add grpc-proxy (github.com/mwitkow/grpc-proxy)

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

proxy/README.md

@@ -0,0 +1,83 @@
+# gRPC Proxy
+
+[![Travis Build](https://travis-ci.org/mwitkow/grpc-proxy.svg?branch=master)](https://travis-ci.org/mwitkow/grpc-proxy)
+[![Go Report Card](https://goreportcard.com/badge/github.com/mwitkow/grpc-proxy)](https://goreportcard.com/report/github.com/mwitkow/grpc-proxy)
+[![Go Reference](https://pkg.go.dev/badge/github.com/mwitkow/grpc-proxy.svg)](https://pkg.go.dev/github.com/mwitkow/grpc-proxy)
+[![Apache 2.0 License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
+
+[gRPC Go](https://github.com/grpc/grpc-go) Proxy server
+
+## Project Goal
+
+Build a transparent reverse proxy for gRPC targets that will make it easy to expose gRPC services
+over the internet. This includes:
+ * no needed knowledge of the semantics of requests exchanged in the call (independent rollouts)
+ * easy, declarative definition of backends and their mappings to frontends
+ * simple round-robin load balancing of inbound requests from a single connection to multiple backends
+
+The project now exists as a **proof of concept**, with the key piece being the `proxy` package that
+is a generic gRPC reverse proxy handler.
+
+## Proxy Handler
+
+The package [`proxy`](proxy/) contains a generic gRPC reverse proxy handler that allows a gRPC server to
+not know about registered handlers or their data types. Please consult the docs, here's an exaple usage.
+
+You can call `proxy.NewProxy` to create a `*grpc.Server` that proxies requests.
+```go
+proxy := proxy.NewProxy(clientConn)
+``` 
+
+More advanced users will want to define a `StreamDirector` that can make more complex decisions on what
+to do with the request.
+```go
+director = func(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error) {
+    md, _ := metadata.FromIncomingContext(ctx)
+    outCtx = metadata.NewOutgoingContext(ctx, md.Copy())
+    return outCtx, cc, nil
+	
+    // Make sure we never forward internal services.
+    if strings.HasPrefix(fullMethodName, "/com.example.internal.") {
+        return outCtx, nil, status.Errorf(codes.Unimplemented, "Unknown method")
+    }
+    
+    if ok {
+        // Decide on which backend to dial
+        if val, exists := md[":authority"]; exists && val[0] == "staging.api.example.com" {
+            // Make sure we use DialContext so the dialing can be cancelled/time out together with the context.
+            return outCtx, grpc.DialContext(ctx, "api-service.staging.svc.local", grpc.WithCodec(proxy.Codec())), nil
+        } else if val, exists := md[":authority"]; exists && val[0] == "api.example.com" {
+            return outCtx, grpc.DialContext(ctx, "api-service.prod.svc.local", grpc.WithCodec(proxy.Codec())), nil
+        }
+    }
+    return outCtx, nil, status.Errorf(codes.Unimplemented, "Unknown method")
+}
+```
+
+Then you need to register it with a `grpc.Server`. The server may have other handlers that will be served
+locally.
+
+```go
+server := grpc.NewServer(
+    grpc.CustomCodec(proxy.Codec()),
+    grpc.UnknownServiceHandler(proxy.TransparentHandler(director)))
+pb_test.RegisterTestServiceServer(server, &testImpl{})
+```
+
+## Testing
+To make debugging a bit simpler, there are some helpers.
+
+`testservice` contains a method `TestTestServiceServerImpl` which performs a complete test against
+the reference implementation of the `TestServiceServer`.
+
+In `proxy_test.go`, the test framework spins up a `TestServiceServer` that it tests the proxy 
+against. To make debugging a bit simpler (eg. if the developer needs to step into 
+`google.golang.org/grpc` methods), this `TestServiceServer` can be provided by a server by 
+passing `-test-backend=addr` to `go test`. A simple, local-only implementation of 
+`TestServiceServer` exists in [`testservice/server`](./testservice/server).
+
+
+## License
+
+`grpc-proxy` is released under the Apache 2.0 license. See [LICENSE.txt](LICENSE.txt).
+

proxy/codec.go

@@ -0,0 +1,69 @@
+package proxy
+
+import (
+	"fmt"
+
+	"google.golang.org/grpc"
+	"google.golang.org/protobuf/proto"
+)
+
+// Codec returns a proxying grpc.Codec with the default protobuf codec as parent.
+//
+// See CodecWithParent.
+//
+// Deprecated: No longer necessary.
+func Codec() grpc.Codec {
+	return CodecWithParent(&protoCodec{})
+}
+
+// CodecWithParent returns a proxying grpc.Codec with a user provided codec as parent.
+//
+// Deprecated: No longer necessary.
+func CodecWithParent(fallback grpc.Codec) grpc.Codec {
+	return &rawCodec{fallback}
+}
+
+type rawCodec struct {
+	parentCodec grpc.Codec
+}
+
+type frame struct {
+	payload []byte
+}
+
+func (c *rawCodec) Marshal(v interface{}) ([]byte, error) {
+	out, ok := v.(*frame)
+	if !ok {
+		return c.parentCodec.Marshal(v)
+	}
+	return out.payload, nil
+
+}
+
+func (c *rawCodec) Unmarshal(data []byte, v interface{}) error {
+	dst, ok := v.(*frame)
+	if !ok {
+		return c.parentCodec.Unmarshal(data, v)
+	}
+	dst.payload = data
+	return nil
+}
+
+func (c *rawCodec) String() string {
+	return fmt.Sprintf("proxy>%s", c.parentCodec.String())
+}
+
+// protoCodec is a Codec implementation with protobuf. It is the default rawCodec for gRPC.
+type protoCodec struct{}
+
+func (protoCodec) Marshal(v interface{}) ([]byte, error) {
+	return proto.Marshal(v.(proto.Message))
+}
+
+func (protoCodec) Unmarshal(data []byte, v interface{}) error {
+	return proto.Unmarshal(data, v.(proto.Message))
+}
+
+func (protoCodec) String() string {
+	return "proto"
+}

proxy/codec_test.go

@@ -0,0 +1,23 @@
+package proxy
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCodec_ReadYourWrites(t *testing.T) {
+	framePtr := &frame{}
+	data := []byte{0xDE, 0xAD, 0xBE, 0xEF}
+	codec := rawCodec{}
+	require.NoError(t, codec.Unmarshal(data, framePtr), "unmarshalling must go ok")
+	out, err := codec.Marshal(framePtr)
+	require.NoError(t, err, "no marshal error")
+	require.Equal(t, data, out, "output and data must be the same")
+
+	// reuse
+	require.NoError(t, codec.Unmarshal([]byte{0x55}, framePtr), "unmarshalling must go ok")
+	out, err = codec.Marshal(framePtr)
+	require.NoError(t, err, "no marshal error")
+	require.Equal(t, []byte{0x55}, out, "output and data must be the same")
+}

proxy/director.go

@@ -0,0 +1,25 @@
+// Copyright 2017 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+package proxy
+
+import (
+	"context"
+
+	"google.golang.org/grpc"
+)
+
+// StreamDirector returns a gRPC ClientConn to be used to forward the call to.
+//
+// The presence of the `Context` allows for rich filtering, e.g. based on Metadata (headers).
+// If no handling is meant to be done, a `codes.NotImplemented` gRPC error should be returned.
+//
+// The context returned from this function should be the context for the *outgoing* (to backend) call. In case you want
+// to forward any Metadata between the inbound request and outbound requests, you should do it manually. However, you
+// *must* propagate the cancel function (`context.WithCancel`) of the inbound context to the one returned.
+//
+// It is worth noting that the StreamDirector will be fired *after* all server-side stream interceptors
+// are invoked. So decisions around authorization, monitoring etc. are better to be handled there.
+//
+// See the rather rich example.
+type StreamDirector func(ctx context.Context, fullMethodName string) (context.Context, grpc.ClientConnInterface, error)

