grpclb: recreate SubConns when switching fallback (#2899)

With pickfirst, the same SubConn is reused, only addresses are updated.
But backends and fallbacks may need different credentials. This change
force-removes all SubConns when switching fallback.

Author: menghanl

Diff for: balancer/grpclb/grpclb_remote_balancer.go

@@ -95,8 +95,6 @@ func (lb *lbBalancer) processServerList(l *lbpb.ServerList) {
 //
 // Caller must hold lb.mu.
 func (lb *lbBalancer) refreshSubConns(backendAddrs []resolver.Address, fallback bool, pickFirst bool) {
-	lb.inFallback = fallback
-
 	opts := balancer.NewSubConnOptions{}
 	if !fallback {
 		opts.CredsBundle = lb.grpclbBackendCreds
@@ -105,17 +103,29 @@ func (lb *lbBalancer) refreshSubConns(backendAddrs []resolver.Address, fallback
 	lb.backendAddrs = backendAddrs
 	lb.backendAddrsWithoutMetadata = nil
 
-	if lb.usePickFirst != pickFirst {
-		// Remove all SubConns when switching modes.
+	fallbackModeChanged := lb.inFallback != fallback
+	lb.inFallback = fallback
+
+	balancingPolicyChanged := lb.usePickFirst != pickFirst
+	oldUsePickFirst := lb.usePickFirst
+	lb.usePickFirst = pickFirst
+
+	if fallbackModeChanged || balancingPolicyChanged {
+		// Remove all SubConns when switching balancing policy or switching
+		// fallback mode.
+		//
+		// For fallback mode switching with pickfirst, we want to recreate the
+		// SubConn because the creds could be different.
 		for a, sc := range lb.subConns {
-			if lb.usePickFirst {
+			if oldUsePickFirst {
+				// If old SubConn were created for pickfirst, bypass cache and
+				// remove directly.
 				lb.cc.cc.RemoveSubConn(sc)
 			} else {
 				lb.cc.RemoveSubConn(sc)
 			}
 			delete(lb.subConns, a)
 		}
-		lb.usePickFirst = pickFirst
 	}
 
 	if lb.usePickFirst {