plugin/grpc: New gRPC plugin (#2667)

* plugin/grpc: New gRPC plugin

* some changes after the first review:

- remove healthcheck. gRPC already has this implicitly implemented
- some naming and stetic changes
- fix some comments
- other minor fixes

* plugin/grpc: New gRPC plugin

* some changes after the first review:

- remove healthcheck. gRPC already has this implicitly implemented
- some naming and stetic changes
- fix some comments
- other minor fixes

* add OWNERS file and change plugin order

* remove Rcode checker

plugin/grpc/grpc_test.go

@@ -0,0 +1,75 @@
+package grpc
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/coredns/coredns/pb"
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+func TestGRPC(t *testing.T) {
+	m := &dns.Msg{}
+	msg, err := m.Pack()
+	if err != nil {
+		t.Fatalf("Error packing response: %s", err.Error())
+	}
+	dnsPacket := &pb.DnsPacket{Msg: msg}
+	tests := map[string]struct {
+		proxies []*Proxy
+		wantErr bool
+	}{
+		"single_proxy_ok": {
+			proxies: []*Proxy{
+				{client: &testServiceClient{dnsPacket: dnsPacket, err: nil}},
+			},
+			wantErr: false,
+		},
+		"multiple_proxies_ok": {
+			proxies: []*Proxy{
+				{client: &testServiceClient{dnsPacket: dnsPacket, err: nil}},
+				{client: &testServiceClient{dnsPacket: dnsPacket, err: nil}},
+				{client: &testServiceClient{dnsPacket: dnsPacket, err: nil}},
+			},
+			wantErr: false,
+		},
+		"single_proxy_ko": {
+			proxies: []*Proxy{
+				{client: &testServiceClient{dnsPacket: nil, err: errors.New("")}},
+			},
+			wantErr: true,
+		},
+		"multiple_proxies_one_ko": {
+			proxies: []*Proxy{
+				{client: &testServiceClient{dnsPacket: dnsPacket, err: nil}},
+				{client: &testServiceClient{dnsPacket: nil, err: errors.New("")}},
+				{client: &testServiceClient{dnsPacket: dnsPacket, err: nil}},
+			},
+			wantErr: false,
+		},
+		"multiple_proxies_ko": {
+			proxies: []*Proxy{
+				{client: &testServiceClient{dnsPacket: nil, err: errors.New("")}},
+				{client: &testServiceClient{dnsPacket: nil, err: errors.New("")}},
+				{client: &testServiceClient{dnsPacket: nil, err: errors.New("")}},
+			},
+			wantErr: true,
+		},
+	}
+
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			g := newGRPC()
+			g.from = "."
+			g.proxies = tt.proxies
+			rec := dnstest.NewRecorder(&test.ResponseWriter{})
+			if _, err := g.ServeDNS(context.TODO(), rec, m); err != nil && !tt.wantErr {
+				t.Fatal("Expected to receive reply, but didn't")
+			}
+		})
+	}
+}