balancer/weightedroundrobin: migrate to StateListener (#6530)

Author: dfawley

balancer/weightedroundrobin/balancer.go

@@ -154,7 +154,12 @@ func (b *wrrBalancer) updateAddresses(addrs []resolver.Address) {
 			wsc = wsci.(*weightedSubConn)
 		} else {
 			// addr is a new address (not existing in b.subConns).
-			sc, err := b.cc.NewSubConn([]resolver.Address{addr}, balancer.NewSubConnOptions{})
+			var sc balancer.SubConn
+			sc, err := b.cc.NewSubConn([]resolver.Address{addr}, balancer.NewSubConnOptions{
+				StateListener: func(state balancer.SubConnState) {
+					b.updateSubConnState(sc, state)
+				},
+			})
 			if err != nil {
 				b.logger.Warningf("Failed to create new SubConn for address %v: %v", addr, err)
 				continue
@@ -205,6 +210,10 @@ func (b *wrrBalancer) ResolverError(err error) {
 }
 
 func (b *wrrBalancer) UpdateSubConnState(sc balancer.SubConn, state balancer.SubConnState) {
+	b.logger.Errorf("UpdateSubConnState(%v, %+v) called unexpectedly", sc, state)
+}
+
+func (b *wrrBalancer) updateSubConnState(sc balancer.SubConn, state balancer.SubConnState) {
 	wsc := b.scMap[sc]
 	if wsc == nil {
 		b.logger.Errorf("UpdateSubConnState called with an unknown SubConn: %p, %v", sc, state)