSpan,
			Lease: reqLease,
		}
	}
	if replica.store.Stopper().RunAsyncTask(func() {
		// Propose a RequestLease command and wait for it to apply.
		var execPErr *roachpb.Error
		ba := roachpb.BatchRequest{}
		ba.Timestamp = replica.store.Clock().Now()
		ba.RangeID = replica.RangeID
		ba.Add(leaseReq)
		// Send lease request directly to raft in order to skip unnecessary
		// checks from normal request machinery, (e.g. the command queue).
		// Note that the command itself isn't traced, but usually the caller
		// waiting for the result has an active Trace.
		ch, _, err := replica.proposeRaftCommand(
			replica.context(context.Background()), ba)
		if err != nil {
			execPErr = roachpb.NewError(err)
		} else {
			// If the command was committed, wait for the range to apply it.
			select {
			case c := <-ch:
				if c.Err != nil {
					if log.V(1) {
						log.Infof("failed to acquire lease for replica %s: %s", replica.store, c.Err)
					}
					execPErr = c.Err
				}
			case <-replica.store.Stopper().ShouldQuiesce():
				execPErr = roachpb.NewError(
					replica.newNotLeaseHolderError(nil, replica.store.StoreID(), replica.Desc()))
			}
		}

		// Send result of lease to all waiter channels.
		replica.mu.Lock()
		defer replica.mu.Unlock()
		for i, llChan := range p.llChans {
			// Don't send the same pErr object twice; this can lead to races. We could
			// clone every time but it's more efficient to send pErr itself to one of
			// the channels (the last one; if we send it earlier the race can still
			// happen).
			if i == len(p.llChans)-1 {
				llChan <- execPErr
			} else {
				llChan <- protoutil.Clone(execPErr).(*roachpb.Error) // works with `nil`
			}
		}
		p.llChans = p.llChans[:0]
		p.nextLease = roachpb.Lease{}
	}) != nil {
		// We failed to start the asynchronous task. Send a blank NotLeaseHolderError
		// back to indicate that we have no idea who the range lease holder might
		// be; we've withdrawn from active duty.
		llChan <- roachpb.NewError(
			replica.newNotLeaseHolderError(nil, replica.store.StoreID(), replica.mu.state.Desc))
		return llChan
	}
	p.llChans = append(p.llChans, llChan)
	p.nextLease = reqLease
	return llChan
}