proxy/doc.go

@@ -0,0 +1,15 @@
+// Copyright 2017 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+/*
+Package proxy provides a reverse proxy handler for gRPC.
+
+The implementation allows a grpc.Server to pass a received ServerStream to a ClientStream without understanding
+the semantics of the messages exchanged. It basically provides a transparent reverse-proxy.
+
+This package is intentionally generic, exposing a StreamDirector function that allows users of this package
+to implement whatever logic of backend-picking, dialing and service verification to perform.
+
+See examples on documented functions.
+*/
+package proxy

proxy/examples_test.go

@@ -0,0 +1,70 @@
+// Copyright 2017 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+package proxy_test
+
+import (
+	"context"
+	"log"
+	"strings"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/status"
+
+	"go.linka.cloud/grpc-toolkit/proxy"
+)
+
+var (
+	director proxy.StreamDirector
+)
+
+func ExampleNewProxy() {
+	dst, err := grpc.Dial("example.com")
+	if err != nil {
+		log.Fatalf("dialing example.org: %v", err)
+	}
+	proxy := proxy.NewProxy(dst)
+	_ = proxy
+}
+
+func ExampleRegisterService() {
+	// A gRPC server with the proxying codec enabled.
+	server := grpc.NewServer()
+	// Register a TestService with 4 of its methods explicitly.
+	proxy.RegisterService(server, director,
+		"mwitkow.testproto.TestService",
+		"PingEmpty", "Ping", "PingError", "PingList")
+}
+
+func ExampleTransparentHandler() {
+	grpc.NewServer(
+		grpc.UnknownServiceHandler(proxy.TransparentHandler(director)))
+}
+
+// Provides a simple example of a director that shields internal services and dials a staging or production backend.
+// This is a *very naive* implementation that creates a new connection on every request. Consider using pooling.
+func ExampleStreamDirector() {
+	director = func(ctx context.Context, fullMethodName string) (context.Context, grpc.ClientConnInterface, error) {
+		// Make sure we never forward internal services.
+		if strings.HasPrefix(fullMethodName, "/com.example.internal.") {
+			return nil, nil, status.Errorf(codes.Unimplemented, "Unknown method")
+		}
+		md, ok := metadata.FromIncomingContext(ctx)
+		// Copy the inbound metadata explicitly.
+		outCtx := metadata.NewOutgoingContext(ctx, md.Copy())
+		if ok {
+			// Decide on which backend to dial
+			if val, exists := md[":authority"]; exists && val[0] == "staging.api.example.com" {
+				// Make sure we use DialContext so the dialing can be cancelled/time out together with the context.
+				conn, err := grpc.DialContext(ctx, "api-service.staging.svc.local")
+				return outCtx, conn, err
+			} else if val, exists := md[":authority"]; exists && val[0] == "api.example.com" {
+				conn, err := grpc.DialContext(ctx, "api-service.prod.svc.local")
+				return outCtx, conn, err
+			}
+		}
+		return nil, nil, status.Errorf(codes.Unimplemented, "Unknown method")
+	}
+}

proxy/handler.go

@@ -0,0 +1,160 @@
+// Copyright 2017 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+package proxy
+
+import (
+	"context"
+	"io"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/types/known/emptypb"
+)
+
+var (
+	clientStreamDescForProxying = &grpc.StreamDesc{
+		ServerStreams: true,
+		ClientStreams: true,
+	}
+)
+
+// RegisterService sets up a proxy handler for a particular gRPC service and method.
+// The behaviour is the same as if you were registering a handler method, e.g. from a generated pb.go file.
+func RegisterService(server *grpc.Server, director StreamDirector, serviceName string, methodNames ...string) {
+	streamer := &handler{director}
+	fakeDesc := &grpc.ServiceDesc{
+		ServiceName: serviceName,
+		HandlerType: (*interface{})(nil),
+	}
+	for _, m := range methodNames {
+		streamDesc := grpc.StreamDesc{
+			StreamName:    m,
+			Handler:       streamer.handler,
+			ServerStreams: true,
+			ClientStreams: true,
+		}
+		fakeDesc.Streams = append(fakeDesc.Streams, streamDesc)
+	}
+	server.RegisterService(fakeDesc, streamer)
+}
+
+// TransparentHandler returns a handler that attempts to proxy all requests that are not registered in the server.
+// The indented use here is as a transparent proxy, where the server doesn't know about the services implemented by the
+// backends. It should be used as a `grpc.UnknownServiceHandler`.
+func TransparentHandler(director StreamDirector) grpc.StreamHandler {
+	streamer := &handler{director: director}
+	return streamer.handler
+}
+
+type handler struct {
+	director StreamDirector
+}
+
+// handler is where the real magic of proxying happens.
+// It is invoked like any gRPC server stream and uses the emptypb.Empty type server
+// to proxy calls between the input and output streams.
+func (s *handler) handler(srv interface{}, serverStream grpc.ServerStream) error {
+	// little bit of gRPC internals never hurt anyone
+	fullMethodName, ok := grpc.MethodFromServerStream(serverStream)
+	if !ok {
+		return status.Errorf(codes.Internal, "lowLevelServerStream not exists in context")
+	}
+	// We require that the director's returned context inherits from the serverStream.Context().
+	outgoingCtx, backendConn, err := s.director(serverStream.Context(), fullMethodName)
+	if err != nil {
+		return err
+	}
+
+	clientCtx, clientCancel := context.WithCancel(outgoingCtx)
+	defer clientCancel()
+	// TODO(mwitkow): Add a `forwarded` header to metadata, https://en.wikipedia.org/wiki/X-Forwarded-For.
+	clientStream, err := backendConn.NewStream(clientCtx, clientStreamDescForProxying, fullMethodName)
+	if err != nil {
+		return err
+	}
+	// Explicitly *do not close* s2cErrChan and c2sErrChan, otherwise the select below will not terminate.
+	// Channels do not have to be closed, it is just a control flow mechanism, see
+	// https://groups.google.com/forum/#!msg/golang-nuts/pZwdYRGxCIk/qpbHxRRPJdUJ
+	s2cErrChan := s.forwardServerToClient(serverStream, clientStream)
+	c2sErrChan := s.forwardClientToServer(clientStream, serverStream)
+	// We don't know which side is going to stop sending first, so we need a select between the two.
+	for i := 0; i < 2; i++ {
+		select {
+		case s2cErr := <-s2cErrChan:
+			if s2cErr == io.EOF {
+				// this is the happy case where the sender has encountered io.EOF, and won't be sending anymore./
+				// the clientStream>serverStream may continue pumping though.
+				clientStream.CloseSend()
+			} else {
+				// however, we may have gotten a receive error (stream disconnected, a read error etc) in which case we need
+				// to cancel the clientStream to the backend, let all of its goroutines be freed up by the CancelFunc and
+				// exit with an error to the stack
+				clientCancel()
+				return status.Errorf(codes.Internal, "failed proxying s2c: %v", s2cErr)
+			}
+		case c2sErr := <-c2sErrChan:
+			// This happens when the clientStream has nothing else to offer (io.EOF), returned a gRPC error. In those two
+			// cases we may have received Trailers as part of the call. In case of other errors (stream closed) the trailers
+			// will be nil.
+			serverStream.SetTrailer(clientStream.Trailer())
+			// c2sErr will contain RPC error from client code. If not io.EOF return the RPC error as server stream error.
+			if c2sErr != io.EOF {
+				return c2sErr
+			}
+			return nil
+		}
+	}
+	return status.Errorf(codes.Internal, "gRPC proxying should never reach this stage.")
+}
+
+func (s *handler) forwardClientToServer(src grpc.ClientStream, dst grpc.ServerStream) chan error {
+	ret := make(chan error, 1)
+	go func() {
+		f := &emptypb.Empty{}
+		for i := 0; ; i++ {
+			if err := src.RecvMsg(f); err != nil {
+				ret <- err // this can be io.EOF which is happy case
+				break
+			}
+			if i == 0 {
+				// This is a bit of a hack, but client to server headers are only readable after first client msg is
+				// received but must be written to server stream before the first msg is flushed.
+				// This is the only place to do it nicely.
+				md, err := src.Header()
+				if err != nil {
+					ret <- err
+					break
+				}
+				if err := dst.SendHeader(md); err != nil {
+					ret <- err
+					break
+				}
+			}
+			if err := dst.SendMsg(f); err != nil {
+				ret <- err
+				break
+			}
+		}
+	}()
+	return ret
+}
+
+func (s *handler) forwardServerToClient(src grpc.ServerStream, dst grpc.ClientStream) chan error {
+	ret := make(chan error, 1)
+	go func() {
+		f := &emptypb.Empty{}
+		for i := 0; ; i++ {
+			if err := src.RecvMsg(f); err != nil {
+				ret <- err // this can be io.EOF which is happy case
+				break
+			}
+			if err := dst.SendMsg(f); err != nil {
+				ret <- err
+				break
+			}
+		}
+	}()
+	return ret
+}

