Add UnimplementedServer for server interface (#785)

This changes protoc-gen-go-grpc to always emit an UnimplementedServer type
that implements the service interface. Users trying to implement
the service interface can embed this type to ensure that new methods
can be added in a forwards compatible manner.

protoc-gen-go/grpc/grpc.go

@@ -52,8 +52,10 @@ const generatedCodeVersion = 4
 // Paths for packages used by code generated in this file,
 // relative to the import_prefix of the generator.Generator.
 const (
+	errorPkgPath   = "google.golang.org/grpc/status"
 	contextPkgPath = "context"
 	grpcPkgPath    = "google.golang.org/grpc"
+	codePkgPath    = "google.golang.org/grpc/codes"
 )
 
 func init() {
@@ -77,6 +79,8 @@ func (g *grpc) Name() string {
 var (
 	contextPkg string
 	grpcPkg    string
+	errorPkg   string
+	codePkg    string
 )
 
 // Init initializes the plugin.
@@ -105,13 +109,19 @@ func (g *grpc) Generate(file *generator.FileDescriptor) {
 		return
 	}
 
+	errorPkg = string(g.gen.AddImport(errorPkgPath))
+	codePkg = string(g.gen.AddImport(codePkgPath))
 	contextPkg = string(g.gen.AddImport(contextPkgPath))
 	grpcPkg = string(g.gen.AddImport(grpcPkgPath))
 
 	g.P("// Reference imports to suppress errors if they are not otherwise used.")
 	g.P("var _ ", contextPkg, ".Context")
 	g.P("var _ ", grpcPkg, ".ClientConn")
 	g.P()
+	g.P("func errUnimplemented(methodName string) error {")
+	g.P("\treturn ", errorPkg, ".Errorf(codes.Unimplemented, \"method %s not implemented\", methodName)")
+	g.P("}")
+	g.P()
 
 	// Assert version compatibility.
 	g.P("// This is a compile-time assertion to ensure that this generated file")
@@ -216,6 +226,12 @@ func (g *grpc) generateService(file *generator.FileDescriptor, service *pb.Servi
 	g.P("}")
 	g.P()
 
+	// Server Unimplemented struct for forward compatability.
+	if deprecated {
+		g.P(deprecationComment)
+	}
+	g.generateUnimplementedServer(servName, service)
+
 	// Server registration.
 	if deprecated {
 		g.P(deprecationComment)
@@ -269,6 +285,34 @@ func (g *grpc) generateService(file *generator.FileDescriptor, service *pb.Servi
 	g.P()
 }
 
+// generateUnimplementedServer creates the unimplemented server struct
+func (g *grpc) generateUnimplementedServer(servName string, service *pb.ServiceDescriptorProto) {
+	serverType := servName + "Server"
+	g.P("// Unimplemented", serverType, " can be embedded to have forward compatible implementations.")
+	g.P("type Unimplemented", serverType, " struct {")
+	g.P("}")
+	g.P()
+	// Unimplemented<service_name>Server's concrete methods
+	for _, method := range service.Method {
+		g.P(g.generateServerMethodConcrete(servName, method))
+	}
+	g.P()
+}
+
+// generateServerMethodConcrete returns unimplemented methods which ensure forward compatibility
+func (g *grpc) generateServerMethodConcrete(servName string, method *pb.MethodDescriptorProto) string {
+	header := g.generateServerSignatureWithParamNames(servName, method)
+	implementation := fmt.Sprintf("func (*Unimplemented%sServer) %s {\n", servName, header)
+	implementation += fmt.Sprintf("\treturn ")
+	if !method.GetServerStreaming() && !method.GetClientStreaming() {
+		implementation += "nil, "
+	}
+	origMethName := method.GetName()
+	methName := generator.CamelCase(origMethName)
+	implementation += fmt.Sprintf("errUnimplemented(%q)\n}", methName)
+	return implementation
+}
+
 // generateClientSignature returns the client-side signature for a method.
 func (g *grpc) generateClientSignature(servName string, method *pb.MethodDescriptorProto) string {
 	origMethName := method.GetName()
@@ -368,6 +412,30 @@ func (g *grpc) generateClientMethod(servName, fullServName, serviceDescVar strin
 	}
 }
 
+// generateServerSignatureWithParamNames returns the server-side signature for a method with parameter names.
+func (g *grpc) generateServerSignatureWithParamNames(servName string, method *pb.MethodDescriptorProto) string {
+	origMethName := method.GetName()
+	methName := generator.CamelCase(origMethName)
+	if reservedClientName[methName] {
+		methName += "_"
+	}
+
+	var reqArgs []string
+	ret := "error"
+	if !method.GetServerStreaming() && !method.GetClientStreaming() {
+		reqArgs = append(reqArgs, "ctx "+contextPkg+".Context")
+		ret = "(*" + g.typeName(method.GetOutputType()) + ", error)"
+	}
+	if !method.GetClientStreaming() {
+		reqArgs = append(reqArgs, "req *"+g.typeName(method.GetInputType()))
+	}
+	if method.GetServerStreaming() || method.GetClientStreaming() {
+		reqArgs = append(reqArgs, "srv "+servName+"_"+generator.CamelCase(origMethName)+"Server")
+	}
+
+	return methName + "(" + strings.Join(reqArgs, ", ") + ") " + ret
+}
+
 // generateServerSignature returns the server-side signature for a method.
 func (g *grpc) generateServerSignature(servName string, method *pb.MethodDescriptorProto) string {
 	origMethName := method.GetName()