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/proxy_test.go

@@ -0,0 +1,66 @@
+package grpc
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/coredns/coredns/pb"
+
+	"github.com/miekg/dns"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials"
+)
+
+func TestProxy(t *testing.T) {
+	tests := map[string]struct {
+		p       *Proxy
+		res     *dns.Msg
+		wantErr bool
+	}{
+		"response_ok": {
+			p:       &Proxy{},
+			res:     &dns.Msg{},
+			wantErr: false,
+		},
+		"nil_response": {
+			p:       &Proxy{},
+			res:     nil,
+			wantErr: true,
+		},
+		"tls": {
+			p:       &Proxy{dialOpts: []grpc.DialOption{grpc.WithTransportCredentials(credentials.NewTLS(nil))}},
+			res:     &dns.Msg{},
+			wantErr: false,
+		},
+	}
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			var mock *testServiceClient
+			if tt.res != nil {
+				msg, err := tt.res.Pack()
+				if err != nil {
+					t.Fatalf("Error packing response: %s", err.Error())
+				}
+				mock = &testServiceClient{&pb.DnsPacket{Msg: msg}, nil}
+			} else {
+				mock = &testServiceClient{nil, errors.New("server error")}
+			}
+			tt.p.client = mock
+
+			_, err := tt.p.query(context.TODO(), new(dns.Msg))
+			if err != nil && !tt.wantErr {
+				t.Fatalf("Error query(): %s", err.Error())
+			}
+		})
+	}
+}
+
+type testServiceClient struct {
+	dnsPacket *pb.DnsPacket
+	err       error
+}
+
+func (m testServiceClient) Query(ctx context.Context, in *pb.DnsPacket, opts ...grpc.CallOption) (*pb.DnsPacket, error) {
+	return m.dnsPacket, m.err
+}