kvserver: improve a comment in TransferLeaseTarget

Release justification: adds no-op safeguard

Release note: None

pkg/kv/kvserver/allocator.go

@@ -1451,7 +1451,7 @@ func (a *Allocator) TransferLeaseTarget(
  leaseholderReplQPS, _ := stats.avgQPS()
  currentDelta := getQPSDelta(storeQPSMap, existing)
  bestOption := getCandidateWithMinQPS(storeQPSMap, existing)
- if bestOption != (roachpb.ReplicaDescriptor{}) &&
+ if bestOption != (roachpb.ReplicaDescriptor{}) && leaseholderReplQPS > 0 &&
  // It is always beneficial to transfer the lease to the coldest candidate
  // if the range's own qps is smaller than the difference between the
  // leaseholder store and the candidate store. This will always drive down
@@ -1466,6 +1466,11 @@ func (a *Allocator) TransferLeaseTarget(
  // ranges with low QPS. This can add up and prevent us from achieving
  // convergence in cases where we're dealing with a ton of very low-QPS
  // ranges.
+ //
+ // NB: If the `bestOption` ends up being the current leaseholder, then the
+ // check below will fail since we know that `leaseholderReplQPS` must be
+ // greater than 0 (the `StoreRebalancer` only bothers rebalancing leases /
+ // replicas for ranges serving at least some traffic).
  (leaseholderStoreQPS-leaseholderReplQPS) > storeQPSMap[bestOption.StoreID] {
  storeQPSMap[leaseRepl.StoreID()] -= leaseholderReplQPS
  storeQPSMap[bestOption.StoreID] += leaseholderReplQPS

pkg/kv/kvserver/replicate_queue.go

@@ -1284,10 +1284,15 @@ const (
 )
 
 type transferLeaseOptions struct {
- goal transferLeaseGoal
+ goal transferLeaseGoal
+ // checkTransferLeaseSource, when false, tells `TransferLeaseTarget` to
+ // exclude the current leaseholder from consideration as a potential target
+ // (i.e. when the caller explicitly wants to shed its lease away).
  checkTransferLeaseSource bool
- checkCandidateFullness bool
- dryRun bool
+ // checkCandidateFullness, when false, tells `TransferLeaseTarget`
+ // to disregard the existing lease counts on candidates.
+ checkCandidateFullness bool
+ dryRun bool
 }
 
 // leaseTransferOutcome represents the result of shedLease().