client: add support for pickfirst address shuffling from gRFC A62

internal/grpcrand/grpcrand.go

@@ -79,3 +79,10 @@ func Uint32() uint32 {
 	defer mu.Unlock()
 	return r.Uint32()
 }
+
+// Shuffle implements rand.Shuffle on the grpcrand global source.
+var Shuffle = func(n int, f func(int, int)) {
+	mu.Lock()
+	defer mu.Unlock()
+	r.Shuffle(n, f)
+}

pickfirst.go

@@ -19,11 +19,14 @@
 package grpc
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/internal/grpcrand"
+	"google.golang.org/grpc/serviceconfig"
 )
 
 // PickFirstBalancerName is the name of the pick_first balancer.
@@ -43,10 +46,28 @@ func (*pickfirstBuilder) Name() string {
 	return PickFirstBalancerName
 }
 
+type pfConfig struct {
+	serviceconfig.LoadBalancingConfig `json:"-"`
+
+	// If set to true, instructs the LB policy to shuffle the order of the list
+	// of addresses received from the name resolver before attempting to
+	// connect to them.
+	ShuffleAddressList bool `json:"shuffleAddressList"`
+}
+
+func (*pickfirstBuilder) ParseConfig(js json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
+	cfg := &pfConfig{}
+	if err := json.Unmarshal(js, cfg); err != nil {
+		return nil, fmt.Errorf("pickfirst: unable to unmarshal LB policy config: %s, error: %v", string(js), err)
+	}
+	return cfg, nil
+}
+
 type pickfirstBalancer struct {
 	state   connectivity.State
 	cc      balancer.ClientConn
 	subConn balancer.SubConn
+	cfg     *pfConfig
 }
 
 func (b *pickfirstBalancer) ResolverError(err error) {
@@ -69,7 +90,8 @@ func (b *pickfirstBalancer) ResolverError(err error) {
 }
 
 func (b *pickfirstBalancer) UpdateClientConnState(state balancer.ClientConnState) error {
-	if len(state.ResolverState.Addresses) == 0 {
+	addrs := state.ResolverState.Addresses
+	if len(addrs) == 0 {
 		// The resolver reported an empty address list. Treat it like an error by
 		// calling b.ResolverError.
 		if b.subConn != nil {
@@ -82,12 +104,23 @@ func (b *pickfirstBalancer) UpdateClientConnState(state balancer.ClientConnState
 		return balancer.ErrBadResolverState
 	}
 
+	if state.BalancerConfig != nil {
+		cfg, ok := state.BalancerConfig.(*pfConfig)
+		if !ok {
+			return fmt.Errorf("pickfirstBalancer: received nil or illegal BalancerConfig (type %T): %v", state.BalancerConfig, state.BalancerConfig)
+		}
+		b.cfg = cfg
+	}
+
+	if b.cfg != nil && b.cfg.ShuffleAddressList {
+		grpcrand.Shuffle(len(addrs), func(i, j int) { addrs[i], addrs[j] = addrs[j], addrs[i] })
+	}
 	if b.subConn != nil {
-		b.cc.UpdateAddresses(b.subConn, state.ResolverState.Addresses)
+		b.cc.UpdateAddresses(b.subConn, addrs)
 		return nil
 	}
 
-	subConn, err := b.cc.NewSubConn(state.ResolverState.Addresses, balancer.NewSubConnOptions{})
+	subConn, err := b.cc.NewSubConn(addrs, balancer.NewSubConnOptions{})
 	if err != nil {
 		if logger.V(2) {
 			logger.Errorf("pickfirstBalancer: failed to NewSubConn: %v", err)

test/pickfirst_test.go

@@ -30,6 +30,7 @@ import (
 	"google.golang.org/grpc/connectivity"
 	"google.golang.org/grpc/credentials/insecure"
 	"google.golang.org/grpc/internal/channelz"
+	"google.golang.org/grpc/internal/grpcrand"
 	"google.golang.org/grpc/internal/stubserver"
 	"google.golang.org/grpc/internal/testutils"
 	"google.golang.org/grpc/internal/testutils/pickfirst"
@@ -379,3 +380,54 @@ func (s) TestPickFirst_StickyTransientFailure(t *testing.T) {
 	}()
 	wg.Wait()
 }
+
+func (s) TestPickFirst_ShuffleAddressList(t *testing.T) {
+	const serviceConfig = `{"loadBalancingConfig": [{"pick_first":{ "shuffleAddressList": true }}]}`
+
+	// Install a shuffler that always reverses two entries.
+	origShuf := grpcrand.Shuffle
+	defer func() { grpcrand.Shuffle = origShuf }()
+	grpcrand.Shuffle = func(n int, f func(int, int)) {
+		if n != 2 {
+			t.Errorf("Shuffle called with n=%v; want 2", n)
+		}
+		f(0, 1) // reverse the two addresses
+	}
+
+	// Set up our backends.
+	cc, r, backends := setupPickFirst(t, 2)
+	addrs := stubBackendsToResolverAddrs(backends)
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	// Push an update with both addresses and shuffling disabled.  We should
+	// connect to backend 0.
+	r.UpdateState(resolver.State{Addresses: []resolver.Address{addrs[0], addrs[1]}})
+	if err := pickfirst.CheckRPCsToBackend(ctx, cc, addrs[0]); err != nil {
+		t.Fatal(err)
+	}
+
+	// Send a config with shuffling enabled.  This will reverse the addresses,
+	// but the channel should still be connected to backend 0.
+	shufState := resolver.State{
+		ServiceConfig: parseServiceConfig(t, r, serviceConfig),
+		Addresses:     []resolver.Address{addrs[0], addrs[1]},
+	}
+	r.UpdateState(shufState)
+	if err := pickfirst.CheckRPCsToBackend(ctx, cc, addrs[0]); err != nil {
+		t.Fatal(err)
+	}
+
+	// Send a resolver update with no addresses. This should push the channel
+	// into TransientFailure.
+	r.UpdateState(resolver.State{})
+	awaitState(ctx, t, cc, connectivity.TransientFailure)
+
+	// Send the same config as last time with shuffling enabled.  Since we are
+	// not connected to backend 0, we should connect to backend 1.
+	r.UpdateState(shufState)
+	if err := pickfirst.CheckRPCsToBackend(ctx, cc, addrs[1]); err != nil {
+		t.Fatal(err)
+	}
+}