proxy/handler_test.go

@@ -0,0 +1,262 @@
+// Copyright 2017 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+package proxy_test
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/stretchr/testify/suite"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/known/emptypb"
+
+	"go.linka.cloud/grpc-toolkit/proxy"
+	pb "go.linka.cloud/grpc-toolkit/proxy/testservice"
+)
+
+const (
+	pingDefaultValue   = "I like kittens."
+	clientMdKey        = "test-client-header"
+	serverHeaderMdKey  = "test-client-header"
+	serverTrailerMdKey = "test-client-trailer"
+
+	rejectingMdKey = "test-reject-rpc-if-in-context"
+
+	countListResponses = 20
+)
+
+// asserting service is implemented on the server side and serves as a handler for stuff
+type assertingService struct {
+	t *testing.T
+	pb.UnsafeTestServiceServer
+}
+
+var _ pb.TestServiceServer = (*assertingService)(nil)
+
+func (s *assertingService) PingEmpty(ctx context.Context, _ *emptypb.Empty) (*pb.PingResponse, error) {
+	// Check that this call has client's metadata.
+	md, ok := metadata.FromIncomingContext(ctx)
+	assert.True(s.t, ok, "PingEmpty call must have metadata in context")
+	_, ok = md[clientMdKey]
+	assert.True(s.t, ok, "PingEmpty call must have clients's custom headers in metadata")
+	return &pb.PingResponse{Value: pingDefaultValue, Counter: 42}, nil
+}
+
+func (s *assertingService) Ping(ctx context.Context, ping *pb.PingRequest) (*pb.PingResponse, error) {
+	// Send user trailers and headers.
+	grpc.SendHeader(ctx, metadata.Pairs(serverHeaderMdKey, "I like turtles."))
+	grpc.SetTrailer(ctx, metadata.Pairs(serverTrailerMdKey, "I like ending turtles."))
+	return &pb.PingResponse{Value: ping.Value, Counter: 42}, nil
+}
+
+func (s *assertingService) PingError(ctx context.Context, ping *pb.PingRequest) (*emptypb.Empty, error) {
+	return nil, status.Errorf(codes.FailedPrecondition, "Userspace error.")
+}
+
+func (s *assertingService) PingList(ping *pb.PingRequest, stream pb.TestService_PingListServer) error {
+	// Send user trailers and headers.
+	stream.SendHeader(metadata.Pairs(serverHeaderMdKey, "I like turtles."))
+	for i := 0; i < countListResponses; i++ {
+		stream.Send(&pb.PingResponse{Value: ping.Value, Counter: int32(i)})
+	}
+	stream.SetTrailer(metadata.Pairs(serverTrailerMdKey, "I like ending turtles."))
+	return nil
+}
+
+func (s *assertingService) PingStream(stream pb.TestService_PingStreamServer) error {
+	stream.SendHeader(metadata.Pairs(serverHeaderMdKey, "I like turtles."))
+	counter := int32(0)
+	for {
+		ping, err := stream.Recv()
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			require.NoError(s.t, err, "can't fail reading stream")
+			return err
+		}
+		pong := &pb.PingResponse{Value: ping.Value, Counter: counter}
+		if err := stream.Send(pong); err != nil {
+			require.NoError(s.t, err, "can't fail sending back a pong")
+		}
+		counter += 1
+	}
+	stream.SetTrailer(metadata.Pairs(serverTrailerMdKey, "I like ending turtles."))
+	return nil
+}
+
+// ProxyHappySuite tests the "happy" path of handling: that everything works in absence of connection issues.
+type ProxyHappySuite struct {
+	suite.Suite
+
+	serverListener   net.Listener
+	server           *grpc.Server
+	proxyListener    net.Listener
+	proxy            *grpc.Server
+	serverClientConn *grpc.ClientConn
+
+	client     *grpc.ClientConn
+	testClient pb.TestServiceClient
+}
+
+func (s *ProxyHappySuite) TestPingEmptyCarriesClientMetadata() {
+	ctx := metadata.NewOutgoingContext(context.Background(), metadata.Pairs(clientMdKey, "true"))
+	out, err := s.testClient.PingEmpty(ctx, &emptypb.Empty{})
+	require.NoError(s.T(), err, "PingEmpty should succeed without errors")
+	want := &pb.PingResponse{Value: pingDefaultValue, Counter: 42}
+	require.True(s.T(), proto.Equal(want, out))
+}
+
+func (s *ProxyHappySuite) TestPingEmpty_StressTest() {
+	for i := 0; i < 50; i++ {
+		s.TestPingEmptyCarriesClientMetadata()
+	}
+}
+
+func (s *ProxyHappySuite) TestPingCarriesServerHeadersAndTrailers() {
+	headerMd := make(metadata.MD)
+	trailerMd := make(metadata.MD)
+	// This is an awkward calling convention... but meh.
+	out, err := s.testClient.Ping(context.Background(), &pb.PingRequest{Value: "foo"}, grpc.Header(&headerMd), grpc.Trailer(&trailerMd))
+	want := &pb.PingResponse{Value: "foo", Counter: 42}
+	require.NoError(s.T(), err, "Ping should succeed without errors")
+	require.True(s.T(), proto.Equal(want, out))
+	assert.Contains(s.T(), headerMd, serverHeaderMdKey, "server response headers must contain server data")
+	assert.Len(s.T(), trailerMd, 1, "server response trailers must contain server data")
+}
+
+func (s *ProxyHappySuite) TestPingErrorPropagatesAppError() {
+	_, err := s.testClient.PingError(context.Background(), &pb.PingRequest{Value: "foo"})
+	require.Error(s.T(), err, "PingError should never succeed")
+	assert.Equal(s.T(), codes.FailedPrecondition, status.Code(err))
+	assert.Equal(s.T(), "Userspace error.", status.Convert(err).Message())
+}
+
+func (s *ProxyHappySuite) TestDirectorErrorIsPropagated() {
+	// See SetupSuite where the StreamDirector has a special case.
+	ctx := metadata.NewOutgoingContext(context.Background(), metadata.Pairs(rejectingMdKey, "true"))
+	_, err := s.testClient.Ping(ctx, &pb.PingRequest{Value: "foo"})
+	require.Error(s.T(), err, "Director should reject this RPC")
+	assert.Equal(s.T(), codes.PermissionDenied, status.Code(err))
+	assert.Equal(s.T(), "testing rejection", status.Convert(err).Message())
+}
+
+func (s *ProxyHappySuite) TestPingStream_FullDuplexWorks() {
+	stream, err := s.testClient.PingStream(context.Background())
+	require.NoError(s.T(), err, "PingStream request should be successful.")
+
+	for i := 0; i < countListResponses; i++ {
+		ping := &pb.PingRequest{Value: fmt.Sprintf("foo:%d", i)}
+		require.NoError(s.T(), stream.Send(ping), "sending to PingStream must not fail")
+		resp, err := stream.Recv()
+		if err == io.EOF {
+			break
+		}
+		if i == 0 {
+			// Check that the header arrives before all entries.
+			headerMd, err := stream.Header()
+			require.NoError(s.T(), err, "PingStream headers should not error.")
+			assert.Contains(s.T(), headerMd, serverHeaderMdKey, "PingStream response headers user contain metadata")
+		}
+		assert.EqualValues(s.T(), i, resp.Counter, "ping roundtrip must succeed with the correct id")
+	}
+	require.NoError(s.T(), stream.CloseSend(), "no error on close send")
+	_, err = stream.Recv()
+	require.Equal(s.T(), io.EOF, err, "stream should close with io.EOF, meaining OK")
+	// Check that the trailer headers are here.
+	trailerMd := stream.Trailer()
+	assert.Len(s.T(), trailerMd, 1, "PingList trailer headers user contain metadata")
+}
+
+func (s *ProxyHappySuite) TestPingStream_StressTest() {
+	for i := 0; i < 50; i++ {
+		s.TestPingStream_FullDuplexWorks()
+	}
+}
+
+func (s *ProxyHappySuite) SetupSuite() {
+	var err error
+
+	s.proxyListener, err = net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(s.T(), err, "must be able to allocate a port for proxyListener")
+	s.serverListener, err = net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(s.T(), err, "must be able to allocate a port for serverListener")
+
+	s.server = grpc.NewServer()
+	pb.RegisterTestServiceServer(s.server, &assertingService{t: s.T()})
+
+	// Setup of the proxy's Director.
+	//lint:ignore SA1019 regression test
+	s.serverClientConn, err = grpc.Dial(s.serverListener.Addr().String(), grpc.WithInsecure(), grpc.WithCodec(proxy.Codec()))
+	require.NoError(s.T(), err, "must not error on deferred client Dial")
+	director := func(ctx context.Context, fullName string) (context.Context, grpc.ClientConnInterface, error) {
+		md, ok := metadata.FromIncomingContext(ctx)
+		if ok {
+			if _, exists := md[rejectingMdKey]; exists {
+				return ctx, nil, status.Errorf(codes.PermissionDenied, "testing rejection")
+			}
+		}
+		// Explicitly copy the metadata, otherwise the tests will fail.
+		outCtx := metadata.NewOutgoingContext(ctx, md.Copy())
+		return outCtx, s.serverClientConn, nil
+	}
+	s.proxy = grpc.NewServer(
+		//lint:ignore SA1019 regression test
+		grpc.CustomCodec(proxy.Codec()),
+		grpc.UnknownServiceHandler(proxy.TransparentHandler(director)),
+	)
+	// Ping handler is handled as an explicit registration and not as a TransparentHandler.
+	proxy.RegisterService(s.proxy, director,
+		"mwitkow.testproto.TestService",
+		"Ping")
+
+	// Start the serving loops.
+	s.T().Logf("starting grpc.Server at: %v", s.serverListener.Addr().String())
+	go func() {
+		s.server.Serve(s.serverListener)
+	}()
+	s.T().Logf("starting grpc.Proxy at: %v", s.proxyListener.Addr().String())
+	go func() {
+		s.proxy.Serve(s.proxyListener)
+	}()
+
+	dCtx, ccl := context.WithTimeout(context.Background(), time.Second)
+	defer ccl()
+	clientConn, err := grpc.DialContext(dCtx, strings.Replace(s.proxyListener.Addr().String(), "127.0.0.1", "localhost", 1), grpc.WithInsecure())
+	require.NoError(s.T(), err, "must not error on deferred client Dial")
+	s.testClient = pb.NewTestServiceClient(clientConn)
+}
+
+func (s *ProxyHappySuite) TearDownSuite() {
+	if s.client != nil {
+		s.client.Close()
+	}
+	if s.serverClientConn != nil {
+		s.serverClientConn.Close()
+	}
+	// Close all transports so the logs don't get spammy.
+	time.Sleep(10 * time.Millisecond)
+	if s.proxy != nil {
+		s.proxy.Stop()
+		s.proxyListener.Close()
+	}
+	if s.serverListener != nil {
+		s.server.Stop()
+		s.serverListener.Close()
+	}
+}
+
+func TestProxyHappySuite(t *testing.T) {
+	suite.Run(t, &ProxyHappySuite{})
+}

