plugin/forward/grpc: Revert forward/grpc policy dedup (#3919)

* revert de-dup

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* unit test

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* use roundrobin policy in test

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/grpc/grpc.go

@@ -7,7 +7,6 @@ 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"
@@ -18,7 +17,7 @@ import (
 // It has a list of proxies each representing one upstream proxy.
 type GRPC struct {
 	proxies []*Proxy
-	p       policy.Policy
+	p       Policy
 
 	from    string
 	ignored []string
@@ -94,7 +93,7 @@ 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(policy.Random),
+		p: new(random),
 	}
 	return g
 }
@@ -127,11 +126,6 @@ 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 {
-	if len(g.p.List(g.proxies)) == 1 {
-		return g.p.List(g.proxies)[0].([]*Proxy)
-	}
-	return nil
-}
+func (g *GRPC) list() []*Proxy { return g.p.List(g.proxies) }
 
 const defaultTimeout = 5 * time.Second

plugin/grpc/policy.go

@@ -0,0 +1,64 @@
+package grpc
+
+import (
+	"math/rand"
+	"sync/atomic"
+)
+
+// Policy defines a policy we use for selecting upstreams.
+type Policy interface {
+	List([]*Proxy) []*Proxy
+	String() string
+}
+
+// random is a policy that implements random upstream selection.
+type random struct{}
+
+func (r *random) String() string { return "random" }
+
+func (r *random) List(p []*Proxy) []*Proxy {
+	switch len(p) {
+	case 1:
+		return p
+	case 2:
+		if rand.Int()%2 == 0 {
+			return []*Proxy{p[1], p[0]} // swap
+		}
+		return p
+	}
+
+	perms := rand.Perm(len(p))
+	rnd := make([]*Proxy, len(p))
+
+	for i, p1 := range perms {
+		rnd[i] = p[p1]
+	}
+	return rnd
+}
+
+// roundRobin is a policy that selects hosts based on round robin ordering.
+type roundRobin struct {
+	robin uint32
+}
+
+func (r *roundRobin) String() string { return "round_robin" }
+
+func (r *roundRobin) List(p []*Proxy) []*Proxy {
+	poolLen := uint32(len(p))
+	i := atomic.AddUint32(&r.robin, 1) % poolLen
+
+	robin := []*Proxy{p[i]}
+	robin = append(robin, p[:i]...)
+	robin = append(robin, p[i+1:]...)
+
+	return robin
+}
+
+// sequential is a policy that selects hosts based on sequential ordering.
+type sequential struct{}
+
+func (r *sequential) String() string { return "sequential" }
+
+func (r *sequential) List(p []*Proxy) []*Proxy {
+	return p
+}

plugin/grpc/setup.go

@@ -8,7 +8,6 @@ import (
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics"
 	"github.com/coredns/coredns/plugin/pkg/parse"
-	"github.com/coredns/coredns/plugin/pkg/policy"
 	pkgtls "github.com/coredns/coredns/plugin/pkg/tls"
 
 	"github.com/caddyserver/caddy"
@@ -133,11 +132,11 @@ func parseBlock(c *caddy.Controller, g *GRPC) error {
 		}
 		switch x := c.Val(); x {
 		case "random":
-			g.p = &policy.Random{}
+			g.p = &random{}
 		case "round_robin":
-			g.p = &policy.RoundRobin{}
+			g.p = &roundRobin{}
 		case "sequential":
-			g.p = &policy.Sequential{}
+			g.p = &sequential{}
 		default:
 			return c.Errf("unknown policy '%s'", x)
 		}