Prepare `balancer` interfaces for `>=google.golang.org/grpc@1.30.0` upgrade.

client/v3/balancer/balancer.go

@@ -23,6 +23,7 @@ import (
 	"go.etcd.io/etcd/client/v3/balancer/connectivity"
 	"go.etcd.io/etcd/client/v3/balancer/picker"
 
+	"go.uber.org/multierr"
 	"go.uber.org/zap"
 	"google.golang.org/grpc/balancer"
 	grpcconnectivity "google.golang.org/grpc/connectivity"
@@ -31,6 +32,12 @@ import (
 	_ "google.golang.org/grpc/resolver/passthrough" // register passthrough resolver
 )
 
+// NOTE: Ensure
+//       - `baseBalancer` satisfies `balancer.V2Balancer`.
+var (
+	_ balancer.V2Balancer = (*baseBalancer)(nil)
+)
+
 // Config defines balancer configurations.
 type Config struct {
 	// Policy configures balancer policy.
@@ -138,12 +145,29 @@ type baseBalancer struct {
 	picker picker.Picker
 }
 
+// UpdateClientConnState implements "grpc/balancer.V2Balancer" interface.
+func (bb *baseBalancer) UpdateClientConnState(ccs balancer.ClientConnState) error {
+	return bb.handleResolvedWithError(ccs.ResolverState.Addresses, nil)
+}
+
+// ResolverError implements "grpc/balancer.V2Balancer" interface.
+func (bb *baseBalancer) ResolverError(err error) {
+	bb.HandleResolvedAddrs(nil, err)
+}
+
 // HandleResolvedAddrs implements "grpc/balancer.Balancer" interface.
 // gRPC sends initial or updated resolved addresses from "Build".
 func (bb *baseBalancer) HandleResolvedAddrs(addrs []resolver.Address, err error) {
+	_ = bb.handleResolvedWithError(addrs, err)
+}
+
+// handleResolvedWithError is an implementation shared both by `HandleResolvedAddrs()`,
+// which is part of the `Balancer` interface as well as `UpdateClientConnState()`,
+// which is part of the `V2Balancer` interface.
+func (bb *baseBalancer) handleResolvedWithError(addrs []resolver.Address, err error) error {
 	if err != nil {
 		bb.lg.Warn("HandleResolvedAddrs called with error", zap.String("balancer-id", bb.id), zap.Error(err))
-		return
+		return err
 	}
 	bb.lg.Info("resolved",
 		zap.String("picker", bb.picker.String()),
@@ -155,12 +179,14 @@ func (bb *baseBalancer) HandleResolvedAddrs(addrs []resolver.Address, err error)
 	defer bb.mu.Unlock()
 
 	resolved := make(map[resolver.Address]struct{})
+	warnedErrors := []error{}
 	for _, addr := range addrs {
 		resolved[addr] = struct{}{}
 		if _, ok := bb.addrToSc[addr]; !ok {
 			sc, err := bb.currentConn.NewSubConn([]resolver.Address{addr}, balancer.NewSubConnOptions{})
 			if err != nil {
 				bb.lg.Warn("NewSubConn failed", zap.String("picker", bb.picker.String()), zap.String("balancer-id", bb.id), zap.Error(err), zap.String("address", addr.Addr))
+				warnedErrors = append(warnedErrors, err)
 				continue
 			}
 			bb.lg.Info("created subconn", zap.String("address", addr.Addr))
@@ -191,6 +217,15 @@ func (bb *baseBalancer) HandleResolvedAddrs(addrs []resolver.Address, err error)
 			// (DO NOT) delete(bb.scToSt, sc)
 		}
 	}
+
+	// TODO: Consider just returning `ErrBadResolverState` if `warnedErrors` is
+	//       not empty.
+	return multierr.Combine(warnedErrors...)
+}
+
+// UpdateSubConnState implements "grpc/balancer.V2Balancer" interface.
+func (bb *baseBalancer) UpdateSubConnState(sc balancer.SubConn, s balancer.SubConnState) {
+	bb.HandleSubConnStateChange(sc, s.ConnectivityState)
 }
 
 // HandleSubConnStateChange implements "grpc/balancer.Balancer" interface.

client/v3/balancer/picker/err.go

@@ -20,6 +20,12 @@ import (
 	"google.golang.org/grpc/balancer"
 )
 
+// NOTE: Ensure
+//       - `errPickerV2` satisfies `balancer.V2Picker`.
+var (
+	_ balancer.V2Picker = (*errPickerV2)(nil)
+)
+
 // NewErr returns a picker that always returns err on "Pick".
 func NewErr(err error) Picker {
 	return &errPicker{p: Error, err: err}
@@ -37,3 +43,11 @@ func (ep *errPicker) String() string {
 func (ep *errPicker) Pick(context.Context, balancer.PickInfo) (balancer.SubConn, func(balancer.DoneInfo), error) {
 	return nil, nil, ep.err
 }
+
+type errPickerV2 struct {
+	errPicker
+}
+
+func (ep2 *errPickerV2) Pick(opts balancer.PickInfo) (balancer.PickResult, error) {
+	return balancer.PickResult{}, ep2.errPicker.err
+}

client/v3/balancer/picker/roundrobin_balanced.go

@@ -24,6 +24,12 @@ import (
 	"google.golang.org/grpc/resolver"
 )
 
+// NOTE: Ensure
+//       - `rrBalancedV2` satisfies `balancer.V2Picker`.
+var (
+	_ balancer.V2Picker = (*rrBalancedV2)(nil)
+)
+
 // newRoundrobinBalanced returns a new roundrobin balanced picker.
 func newRoundrobinBalanced(cfg Config) Picker {
 	scs := make([]balancer.SubConn, 0, len(cfg.SubConnToResolverAddress))
@@ -52,7 +58,7 @@ type rrBalanced struct {
 func (rb *rrBalanced) String() string { return rb.p.String() }
 
 // Pick is called for every client request.
-func (rb *rrBalanced) Pick(ctx context.Context, opts balancer.PickInfo) (balancer.SubConn, func(balancer.DoneInfo), error) {
+func (rb *rrBalanced) Pick(_ context.Context, opts balancer.PickInfo) (balancer.SubConn, func(balancer.DoneInfo), error) {
 	rb.mu.RLock()
 	n := len(rb.scs)
 	rb.mu.RUnlock()
@@ -93,3 +99,13 @@ func (rb *rrBalanced) Pick(ctx context.Context, opts balancer.PickInfo) (balance
 	}
 	return sc, doneFunc, nil
 }
+
+type rrBalancedV2 struct {
+	rrBalanced
+}
+
+func (rb2 *rrBalancedV2) Pick(opts balancer.PickInfo) (balancer.PickResult, error) {
+	sc, doneFunc, err := rb2.rrBalanced.Pick(context.TODO(), opts)
+	pr := balancer.PickResult{SubConn: sc, Done: doneFunc}
+	return pr, err
+}

client/v3/go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/prometheus/client_golang v1.5.1
 	go.etcd.io/etcd/api/v3 v3.5.0-pre
 	go.etcd.io/etcd/pkg/v3 v3.5.0-pre
+	go.uber.org/multierr v1.5.0
 	go.uber.org/zap v1.16.0
 	google.golang.org/grpc v1.29.1
 	sigs.k8s.io/yaml v1.2.0