proxy/proxy.go

@@ -0,0 +1,33 @@
+// Copyright 2021 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+package proxy
+
+import (
+	"context"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/metadata"
+)
+
+// NewProxy sets up a simple proxy that forwards all requests to dst.
+func NewProxy(dst *grpc.ClientConn, opts ...grpc.ServerOption) *grpc.Server {
+	opts = append(opts, DefaultProxyOpt(dst))
+	// Set up the proxy server and then serve from it like in step one.
+	return grpc.NewServer(opts...)
+}
+
+// DefaultProxyOpt returns an grpc.UnknownServiceHandler with a DefaultDirector.
+func DefaultProxyOpt(cc *grpc.ClientConn) grpc.ServerOption {
+	return grpc.UnknownServiceHandler(TransparentHandler(DefaultDirector(cc)))
+}
+
+// DefaultDirector returns a very simple forwarding StreamDirector that forwards all
+// calls.
+func DefaultDirector(cc *grpc.ClientConn) StreamDirector {
+	return func(ctx context.Context, fullMethodName string) (context.Context, grpc.ClientConnInterface, error) {
+		md, _ := metadata.FromIncomingContext(ctx)
+		ctx = metadata.NewOutgoingContext(ctx, md.Copy())
+		return ctx, cc, nil
+	}
+}

