Break StreamDirector interface, fix metadata propagation for gRPC-Go>1.5. (#20)

Author: Michal Witkowski

proxy/director.go

@@ -13,8 +13,12 @@ import (
 // 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) (*grpc.ClientConn, error)
+type StreamDirector func(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error)

proxy/examples_test.go

@@ -35,21 +35,26 @@ func ExampleTransparentHandler() {
 // Provide sa 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) (*grpc.ClientConn, error) {
+	director = func(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error) {
 		// Make sure we never forward internal services.
 		if strings.HasPrefix(fullMethodName, "/com.example.internal.") {
-			return nil, grpc.Errorf(codes.Unimplemented, "Unknown method")
+			return nil, nil, grpc.Errorf(codes.Unimplemented, "Unknown method")
 		}
-		md, ok := metadata.FromContext(ctx)
+		md, ok := metadata.FromIncomingContext(ctx)
+		// Copy the inbound metadata explicitly.
+		outCtx, _ := context.WithCancel(ctx)
+		outCtx = metadata.NewOutgoingContext(outCtx, 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.
-				return grpc.DialContext(ctx, "api-service.staging.svc.local", grpc.WithCodec(proxy.Codec()))
+				conn, err := grpc.DialContext(ctx, "api-service.staging.svc.local", grpc.WithCodec(proxy.Codec()))
+				return outCtx, conn, err
 			} else if val, exists := md[":authority"]; exists && val[0] == "api.example.com" {
-				return grpc.DialContext(ctx, "api-service.prod.svc.local", grpc.WithCodec(proxy.Codec()))
+				conn, err := grpc.DialContext(ctx, "api-service.prod.svc.local", grpc.WithCodec(proxy.Codec()))
+				return outCtx, conn, err
 			}
 		}
-		return nil, grpc.Errorf(codes.Unimplemented, "Unknown method")
+		return nil, nil, grpc.Errorf(codes.Unimplemented, "Unknown method")
 	}
 }

proxy/handler.go

@@ -65,8 +65,9 @@ func (s *handler) handler(srv interface{}, serverStream grpc.ServerStream) error
 		return grpc.Errorf(codes.Internal, "lowLevelServerStream not exists in context")
 	}
 	fullMethodName := lowLevelServerStream.Method()
-	clientCtx, clientCancel := context.WithCancel(serverStream.Context())
-	backendConn, err := s.director(serverStream.Context(), fullMethodName)
+	// We require that the director's returned context inherits from the serverStream.Context().
+	outgoingCtx, backendConn, err := s.director(serverStream.Context(), fullMethodName)
+	clientCtx, clientCancel := context.WithCancel(outgoingCtx)
 	if err != nil {
 		return err
 	}

proxy/handler_test.go

@@ -46,7 +46,7 @@ type assertingService struct {
 
 func (s *assertingService) PingEmpty(ctx context.Context, _ *pb.Empty) (*pb.PingResponse, error) {
 	// Check that this call has client's metadata.
-	md, ok := metadata.FromContext(ctx)
+	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")
@@ -116,7 +116,7 @@ func (s *ProxyHappySuite) ctx() context.Context {
 }
 
 func (s *ProxyHappySuite) TestPingEmptyCarriesClientMetadata() {
-	ctx := metadata.NewContext(s.ctx(), metadata.Pairs(clientMdKey, "true"))
+	ctx := metadata.NewOutgoingContext(s.ctx(), metadata.Pairs(clientMdKey, "true"))
 	out, err := s.testClient.PingEmpty(ctx, &pb.Empty{})
 	require.NoError(s.T(), err, "PingEmpty should succeed without errors")
 	require.Equal(s.T(), &pb.PingResponse{Value: pingDefaultValue, Counter: 42}, out)
@@ -148,7 +148,7 @@ func (s *ProxyHappySuite) TestPingErrorPropagatesAppError() {
 
 func (s *ProxyHappySuite) TestDirectorErrorIsPropagated() {
 	// See SetupSuite where the StreamDirector has a special case.
-	ctx := metadata.NewContext(s.ctx(), metadata.Pairs(rejectingMdKey, "true"))
+	ctx := metadata.NewOutgoingContext(s.ctx(), 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, grpc.Code(err))
@@ -204,14 +204,17 @@ func (s *ProxyHappySuite) SetupSuite() {
 	// Setup of the proxy's Director.
 	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) (*grpc.ClientConn, error) {
-		md, ok := metadata.FromContext(ctx)
+	director := func(ctx context.Context, fullName string) (context.Context, *grpc.ClientConn, error) {
+		md, ok := metadata.FromIncomingContext(ctx)
 		if ok {
 			if _, exists := md[rejectingMdKey]; exists {
-				return nil, grpc.Errorf(codes.PermissionDenied, "testing rejection")
+				return ctx, nil, grpc.Errorf(codes.PermissionDenied, "testing rejection")
 			}
 		}
-		return s.serverClientConn, nil
+		// Explicitly copy the metadata, otherwise the tests will fail.
+		outCtx, _ := context.WithCancel(ctx)
+		outCtx = metadata.NewOutgoingContext(outCtx, md.Copy())
+		return outCtx, s.serverClientConn, nil
 	}
 	s.proxy = grpc.NewServer(
 		grpc.CustomCodec(proxy.Codec()),