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/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
+}