proxy/proxy_test.go

@@ -0,0 +1,211 @@
+package proxy_test
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"net"
+	"testing"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/test/bufconn"
+
+	"go.linka.cloud/grpc-toolkit/proxy"
+	"go.linka.cloud/grpc-toolkit/proxy/testservice"
+)
+
+var testBackend = flag.String("test-backend", "", "Service providing TestServiceServer")
+
+// TestIntegrationV1 is a regression test of the proxy.
+func TestLegacyBehaviour(t *testing.T) {
+	// These bufconns are test listeners used to make connections between our
+	// services. This test actually starts two fully functional grpc services.
+	proxyBc := bufconn.Listen(10)
+
+	// Setup is a little thorough, but here's the gist of it:
+	// 1. Create the test backend using testservice.DefaultTestServiceServer
+	// 2. Create the proxy backend using this package
+	// 3. Make calls to 1 via 2.
+
+	// 1.
+	//lint:ignore SA1019 regression test
+	testCC, err := backendDialer(t, grpc.WithCodec(proxy.Codec()))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// 2.
+	go func() {
+		// Second, we need to implement the SteamDirector.
+		directorFn := func(ctx context.Context, fullMethodName string) (context.Context, grpc.ClientConnInterface, error) {
+			md, _ := metadata.FromIncomingContext(ctx)
+			outCtx := metadata.NewOutgoingContext(ctx, md.Copy())
+			return outCtx, testCC, nil
+		}
+
+		// Set up the proxy server and then serve from it like in step one.
+		proxySrv := grpc.NewServer(
+			//lint:ignore SA1019 regression test
+			grpc.CustomCodec(proxy.Codec()), // was previously needed for proxy to function.
+			grpc.UnknownServiceHandler(proxy.TransparentHandler(directorFn)),
+		)
+		// run the proxy backend
+		go func() {
+			t.Log("Running proxySrv")
+			if err := proxySrv.Serve(proxyBc); err != nil {
+				if err == grpc.ErrServerStopped {
+					return
+				}
+				t.Logf("running proxy server: %v", err)
+			}
+		}()
+		t.Cleanup(func() {
+			t.Log("Gracefully stopping proxySrv")
+			proxySrv.GracefulStop()
+		})
+	}()
+
+	// 3.
+	// Connect to the proxy. We should not need to do anything special here -
+	// users do not need to know they're talking to a proxy.
+	proxyCC, err := grpc.Dial(
+		"bufnet",
+		grpc.WithInsecure(),
+		grpc.WithBlock(),
+		grpc.WithContextDialer(func(ctx context.Context, s string) (net.Conn, error) {
+			return proxyBc.Dial()
+		}),
+	)
+	if err != nil {
+		t.Fatalf("dialing proxy: %v", err)
+	}
+	proxyClient := testservice.NewTestServiceClient(proxyCC)
+
+	// 4. Run the tests!
+	testservice.TestTestServiceServerImpl(t, proxyClient)
+}
+
+func TestNewProxy(t *testing.T) {
+	proxyBc := bufconn.Listen(10)
+
+	// Setup is a little thorough, but here's the gist of it:
+	// 1. Create the test backend using testservice.DefaultTestServiceServer
+	// 2. Create the proxy backend using this package
+	// 3. Make calls to 1 via 2.
+
+	// 1.
+	// First, we need to create a client connection to this backend.
+	testCC, err := backendDialer(t)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// 2.
+	go func() {
+		t.Helper()
+
+		// First, we need to create a client connection to this backend.
+		proxySrv := proxy.NewProxy(testCC)
+
+		// run the proxy backend
+		go func() {
+			t.Log("Running proxySrv")
+			if err := proxySrv.Serve(proxyBc); err != nil {
+				if err == grpc.ErrServerStopped {
+					return
+				}
+				t.Logf("running proxy server: %v", err)
+			}
+		}()
+		t.Cleanup(func() {
+			t.Log("Gracefully stopping proxySrv")
+			proxySrv.GracefulStop()
+		})
+	}()
+
+	// 3.
+	// Connect to the proxy. We should not need to do anything special here -
+	// users do not need to know they're talking to a proxy.
+	t.Logf("dialing %s", proxyBc.Addr())
+	proxyCC, err := grpc.Dial(
+		proxyBc.Addr().String(),
+		grpc.WithInsecure(),
+		grpc.WithBlock(),
+		grpc.WithContextDialer(func(ctx context.Context, s string) (net.Conn, error) {
+			return proxyBc.Dial()
+		}),
+	)
+	if err != nil {
+		t.Fatalf("dialing proxy: %v", err)
+	}
+	proxyClient := testservice.NewTestServiceClient(proxyCC)
+
+	// 4. Run the tests!
+	testservice.TestTestServiceServerImpl(t, proxyClient)
+}
+
+// backendDialer dials the testservice.TestServiceServer either by connecting
+// to the user-supplied server, or by creating a mock server using bufconn.
+func backendDialer(t *testing.T, opts ...grpc.DialOption) (*grpc.ClientConn, error) {
+	t.Helper()
+
+	if *testBackend != "" {
+		return backendSvcDialer(t, *testBackend, opts...)
+	}
+
+	backendBc := bufconn.Listen(10)
+	// set up the backend using a "real" server over a bufconn
+	testSrv := grpc.NewServer()
+	testservice.RegisterTestServiceServer(testSrv, testservice.DefaultTestServiceServer)
+
+	// run the test backend
+	go func() {
+		t.Log("Running testSrv")
+		if err := testSrv.Serve(backendBc); err != nil {
+			if err == grpc.ErrServerStopped {
+				return
+			}
+			t.Logf("running test server: %v", err)
+		}
+	}()
+	t.Cleanup(func() {
+		t.Log("Gracefully stopping testSrv")
+		testSrv.GracefulStop()
+	})
+
+	opts = append(opts,
+		grpc.WithInsecure(),
+		grpc.WithBlock(),
+		grpc.WithContextDialer(func(ctx context.Context, s string) (net.Conn, error) {
+			return backendBc.Dial()
+		}),
+	)
+
+	backendCC, err := grpc.Dial(
+		"bufnet",
+		opts...,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("dialing backend: %v", err)
+	}
+	return backendCC, nil
+}
+
+func backendSvcDialer(t *testing.T, addr string, opts ...grpc.DialOption) (*grpc.ClientConn, error) {
+	opts = append(opts,
+		grpc.WithInsecure(),
+		grpc.WithBlock(),
+	)
+
+	t.Logf("connecting to %s", addr)
+	cc, err := grpc.Dial(
+		addr,
+		opts...,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("dialing backend: %v", err)
+	}
+
+	return cc, nil
+}

proxy/testservice/Makefile

@@ -0,0 +1,9 @@
+
+all: test_go
+
+test_go: test.proto
+	 protoc --go_out=. --go_opt=paths=source_relative \
+        --go-grpc_out=. --go-grpc_opt=paths=source_relative \
+        test.proto
+
+

proxy/testservice/ping.go

@@ -0,0 +1,168 @@
+package testservice
+
+import (
+	"context"
+	"fmt"
+	"io"
+
+	"golang.org/x/sync/errgroup"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/types/known/emptypb"
+)
+
+var DefaultTestServiceServer = defaultPingServer{}
+
+const (
+	PingHeader      = "ping-header"
+	PingHeaderCts   = "Arbitrary header text"
+	PingTrailer     = "ping-trailer"
+	PingTrailerCts  = "Arbitrary trailer text"
+	PingEchoHeader  = "ping-echo-header"
+	PingEchoTrailer = "ping-echo-trailer"
+)
+
+// defaultPingServer is the canonical implementation of a TestServiceServer.
+type defaultPingServer struct {
+	UnsafeTestServiceServer
+}
+
+func (s defaultPingServer) PingEmpty(ctx context.Context, empty *emptypb.Empty) (*PingResponse, error) {
+	if err := s.sendHeader(ctx); err != nil {
+		return nil, err
+	}
+	if err := s.setTrailer(ctx); err != nil {
+		return nil, err
+	}
+	return &PingResponse{}, nil
+}
+
+func (s defaultPingServer) Ping(ctx context.Context, request *PingRequest) (*PingResponse, error) {
+	if err := s.sendHeader(ctx); err != nil {
+		return nil, err
+	}
+	if err := s.setTrailer(ctx); err != nil {
+		return nil, err
+	}
+
+	return &PingResponse{Value: request.Value}, nil
+}
+
+func (s defaultPingServer) PingError(ctx context.Context, request *PingRequest) (*emptypb.Empty, error) {
+	if err := s.sendHeader(ctx); err != nil {
+		return nil, err
+	}
+	if err := s.setTrailer(ctx); err != nil {
+		return nil, err
+	}
+	return nil, status.Error(codes.Unknown, "Something is wrong and this is a message that describes it")
+}
+
+func (s defaultPingServer) PingList(request *PingRequest, server TestService_PingListServer) error {
+	if err := s.sendHeader(server.Context()); err != nil {
+		return err
+	}
+	s.setStreamTrailer(server)
+	for i := 0; i < 10; i++ {
+		if err := server.Send(&PingResponse{
+			Value:   request.Value,
+			Counter: int32(i),
+		}); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (s defaultPingServer) PingStream(server TestService_PingStreamServer) error {
+	g, ctx := errgroup.WithContext(context.Background())
+
+	if err := s.sendHeader(server.Context()); err != nil {
+		return err
+	}
+
+	pings := make(chan *PingRequest)
+	g.Go(func() error {
+		defer close(pings)
+		for {
+			m, err := server.Recv()
+			if err != nil {
+				if err == io.EOF {
+					return nil
+				}
+				return err
+			}
+			select {
+			case pings <- m:
+			case <-ctx.Done():
+				return ctx.Err()
+			}
+		}
+	})
+	g.Go(func() error {
+		var i int32
+		for m := range pings {
+			if err := server.Send(&PingResponse{
+				Value:   m.Value,
+				Counter: i,
+			}); err != nil {
+				return err
+			}
+			i++
+		}
+		return nil
+	})
+
+	return g.Wait()
+}
+
+func (s *defaultPingServer) sendHeader(ctx context.Context) error {
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
+		md = metadata.New(nil)
+	}
+
+	if tvs := md.Get(PingEchoHeader); len(tvs) > 0 {
+		md.Append(PingEchoHeader, tvs...)
+	}
+
+	md.Append(PingHeader, PingHeaderCts)
+
+	if err := grpc.SendHeader(ctx, md); err != nil {
+		return fmt.Errorf("setting header: %w", err)
+	}
+	return nil
+}
+
+func (s *defaultPingServer) setTrailer(ctx context.Context) error {
+	md := s.buildTrailer(ctx)
+
+	if err := grpc.SetTrailer(ctx, md); err != nil {
+		return fmt.Errorf("setting trailer: %w", err)
+	}
+
+	return nil
+}
+
+func (s *defaultPingServer) buildTrailer(ctx context.Context) metadata.MD {
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
+		md = metadata.New(nil)
+	}
+
+	if tvs := md.Get(PingEchoTrailer); len(tvs) > 0 {
+		md.Append(PingEchoTrailer, tvs...)
+	}
+
+	md.Append(PingTrailer, PingTrailerCts)
+
+	return md
+}
+
+func (s defaultPingServer) setStreamTrailer(server grpc.ServerStream) {
+	server.SetTrailer(s.buildTrailer(server.Context()))
+}
+
+var _ TestServiceServer = (*defaultPingServer)(nil)

