pickfirstleaf: Fix shuffling of addresses in resolver updates without endpoints (#8610)

The new `pick_first`, which is the default, doesn't shuffle the
addresses at all for resolver updates that are missing the `Endpoints`
field. This change fixes that. Since [gRPC automatically sets the the
missing
`Endpoints`](https://github.com/grpc/grpc-go/blob/1059e84f885bf7ed65b3b1a4fbe914360d8ab5b1/resolver_wrapper.go#L136-L138),
occurrence of this bug should be uncommon in practice.

RELEASE NOTES:
* balancer/pick_first: When configured, shuffle addresses in resolver
updates that lack endpoints. Since gRPC automatically adds endpoints to
resolver updates, this bug should only affect implementers of custom LB
policies that use pick_first for delegation but don't forward the
endpoints.

Author: arjan-bal

balancer/pickfirst/pickfirst.go

@@ -169,7 +169,7 @@ func (b *pickfirstBalancer) UpdateClientConnState(state balancer.ClientConnState
 		addrs = state.ResolverState.Addresses
 		if cfg.ShuffleAddressList {
 			addrs = append([]resolver.Address{}, addrs...)
-			rand.Shuffle(len(addrs), func(i, j int) { addrs[i], addrs[j] = addrs[j], addrs[i] })
+			internal.RandShuffle(len(addrs), func(i, j int) { addrs[i], addrs[j] = addrs[j], addrs[i] })
 		}
 	}
 

balancer/pickfirst/pickfirst_ext_test.go

@@ -20,6 +20,7 @@ package pickfirst_test
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"strings"
@@ -28,11 +29,14 @@ import (
 
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/backoff"
+	"google.golang.org/grpc/balancer"
+	pfbalancer "google.golang.org/grpc/balancer/pickfirst"
 	pfinternal "google.golang.org/grpc/balancer/pickfirst/internal"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/connectivity"
 	"google.golang.org/grpc/credentials/insecure"
 	"google.golang.org/grpc/internal"
+	"google.golang.org/grpc/internal/balancer/stub"
 	"google.golang.org/grpc/internal/channelz"
 	"google.golang.org/grpc/internal/grpctest"
 	"google.golang.org/grpc/internal/stubserver"
@@ -463,6 +467,85 @@ func (s) TestPickFirst_ShuffleAddressList(t *testing.T) {
 	}
 }
 
+// Tests the PF LB policy with shuffling enabled. It explicitly unsets the
+// Endpoints field in the resolver update to test the shuffling of the
+// Addresses.
+func (s) TestPickFirst_ShuffleAddressListNoEndpoints(t *testing.T) {
+	// Install a shuffler that always reverses two entries.
+	origShuf := pfinternal.RandShuffle
+	defer func() { pfinternal.RandShuffle = origShuf }()
+	pfinternal.RandShuffle = func(n int, f func(int, int)) {
+		if n != 2 {
+			t.Errorf("Shuffle called with n=%v; want 2", n)
+			return
+		}
+		f(0, 1) // reverse the two addresses
+	}
+
+	pfBuilder := balancer.Get(pfbalancer.Name)
+	shuffleConfig, err := pfBuilder.(balancer.ConfigParser).ParseConfig(json.RawMessage(`{ "shuffleAddressList": true }`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	noShuffleConfig, err := pfBuilder.(balancer.ConfigParser).ParseConfig(json.RawMessage(`{ "shuffleAddressList": false }`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var activeCfg serviceconfig.LoadBalancingConfig
+
+	bf := stub.BalancerFuncs{
+		Init: func(bd *stub.BalancerData) {
+			bd.ChildBalancer = pfBuilder.Build(bd.ClientConn, bd.BuildOptions)
+		},
+		Close: func(bd *stub.BalancerData) {
+			bd.ChildBalancer.Close()
+		},
+		UpdateClientConnState: func(bd *stub.BalancerData, ccs balancer.ClientConnState) error {
+			ccs.BalancerConfig = activeCfg
+			ccs.ResolverState.Endpoints = nil
+			return bd.ChildBalancer.UpdateClientConnState(ccs)
+		},
+	}
+
+	stub.Register(t.Name(), bf)
+	svcCfg := fmt.Sprintf(`{ "loadBalancingConfig": [{%q: {}}] }`, t.Name())
+	// Set up our backends.
+	cc, r, backends := setupPickFirst(t, 2, grpc.WithDefaultServiceConfig(svcCfg))
+	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.
+	activeCfg = noShuffleConfig
+	resolverState := resolver.State{Addresses: addrs}
+	r.UpdateState(resolverState)
+	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.
+	activeCfg = shuffleConfig
+	r.UpdateState(resolverState)
+	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{})
+	testutils.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(resolverState)
+	if err := pickfirst.CheckRPCsToBackend(ctx, cc, addrs[1]); err != nil {
+		t.Fatal(err)
+	}
+}
+
 // Test config parsing with the env var turned on and off for various scenarios.
 func (s) TestPickFirst_ParseConfig_Success(t *testing.T) {
 	// Install a shuffler that always reverses two entries.

balancer/pickfirst/pickfirstleaf/pickfirstleaf.go

@@ -283,7 +283,7 @@ func (b *pickfirstBalancer) UpdateClientConnState(state balancer.ClientConnState
 		newAddrs = state.ResolverState.Addresses
 		if cfg.ShuffleAddressList {
 			newAddrs = append([]resolver.Address{}, newAddrs...)
-			internal.RandShuffle(len(endpoints), func(i, j int) { endpoints[i], endpoints[j] = endpoints[j], endpoints[i] })
+			internal.RandShuffle(len(newAddrs), func(i, j int) { newAddrs[i], newAddrs[j] = newAddrs[j], newAddrs[i] })
 		}
 	}