Dedup policy implement between grpc and proxy plugin (#3537)

Signed-off-by: zouyee <zounengren@cmss.chinamobile.com>

plugin/grpc/grpc.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/debug"
+	"github.com/coredns/coredns/plugin/pkg/policy"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -17,7 +18,7 @@ import (
 // It has a list of proxies each representing one upstream proxy.
 type GRPC struct {
 	proxies []*Proxy
-	p       Policy
+	p       policy.Policy
 
 	from    string
 	ignored []string
@@ -93,7 +94,7 @@ func (g *GRPC) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (
 // NewGRPC returns a new GRPC.
 func newGRPC() *GRPC {
 	g := &GRPC{
-		p: new(random),
+		p: new(policy.Random),
 	}
 	return g
 }
@@ -126,6 +127,11 @@ func (g *GRPC) isAllowedDomain(name string) bool {
 }
 
 // List returns a set of proxies to be used for this client depending on the policy in p.
-func (g *GRPC) list() []*Proxy { return g.p.List(g.proxies) }
+func (g *GRPC) list() []*Proxy {
+	if len(g.p.List(g.proxies)) == 1 {
+		return g.p.List(g.proxies)[0].([]*Proxy)
+	}
+	return nil
+}
 
 const defaultTimeout = 5 * time.Second