proxy/testservice/server/main.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"google.golang.org/grpc"
+
+	"go.linka.cloud/grpc-toolkit/proxy/testservice"
+)
+
+var (
+	port = flag.Uint("port", 8080, "Port to listen to")
+)
+
+func main() {
+	srv := grpc.NewServer()
+	lis, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", *port))
+	if err != nil {
+		panic(err)
+	}
+	testservice.RegisterTestServiceServer(srv, testservice.DefaultTestServiceServer)
+
+	errs := make(chan error)
+
+	go func() {
+		log.Printf("listening on %s", lis.Addr().String())
+		errs <- srv.Serve(lis)
+	}()
+
+	sigs := make(chan os.Signal, 1)
+	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		sig := <-sigs
+		log.Printf("shutdown due to %s", sig)
+		srv.GracefulStop()
+	}()
+
+	if err := <-errs; err != nil {
+		log.Println(err)
+		os.Exit(1)
+	}
+	os.Exit(0)
+}
+
+func init() {
+	flag.Parse()
+}

proxy/testservice/test.pb.go

@@ -0,0 +1,255 @@
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// versions:
+// 	protoc-gen-go v1.25.0-devel
+// 	protoc        v3.15.5
+// source: test.proto
+
+package testservice
+
+import (
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	emptypb "google.golang.org/protobuf/types/known/emptypb"
+	reflect "reflect"
+	sync "sync"
+)
+
+const (
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(20 - protoimpl.MinVersion)
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
+)
+
+type PingRequest struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Value string `protobuf:"bytes,1,opt,name=value,proto3" json:"value,omitempty"`
+}
+
+func (x *PingRequest) Reset() {
+	*x = PingRequest{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_test_proto_msgTypes[0]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *PingRequest) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*PingRequest) ProtoMessage() {}
+
+func (x *PingRequest) ProtoReflect() protoreflect.Message {
+	mi := &file_test_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use PingRequest.ProtoReflect.Descriptor instead.
+func (*PingRequest) Descriptor() ([]byte, []int) {
+	return file_test_proto_rawDescGZIP(), []int{0}
+}
+
+func (x *PingRequest) GetValue() string {
+	if x != nil {
+		return x.Value
+	}
+	return ""
+}
+
+type PingResponse struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Value   string `protobuf:"bytes,1,opt,name=value,proto3" json:"value,omitempty"`
+	Counter int32  `protobuf:"varint,2,opt,name=counter,proto3" json:"counter,omitempty"`
+}
+
+func (x *PingResponse) Reset() {
+	*x = PingResponse{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_test_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *PingResponse) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*PingResponse) ProtoMessage() {}
+
+func (x *PingResponse) ProtoReflect() protoreflect.Message {
+	mi := &file_test_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use PingResponse.ProtoReflect.Descriptor instead.
+func (*PingResponse) Descriptor() ([]byte, []int) {
+	return file_test_proto_rawDescGZIP(), []int{1}
+}
+
+func (x *PingResponse) GetValue() string {
+	if x != nil {
+		return x.Value
+	}
+	return ""
+}
+
+func (x *PingResponse) GetCounter() int32 {
+	if x != nil {
+		return x.Counter
+	}
+	return 0
+}
+
+var File_test_proto protoreflect.FileDescriptor
+
+var file_test_proto_rawDesc = []byte{
+	0x0a, 0x0a, 0x74, 0x65, 0x73, 0x74, 0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x12, 0x11, 0x6d, 0x77,
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+	0x69, 0x6e, 0x67, 0x52, 0x65, 0x71, 0x75, 0x65, 0x73, 0x74, 0x1a, 0x1f, 0x2e, 0x6d, 0x77, 0x69,
+	0x74, 0x6b, 0x6f, 0x77, 0x2e, 0x74, 0x65, 0x73, 0x74, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2e, 0x50,
+	0x69, 0x6e, 0x67, 0x52, 0x65, 0x73, 0x70, 0x6f, 0x6e, 0x73, 0x65, 0x22, 0x00, 0x28, 0x01, 0x30,
+	0x01, 0x42, 0x2b, 0x5a, 0x29, 0x67, 0x69, 0x74, 0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f,
+	0x6d, 0x77, 0x69, 0x74, 0x6b, 0x6f, 0x77, 0x2f, 0x67, 0x72, 0x70, 0x63, 0x2d, 0x70, 0x72, 0x6f,
+	0x78, 0x79, 0x2f, 0x74, 0x65, 0x73, 0x74, 0x73, 0x65, 0x72, 0x76, 0x69, 0x63, 0x65, 0x62, 0x06,
+	0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
+}
+
+var (
+	file_test_proto_rawDescOnce sync.Once
+	file_test_proto_rawDescData = file_test_proto_rawDesc
+)
+
+func file_test_proto_rawDescGZIP() []byte {
+	file_test_proto_rawDescOnce.Do(func() {
+		file_test_proto_rawDescData = protoimpl.X.CompressGZIP(file_test_proto_rawDescData)
+	})
+	return file_test_proto_rawDescData
+}
+
+var file_test_proto_msgTypes = make([]protoimpl.MessageInfo, 2)
+var file_test_proto_goTypes = []interface{}{
+	(*PingRequest)(nil),   // 0: mwitkow.testproto.PingRequest
+	(*PingResponse)(nil),  // 1: mwitkow.testproto.PingResponse
+	(*emptypb.Empty)(nil), // 2: google.protobuf.Empty
+}
+var file_test_proto_depIdxs = []int32{
+	2, // 0: mwitkow.testproto.TestService.PingEmpty:input_type -> google.protobuf.Empty
+	0, // 1: mwitkow.testproto.TestService.Ping:input_type -> mwitkow.testproto.PingRequest
+	0, // 2: mwitkow.testproto.TestService.PingError:input_type -> mwitkow.testproto.PingRequest
+	0, // 3: mwitkow.testproto.TestService.PingList:input_type -> mwitkow.testproto.PingRequest
+	0, // 4: mwitkow.testproto.TestService.PingStream:input_type -> mwitkow.testproto.PingRequest
+	1, // 5: mwitkow.testproto.TestService.PingEmpty:output_type -> mwitkow.testproto.PingResponse
+	1, // 6: mwitkow.testproto.TestService.Ping:output_type -> mwitkow.testproto.PingResponse
+	2, // 7: mwitkow.testproto.TestService.PingError:output_type -> google.protobuf.Empty
+	1, // 8: mwitkow.testproto.TestService.PingList:output_type -> mwitkow.testproto.PingResponse
+	1, // 9: mwitkow.testproto.TestService.PingStream:output_type -> mwitkow.testproto.PingResponse
+	5, // [5:10] is the sub-list for method output_type
+	0, // [0:5] is the sub-list for method input_type
+	0, // [0:0] is the sub-list for extension type_name
+	0, // [0:0] is the sub-list for extension extendee
+	0, // [0:0] is the sub-list for field type_name
+}
+
+func init() { file_test_proto_init() }
+func file_test_proto_init() {
+	if File_test_proto != nil {
+		return
+	}
+	if !protoimpl.UnsafeEnabled {
+		file_test_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*PingRequest); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_test_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*PingResponse); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+	}
+	type x struct{}
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
+			RawDescriptor: file_test_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   2,
+			NumExtensions: 0,
+			NumServices:   1,
+		},
+		GoTypes:           file_test_proto_goTypes,
+		DependencyIndexes: file_test_proto_depIdxs,
+		MessageInfos:      file_test_proto_msgTypes,
+	}.Build()
+	File_test_proto = out.File
+	file_test_proto_rawDesc = nil
+	file_test_proto_goTypes = nil
+	file_test_proto_depIdxs = nil
+}

