leastrequest: fix data race in leastrequest picker (#6606)

Co-authored-by: Huang Chong <hchtgh315@gmail.com>

balancer/leastrequest/leastrequest.go

@@ -80,7 +80,7 @@ func (bb) Name() string {
 }
 
 func (bb) Build(cc balancer.ClientConn, bOpts balancer.BuildOptions) balancer.Balancer {
-	b := &leastRequestBalancer{scRPCCounts: make(map[balancer.SubConn]*int32)}
+	b := &leastRequestBalancer{scRPCCounts: make(map[balancer.SubConn]*atomic.Int32)}
 	baseBuilder := base.NewBalancerBuilder(Name, b, base.Config{HealthCheck: true})
 	b.Balancer = baseBuilder.Build(cc, bOpts)
 	return b
@@ -92,7 +92,7 @@ type leastRequestBalancer struct {
 	balancer.Balancer
 
 	choiceCount uint32
-	scRPCCounts map[balancer.SubConn]*int32 // Hold onto RPC counts to keep track for subsequent picker updates.
+	scRPCCounts map[balancer.SubConn]*atomic.Int32 // Hold onto RPC counts to keep track for subsequent picker updates.
 }
 
 func (lrb *leastRequestBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
@@ -108,7 +108,7 @@ func (lrb *leastRequestBalancer) UpdateClientConnState(s balancer.ClientConnStat
 
 type scWithRPCCount struct {
 	sc      balancer.SubConn
-	numRPCs *int32
+	numRPCs *atomic.Int32
 }
 
 func (lrb *leastRequestBalancer) Build(info base.PickerBuildInfo) balancer.Picker {
@@ -126,7 +126,7 @@ func (lrb *leastRequestBalancer) Build(info base.PickerBuildInfo) balancer.Picke
 	// Create new refs if needed.
 	for sc := range info.ReadySCs {
 		if _, ok := lrb.scRPCCounts[sc]; !ok {
-			lrb.scRPCCounts[sc] = new(int32)
+			lrb.scRPCCounts[sc] = new(atomic.Int32)
 		}
 	}
 
@@ -162,18 +162,18 @@ func (p *picker) Pick(balancer.PickInfo) (balancer.PickResult, error) {
 			pickedSC = &sc
 			continue
 		}
-		if *sc.numRPCs < *pickedSC.numRPCs {
+		if sc.numRPCs.Load() < pickedSC.numRPCs.Load() {
 			pickedSC = &sc
 		}
 	}
 	// "The counter for a subchannel should be atomically incremented by one
 	// after it has been successfully picked by the picker." - A48
-	atomic.AddInt32(pickedSC.numRPCs, 1)
+	pickedSC.numRPCs.Add(1)
 	// "the picker should add a callback for atomically decrementing the
 	// subchannel counter once the RPC finishes (regardless of Status code)." -
 	// A48.
 	done := func(balancer.DoneInfo) {
-		atomic.AddInt32(pickedSC.numRPCs, -1)
+		pickedSC.numRPCs.Add(-1)
 	}
 	return balancer.PickResult{
 		SubConn: pickedSC.sc,