Added support for Haystack tracing

vendor/google.golang.org/grpc/balancer/roundrobin/roundrobin.go

@@ -22,12 +22,13 @@
 package roundrobin
 
 import (
+	"context"
 	"sync"
 
-	"golang.org/x/net/context"
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/balancer/base"
 	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/internal/grpcrand"
 	"google.golang.org/grpc/resolver"
 )
 
@@ -36,7 +37,7 @@ const Name = "round_robin"
 
 // newBuilder creates a new roundrobin balancer builder.
 func newBuilder() balancer.Builder {
-	return base.NewBalancerBuilder(Name, &rrPickerBuilder{})
+	return base.NewBalancerBuilderWithConfig(Name, &rrPickerBuilder{}, base.Config{HealthCheck: true})
 }
 
 func init() {
@@ -47,12 +48,19 @@ type rrPickerBuilder struct{}
 
 func (*rrPickerBuilder) Build(readySCs map[resolver.Address]balancer.SubConn) balancer.Picker {
 	grpclog.Infof("roundrobinPicker: newPicker called with readySCs: %v", readySCs)
+	if len(readySCs) == 0 {
+		return base.NewErrPicker(balancer.ErrNoSubConnAvailable)
+	}
 	var scs []balancer.SubConn
 	for _, sc := range readySCs {
 		scs = append(scs, sc)
 	}
 	return &rrPicker{
 		subConns: scs,
+		// Start at a random index, as the same RR balancer rebuilds a new
+		// picker when SubConn states change, and we don't want to apply excess
+		// load to the first server in the list.
+		next: grpcrand.Intn(len(scs)),
 	}
 }
 
@@ -67,10 +75,6 @@ type rrPicker struct {
 }
 
 func (p *rrPicker) Pick(ctx context.Context, opts balancer.PickOptions) (balancer.SubConn, func(balancer.DoneInfo), error) {
-	if len(p.subConns) <= 0 {
-		return nil, nil, balancer.ErrNoSubConnAvailable
-	}
-
 	p.mu.Lock()
 	sc := p.subConns[p.next]
 	p.next = (p.next + 1) % len(p.subConns)