proxy/testservice/test.proto

@@ -0,0 +1,30 @@
+syntax = "proto3";
+
+package mwitkow.testproto;
+
+option go_package="go.linka.cloud/grpc-toolkit/proxy/testservice";
+
+import "google/protobuf/empty.proto";
+
+message PingRequest {
+  string value = 1;
+}
+
+message PingResponse {
+  string value = 1;
+  int32 counter = 2;
+}
+
+service TestService {
+  rpc PingEmpty(google.protobuf.Empty) returns (PingResponse) {}
+
+  rpc Ping(PingRequest) returns (PingResponse) {}
+
+  rpc PingError(PingRequest) returns (google.protobuf.Empty) {}
+
+  rpc PingList(PingRequest) returns (stream PingResponse) {}
+
+  rpc PingStream(stream PingRequest) returns (stream PingResponse) {}
+
+}
+

proxy/testservice/test_grpc.pb.go

@@ -0,0 +1,306 @@
+// Code generated by protoc-gen-go-grpc. DO NOT EDIT.
+
+package testservice
+
+import (
+	context "context"
+	grpc "google.golang.org/grpc"
+	codes "google.golang.org/grpc/codes"
+	status "google.golang.org/grpc/status"
+	emptypb "google.golang.org/protobuf/types/known/emptypb"
+)
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the grpc package it is being compiled against.
+// Requires gRPC-Go v1.32.0 or later.
+const _ = grpc.SupportPackageIsVersion7
+
+// TestServiceClient is the client API for TestService service.
+//
+// For semantics around ctx use and closing/ending streaming RPCs, please refer to https://pkg.go.dev/google.golang.org/grpc/?tab=doc#ClientConn.NewStream.
+type TestServiceClient interface {
+	PingEmpty(ctx context.Context, in *emptypb.Empty, opts ...grpc.CallOption) (*PingResponse, error)
+	Ping(ctx context.Context, in *PingRequest, opts ...grpc.CallOption) (*PingResponse, error)
+	PingError(ctx context.Context, in *PingRequest, opts ...grpc.CallOption) (*emptypb.Empty, error)
+	PingList(ctx context.Context, in *PingRequest, opts ...grpc.CallOption) (TestService_PingListClient, error)
+	PingStream(ctx context.Context, opts ...grpc.CallOption) (TestService_PingStreamClient, error)
+}
+
+type testServiceClient struct {
+	cc grpc.ClientConnInterface
+}
+
+func NewTestServiceClient(cc grpc.ClientConnInterface) TestServiceClient {
+	return &testServiceClient{cc}
+}
+
+func (c *testServiceClient) PingEmpty(ctx context.Context, in *emptypb.Empty, opts ...grpc.CallOption) (*PingResponse, error) {
+	out := new(PingResponse)
+	err := c.cc.Invoke(ctx, "/mwitkow.testproto.TestService/PingEmpty", in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *testServiceClient) Ping(ctx context.Context, in *PingRequest, opts ...grpc.CallOption) (*PingResponse, error) {
+	out := new(PingResponse)
+	err := c.cc.Invoke(ctx, "/mwitkow.testproto.TestService/Ping", in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *testServiceClient) PingError(ctx context.Context, in *PingRequest, opts ...grpc.CallOption) (*emptypb.Empty, error) {
+	out := new(emptypb.Empty)
+	err := c.cc.Invoke(ctx, "/mwitkow.testproto.TestService/PingError", in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *testServiceClient) PingList(ctx context.Context, in *PingRequest, opts ...grpc.CallOption) (TestService_PingListClient, error) {
+	stream, err := c.cc.NewStream(ctx, &TestService_ServiceDesc.Streams[0], "/mwitkow.testproto.TestService/PingList", opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &testServicePingListClient{stream}
+	if err := x.ClientStream.SendMsg(in); err != nil {
+		return nil, err
+	}
+	if err := x.ClientStream.CloseSend(); err != nil {
+		return nil, err
+	}
+	return x, nil
+}
+
+type TestService_PingListClient interface {
+	Recv() (*PingResponse, error)
+	grpc.ClientStream
+}
+
+type testServicePingListClient struct {
+	grpc.ClientStream
+}
+
+func (x *testServicePingListClient) Recv() (*PingResponse, error) {
+	m := new(PingResponse)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+func (c *testServiceClient) PingStream(ctx context.Context, opts ...grpc.CallOption) (TestService_PingStreamClient, error) {
+	stream, err := c.cc.NewStream(ctx, &TestService_ServiceDesc.Streams[1], "/mwitkow.testproto.TestService/PingStream", opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &testServicePingStreamClient{stream}
+	return x, nil
+}
+
+type TestService_PingStreamClient interface {
+	Send(*PingRequest) error
+	Recv() (*PingResponse, error)
+	grpc.ClientStream
+}
+
+type testServicePingStreamClient struct {
+	grpc.ClientStream
+}
+
+func (x *testServicePingStreamClient) Send(m *PingRequest) error {
+	return x.ClientStream.SendMsg(m)
+}
+
+func (x *testServicePingStreamClient) Recv() (*PingResponse, error) {
+	m := new(PingResponse)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// TestServiceServer is the server API for TestService service.
+// All implementations must embed UnimplementedTestServiceServer
+// for forward compatibility
+type TestServiceServer interface {
+	PingEmpty(context.Context, *emptypb.Empty) (*PingResponse, error)
+	Ping(context.Context, *PingRequest) (*PingResponse, error)
+	PingError(context.Context, *PingRequest) (*emptypb.Empty, error)
+	PingList(*PingRequest, TestService_PingListServer) error
+	PingStream(TestService_PingStreamServer) error
+	mustEmbedUnimplementedTestServiceServer()
+}
+
+// UnimplementedTestServiceServer must be embedded to have forward compatible implementations.
+type UnimplementedTestServiceServer struct {
+}
+
+func (UnimplementedTestServiceServer) PingEmpty(context.Context, *emptypb.Empty) (*PingResponse, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method PingEmpty not implemented")
+}
+func (UnimplementedTestServiceServer) Ping(context.Context, *PingRequest) (*PingResponse, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method Ping not implemented")
+}
+func (UnimplementedTestServiceServer) PingError(context.Context, *PingRequest) (*emptypb.Empty, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method PingError not implemented")
+}
+func (UnimplementedTestServiceServer) PingList(*PingRequest, TestService_PingListServer) error {
+	return status.Errorf(codes.Unimplemented, "method PingList not implemented")
+}
+func (UnimplementedTestServiceServer) PingStream(TestService_PingStreamServer) error {
+	return status.Errorf(codes.Unimplemented, "method PingStream not implemented")
+}
+func (UnimplementedTestServiceServer) mustEmbedUnimplementedTestServiceServer() {}
+
+// UnsafeTestServiceServer may be embedded to opt out of forward compatibility for this service.
+// Use of this interface is not recommended, as added methods to TestServiceServer will
+// result in compilation errors.
+type UnsafeTestServiceServer interface {
+	mustEmbedUnimplementedTestServiceServer()
+}
+
+func RegisterTestServiceServer(s grpc.ServiceRegistrar, srv TestServiceServer) {
+	s.RegisterService(&TestService_ServiceDesc, srv)
+}
+
+func _TestService_PingEmpty_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(emptypb.Empty)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(TestServiceServer).PingEmpty(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/mwitkow.testproto.TestService/PingEmpty",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(TestServiceServer).PingEmpty(ctx, req.(*emptypb.Empty))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _TestService_Ping_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(PingRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(TestServiceServer).Ping(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/mwitkow.testproto.TestService/Ping",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(TestServiceServer).Ping(ctx, req.(*PingRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _TestService_PingError_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(PingRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(TestServiceServer).PingError(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/mwitkow.testproto.TestService/PingError",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(TestServiceServer).PingError(ctx, req.(*PingRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _TestService_PingList_Handler(srv interface{}, stream grpc.ServerStream) error {
+	m := new(PingRequest)
+	if err := stream.RecvMsg(m); err != nil {
+		return err
+	}
+	return srv.(TestServiceServer).PingList(m, &testServicePingListServer{stream})
+}
+
+type TestService_PingListServer interface {
+	Send(*PingResponse) error
+	grpc.ServerStream
+}
+
+type testServicePingListServer struct {
+	grpc.ServerStream
+}
+
+func (x *testServicePingListServer) Send(m *PingResponse) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+func _TestService_PingStream_Handler(srv interface{}, stream grpc.ServerStream) error {
+	return srv.(TestServiceServer).PingStream(&testServicePingStreamServer{stream})
+}
+
+type TestService_PingStreamServer interface {
+	Send(*PingResponse) error
+	Recv() (*PingRequest, error)
+	grpc.ServerStream
+}
+
+type testServicePingStreamServer struct {
+	grpc.ServerStream
+}
+
+func (x *testServicePingStreamServer) Send(m *PingResponse) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+func (x *testServicePingStreamServer) Recv() (*PingRequest, error) {
+	m := new(PingRequest)
+	if err := x.ServerStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// TestService_ServiceDesc is the grpc.ServiceDesc for TestService service.
+// It's only intended for direct use with grpc.RegisterService,
+// and not to be introspected or modified (even as a copy)
+var TestService_ServiceDesc = grpc.ServiceDesc{
+	ServiceName: "mwitkow.testproto.TestService",
+	HandlerType: (*TestServiceServer)(nil),
+	Methods: []grpc.MethodDesc{
+		{
+			MethodName: "PingEmpty",
+			Handler:    _TestService_PingEmpty_Handler,
+		},
+		{
+			MethodName: "Ping",
+			Handler:    _TestService_Ping_Handler,
+		},
+		{
+			MethodName: "PingError",
+			Handler:    _TestService_PingError_Handler,
+		},
+	},
+	Streams: []grpc.StreamDesc{
+		{
+			StreamName:    "PingList",
+			Handler:       _TestService_PingList_Handler,
+			ServerStreams: true,
+		},
+		{
+			StreamName:    "PingStream",
+			Handler:       _TestService_PingStream_Handler,
+			ServerStreams: true,
+			ClientStreams: true,
+		},
+	},
+	Metadata: "test.proto",
+}

