Only support single signature for subscriber and validate

examples/server/main.go

@@ -39,19 +39,23 @@ func main() {
 	)
 
 	// Register Subscribers
-	server.Subscribe(
+	if err := server.Subscribe(
 		server.NewSubscriber(
 			"topic.go.micro.srv.example",
 			new(subscriber.Example),
 		),
-	)
+	); err != nil {
+		log.Fatal(err)
+	}
 
-	server.Subscribe(
+	if err := server.Subscribe(
 		server.NewSubscriber(
 			"topic.go.micro.srv.example",
 			subscriber.Handler,
 		),
-	)
+	); err != nil {
+		log.Fatal(err)
+	}
 
 	// Run server
 	if err := server.Run(); err != nil {

examples/server/subscriber/subscriber.go

@@ -13,6 +13,7 @@ func (e *Example) Handle(ctx context.Context, msg *example.Message) error {
 	return nil
 }
 
-func Handler(msg *example.Message) {
+func Handler(ctx context.Context, msg *example.Message) error {
 	log.Info("Function Received message: ", msg.Say)
+	return nil
 }

server/rpc_server.go

@@ -132,6 +132,10 @@ func (s *rpcServer) Subscribe(sb Subscriber) error {
 		return fmt.Errorf("invalid subscriber: no handler functions")
 	}
 
+	if err := validateSubscriber(sb); err != nil {
+		return err
+	}
+
 	s.Lock()
 	_, ok = s.subscribers[sub]
 	if ok {

server/subscriber.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"bytes"
+	"fmt"
 	"reflect"
 
 	"github.com/micro/go-micro/broker"
@@ -11,6 +12,10 @@ import (
 	"golang.org/x/net/context"
 )
 
+const (
+	subSig = "func(context.Context, interface{}) error"
+)
+
 type handler struct {
 	method  reflect.Value
 	reqType reflect.Type
@@ -94,6 +99,61 @@ func newSubscriber(topic string, sub interface{}) Subscriber {
 	}
 }
 
+func validateSubscriber(sub Subscriber) error {
+	typ := reflect.TypeOf(sub.Subscriber())
+	var argType reflect.Type
+
+	if typ.Kind() == reflect.Func {
+		name := "Func"
+		switch typ.NumIn() {
+		case 2:
+			argType = typ.In(1)
+		default:
+			return fmt.Errorf("subscriber %v takes wrong number of args: %v required signature %s", name, typ.NumIn(), subSig)
+		}
+		if !isExportedOrBuiltinType(argType) {
+			return fmt.Errorf("subscriber %v argument type not exported: %v", name, argType)
+		}
+		if typ.NumOut() != 1 {
+			return fmt.Errorf(
+				"subscriber %v.%v has wrong number of outs: %v require signature %s",
+				name, typ.NumOut(), subSig)
+		}
+		if returnType := typ.Out(0); returnType != typeOfError {
+			return fmt.Errorf("subscriber %v returns %v not error", name, returnType.String())
+		}
+	} else {
+		hdlr := reflect.ValueOf(sub.Subscriber())
+		name := reflect.Indirect(hdlr).Type().Name()
+
+		for m := 0; m < typ.NumMethod(); m++ {
+			method := typ.Method(m)
+
+			switch method.Type.NumIn() {
+			case 3:
+				argType = method.Type.In(2)
+			default:
+				return fmt.Errorf("subscriber %v.%v takes wrong number of args: %v required signature %s",
+					name, method.Name, method.Type.NumIn(), subSig)
+			}
+
+			if !isExportedOrBuiltinType(argType) {
+				return fmt.Errorf("%v argument type not exported: %v", name, argType)
+			}
+			if method.Type.NumOut() != 1 {
+				return fmt.Errorf(
+					"subscriber %v.%v has wrong number of outs: %v require signature %s",
+					name, method.Name, method.Type.NumOut(), subSig)
+			}
+			if returnType := method.Type.Out(0); returnType != typeOfError {
+				return fmt.Errorf("subscriber %v.%v returns %v not error", name, method.Name, returnType.String())
+			}
+		}
+	}
+
+	return nil
+}
+
 func (s *rpcServer) createSubHandler(sb *subscriber) broker.Handler {
 	return func(msg *broker.Message) {
 		cf, err := s.newCodec(msg.Header["Content-Type"])