Client-side of gRPC proxy (#511)

* WIP: Client-side of gRPC proxy

* Add tests

* gofmt

* Implement OnShutdown; add a little logging

* Update for context in Exchange change

* go fmt

* Update README

* Review comments

* Compiling is good

* More README improvements

middleware/proxy/pb/dns.pb.go

@@ -0,0 +1,140 @@
+// Code generated by protoc-gen-go.
+// source: dns.proto
+// DO NOT EDIT!
+
+/*
+Package pb is a generated protocol buffer package.
+
+It is generated from these files:
+	dns.proto
+
+It has these top-level messages:
+	DnsPacket
+*/
+package pb
+
+import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
+import math "math"
+
+import (
+	context "golang.org/x/net/context"
+	grpc "google.golang.org/grpc"
+)
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
+type DnsPacket struct {
+	Msg []byte `protobuf:"bytes,1,opt,name=msg,proto3" json:"msg,omitempty"`
+}
+
+func (m *DnsPacket) Reset()                    { *m = DnsPacket{} }
+func (m *DnsPacket) String() string            { return proto.CompactTextString(m) }
+func (*DnsPacket) ProtoMessage()               {}
+func (*DnsPacket) Descriptor() ([]byte, []int) { return fileDescriptor0, []int{0} }
+
+func (m *DnsPacket) GetMsg() []byte {
+	if m != nil {
+		return m.Msg
+	}
+	return nil
+}
+
+func init() {
+	proto.RegisterType((*DnsPacket)(nil), "coredns.dns.DnsPacket")
+}
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ context.Context
+var _ grpc.ClientConn
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the grpc package it is being compiled against.
+const _ = grpc.SupportPackageIsVersion4
+
+// Client API for DnsService service
+
+type DnsServiceClient interface {
+	Query(ctx context.Context, in *DnsPacket, opts ...grpc.CallOption) (*DnsPacket, error)
+}
+
+type dnsServiceClient struct {
+	cc *grpc.ClientConn
+}
+
+func NewDnsServiceClient(cc *grpc.ClientConn) DnsServiceClient {
+	return &dnsServiceClient{cc}
+}
+
+func (c *dnsServiceClient) Query(ctx context.Context, in *DnsPacket, opts ...grpc.CallOption) (*DnsPacket, error) {
+	out := new(DnsPacket)
+	err := grpc.Invoke(ctx, "/coredns.dns.DnsService/Query", in, out, c.cc, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+// Server API for DnsService service
+
+type DnsServiceServer interface {
+	Query(context.Context, *DnsPacket) (*DnsPacket, error)
+}
+
+func RegisterDnsServiceServer(s *grpc.Server, srv DnsServiceServer) {
+	s.RegisterService(&_DnsService_serviceDesc, srv)
+}
+
+func _DnsService_Query_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(DnsPacket)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(DnsServiceServer).Query(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/coredns.dns.DnsService/Query",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(DnsServiceServer).Query(ctx, req.(*DnsPacket))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+var _DnsService_serviceDesc = grpc.ServiceDesc{
+	ServiceName: "coredns.dns.DnsService",
+	HandlerType: (*DnsServiceServer)(nil),
+	Methods: []grpc.MethodDesc{
+		{
+			MethodName: "Query",
+			Handler:    _DnsService_Query_Handler,
+		},
+	},
+	Streams:  []grpc.StreamDesc{},
+	Metadata: "dns.proto",
+}
+
+func init() { proto.RegisterFile("dns.proto", fileDescriptor0) }
+
+var fileDescriptor0 = []byte{
+	// 120 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0xe2, 0xe2, 0x4c, 0xc9, 0x2b, 0xd6,
+	0x2b, 0x28, 0xca, 0x2f, 0xc9, 0x17, 0xe2, 0x4e, 0xce, 0x2f, 0x4a, 0x05, 0x71, 0x53, 0xf2, 0x8a,
+	0x95, 0x64, 0xb9, 0x38, 0x5d, 0xf2, 0x8a, 0x03, 0x12, 0x93, 0xb3, 0x53, 0x4b, 0x84, 0x04, 0xb8,
+	0x98, 0x73, 0x8b, 0xd3, 0x25, 0x18, 0x15, 0x18, 0x35, 0x78, 0x82, 0x40, 0x4c, 0x23, 0x57, 0x2e,
+	0x2e, 0x97, 0xbc, 0xe2, 0xe0, 0xd4, 0xa2, 0xb2, 0xcc, 0xe4, 0x54, 0x21, 0x73, 0x2e, 0xd6, 0xc0,
+	0xd2, 0xd4, 0xa2, 0x4a, 0x21, 0x31, 0x3d, 0x24, 0x33, 0xf4, 0xe0, 0x06, 0x48, 0xe1, 0x10, 0x77,
+	0x62, 0x89, 0x62, 0x2a, 0x48, 0x4a, 0x62, 0x03, 0xdb, 0x6f, 0x0c, 0x08, 0x00, 0x00, 0xff, 0xff,
+	0xf5, 0xd1, 0x3f, 0x26, 0x8c, 0x00, 0x00, 0x00,
+}

middleware/proxy/pb/dns.proto

@@ -0,0 +1,12 @@
+syntax = "proto3";
+
+package coredns.dns;
+option go_package = "pb";
+
+message DnsPacket {
+	bytes msg = 1;
+}
+
+service DnsService {
+	rpc Query (DnsPacket) returns (DnsPacket);
+}