proxy/testservice/testping.go

@@ -0,0 +1,158 @@
+package testservice
+
+import (
+	"context"
+	"io"
+	"reflect"
+	"testing"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/metadata"
+)
+
+const (
+	returnHeader = "test-client-header"
+)
+
+// TestTestServiceServerImpl can be called to test the underlying TestServiceServer.
+func TestTestServiceServerImpl(t *testing.T, client TestServiceClient) {
+	t.Run("Unary ping", func(t *testing.T) {
+		want := "hello, world"
+		hdr := metadata.MD{}
+		res, err := client.Ping(context.TODO(), &PingRequest{Value: want}, grpc.Header(&hdr))
+		if err != nil {
+			t.Errorf("want no err; got %v", err)
+			return
+		}
+		checkHeaders(t, hdr)
+		t.Logf("got %v (%d)", res.Value, res.Counter)
+		if got := res.Value; got != want {
+			t.Errorf("res.Value = %q; want %q", got, want)
+		}
+	})
+
+	t.Run("Error ping", func(t *testing.T) {
+		_, err := client.PingError(context.TODO(), &PingRequest{})
+		if err == nil {
+			t.Errorf("want err; got %v", err)
+		}
+	})
+
+	t.Run("Server streaming ping", func(t *testing.T) {
+		want := "hello, world"
+		stream, err := client.PingList(context.TODO(), &PingRequest{Value: want})
+		if err != nil {
+			t.Errorf("want no err; got %v", err)
+			if err := stream.CloseSend(); err != nil {
+				t.Fatalf("closing send channel: %v", err)
+			}
+			return
+		}
+		hdr, err := stream.Header()
+		if err != nil {
+			t.Errorf("reading headers: %v", err)
+		}
+		checkHeaders(t, hdr)
+
+		for {
+			res, err := stream.Recv()
+			if err != nil {
+				if err == io.EOF {
+					checkTrailers(t, stream.Trailer())
+					return
+				}
+				t.Errorf("want no err; got %v", err)
+				return
+			}
+			t.Logf("got %v (%d)", res.Value, res.Counter)
+			if got := res.Value; got != want {
+				t.Errorf("res.Value = %q; want %q", got, want)
+			}
+		}
+	})
+
+	t.Run("Bidirectional pinging", func(t *testing.T) {
+		want := "hello, world"
+		stream, err := client.PingStream(context.TODO())
+		if err != nil {
+			t.Errorf("want no err; got %v", err)
+			if err := stream.CloseSend(); err != nil {
+				t.Fatalf("closing send channel: %v", err)
+			}
+			return
+		}
+
+		d := make(chan struct{})
+		go func() {
+			hdr, err := stream.Header()
+			if err != nil {
+				t.Errorf("reading headers: %v", err)
+			}
+			checkHeaders(t, hdr)
+			close(d)
+		}()
+
+		for i := 0; i < 25; i++ {
+			if err := stream.Send(&PingRequest{Value: want}); err != nil {
+				t.Errorf("want no err; got %v", err)
+				return
+			}
+			res, err := stream.Recv()
+			if err != nil {
+				t.Errorf("receiving full duplex stream: %w", err)
+				return
+			}
+			t.Logf("got %v (%d)", res.Value, res.Counter)
+			if got := res.Value; got != want {
+				t.Errorf("res.Value = %q; want %q", got, want)
+			}
+			if got, want := res.Counter, int32(i); got != want {
+				t.Errorf("res.Counter = %d; want %d", got, want)
+			}
+		}
+		if err := stream.CloseSend(); err != nil {
+			t.Errorf("closing full duplex stream: %v", err)
+		}
+		<-d
+	})
+
+	t.Run("Unary ping with headers", func(t *testing.T) {
+		want := "hello, world"
+		req := &PingRequest{Value: want}
+
+		ctx := metadata.AppendToOutgoingContext(context.Background(), returnHeader, "I like turtles.")
+		inHeader := make(metadata.MD)
+
+		res, err := client.Ping(ctx, req, grpc.Header(&inHeader))
+		if err != nil {
+			t.Errorf("want no err; got %v", err)
+			return
+		}
+		t.Logf("got %v (%d)", res.Value, res.Counter)
+		if !reflect.DeepEqual(inHeader.Get(returnHeader), []string{"I like turtles."}) {
+			t.Errorf("did not receive correct return headers")
+		}
+	})
+}
+
+func checkTrailers(t *testing.T, md metadata.MD) {
+	vs := md.Get(PingTrailer)
+	if want, got := 1, len(vs); want != got {
+		t.Errorf("trailer %q not present", PingTrailer)
+		return
+	}
+	if want, got := []string{PingTrailerCts}, vs; !reflect.DeepEqual(got, want) {
+		t.Errorf("trailer mismatch; want %q, got %q", want, got)
+	}
+}
+
+func checkHeaders(t *testing.T, md metadata.MD) {
+	vs := md.Get(PingHeader)
+	if want, got := 1, len(vs); want != got {
+		t.Errorf("header %q not present", PingHeader)
+		return
+	}
+	if want, got := []string{PingHeaderCts}, vs; !reflect.DeepEqual(got, want) {
+		t.Errorf("header mismatch; want %q, got %q", want, got)
+	}
+}