net/rpc: Create empty maps and slices as return type

When a map or slice is used as a return type create an empty value
rather than a nil value.

Fixes #19588

Change-Id: I577fd74956172329745d614ac37d4db8f737efb8
Reviewed-on: https://go-review.googlesource.com/38474
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/rpc/jsonrpc/all_test.go

@@ -13,6 +13,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/rpc"
+	"reflect"
 	"strings"
 	"testing"
 )
@@ -55,8 +56,26 @@ func (t *Arith) Error(args *Args, reply *Reply) error {
 	panic("ERROR")
 }
 
+type BuiltinTypes struct{}
+
+func (BuiltinTypes) Map(i int, reply *map[int]int) error {
+	(*reply)[i] = i
+	return nil
+}
+
+func (BuiltinTypes) Slice(i int, reply *[]int) error {
+	*reply = append(*reply, i)
+	return nil
+}
+
+func (BuiltinTypes) Array(i int, reply *[1]int) error {
+	(*reply)[0] = i
+	return nil
+}
+
 func init() {
 	rpc.Register(new(Arith))
+	rpc.Register(BuiltinTypes{})
 }
 
 func TestServerNoParams(t *testing.T) {
@@ -182,6 +201,45 @@ func TestClient(t *testing.T) {
 	}
 }
 
+func TestBuiltinTypes(t *testing.T) {
+	cli, srv := net.Pipe()
+	go ServeConn(srv)
+
+	client := NewClient(cli)
+	defer client.Close()
+
+	// Map
+	arg := 7
+	replyMap := map[int]int{}
+	err := client.Call("BuiltinTypes.Map", arg, &replyMap)
+	if err != nil {
+		t.Errorf("Map: expected no error but got string %q", err.Error())
+	}
+	if replyMap[arg] != arg {
+		t.Errorf("Map: expected %d got %d", arg, replyMap[arg])
+	}
+
+	// Slice
+	replySlice := []int{}
+	err = client.Call("BuiltinTypes.Slice", arg, &replySlice)
+	if err != nil {
+		t.Errorf("Slice: expected no error but got string %q", err.Error())
+	}
+	if e := []int{arg}; !reflect.DeepEqual(replySlice, e) {
+		t.Errorf("Slice: expected %v got %v", e, replySlice)
+	}
+
+	// Array
+	replyArray := [1]int{}
+	err = client.Call("BuiltinTypes.Array", arg, &replyArray)
+	if err != nil {
+		t.Errorf("Array: expected no error but got string %q", err.Error())
+	}
+	if e := [1]int{arg}; !reflect.DeepEqual(replyArray, e) {
+		t.Errorf("Array: expected %v got %v", e, replyArray)
+	}
+}
+
 func TestMalformedInput(t *testing.T) {
 	cli, srv := net.Pipe()
 	go cli.Write([]byte(`{id:1}`)) // invalid json

src/net/rpc/server.go

@@ -571,6 +571,13 @@ func (server *Server) readRequest(codec ServerCodec) (service *service, mtype *m
 	}
 
 	replyv = reflect.New(mtype.ReplyType.Elem())
+
+	switch mtype.ReplyType.Elem().Kind() {
+	case reflect.Map:
+		replyv.Elem().Set(reflect.MakeMap(mtype.ReplyType.Elem()))
+	case reflect.Slice:
+		replyv.Elem().Set(reflect.MakeSlice(mtype.ReplyType.Elem(), 0, 0))
+	}
 	return
 }
 

src/net/rpc/server_test.go

@@ -11,6 +11,7 @@ import (
 	"log"
 	"net"
 	"net/http/httptest"
+	"reflect"
 	"runtime"
 	"strings"
 	"sync"
@@ -85,6 +86,24 @@ type Embed struct {
 	hidden
 }
 
+type BuiltinTypes struct{}
+
+func (BuiltinTypes) Map(args *Args, reply *map[int]int) error {
+	(*reply)[args.A] = args.B
+	return nil
+}
+
+func (BuiltinTypes) Slice(args *Args, reply *[]int) error {
+	*reply = append(*reply, args.A, args.B)
+	return nil
+}
+
+func (BuiltinTypes) Array(args *Args, reply *[2]int) error {
+	(*reply)[0] = args.A
+	(*reply)[1] = args.B
+	return nil
+}
+
 func listenTCP() (net.Listener, string) {
 	l, e := net.Listen("tcp", "127.0.0.1:0") // any available address
 	if e != nil {
@@ -97,6 +116,7 @@ func startServer() {
 	Register(new(Arith))
 	Register(new(Embed))
 	RegisterName("net.rpc.Arith", new(Arith))
+	Register(BuiltinTypes{})
 
 	var l net.Listener
 	l, serverAddr = listenTCP()
@@ -326,6 +346,49 @@ func testHTTPRPC(t *testing.T, path string) {
 	}
 }
 
+func TestBuiltinTypes(t *testing.T) {
+	once.Do(startServer)
+
+	client, err := DialHTTP("tcp", httpServerAddr)
+	if err != nil {
+		t.Fatal("dialing", err)
+	}
+	defer client.Close()
+
+	// Map
+	args := &Args{7, 8}
+	replyMap := map[int]int{}
+	err = client.Call("BuiltinTypes.Map", args, &replyMap)
+	if err != nil {
+		t.Errorf("Map: expected no error but got string %q", err.Error())
+	}
+	if replyMap[args.A] != args.B {
+		t.Errorf("Map: expected %d got %d", args.B, replyMap[args.A])
+	}
+
+	// Slice
+	args = &Args{7, 8}
+	replySlice := []int{}
+	err = client.Call("BuiltinTypes.Slice", args, &replySlice)
+	if err != nil {
+		t.Errorf("Slice: expected no error but got string %q", err.Error())
+	}
+	if e := []int{args.A, args.B}; !reflect.DeepEqual(replySlice, e) {
+		t.Errorf("Slice: expected %v got %v", e, replySlice)
+	}
+
+	// Array
+	args = &Args{7, 8}
+	replyArray := [2]int{}
+	err = client.Call("BuiltinTypes.Array", args, &replyArray)
+	if err != nil {
+		t.Errorf("Array: expected no error but got string %q", err.Error())
+	}
+	if e := [2]int{args.A, args.B}; !reflect.DeepEqual(replyArray, e) {
+		t.Errorf("Array: expected %v got %v", e, replyArray)
+	}
+}
+
 // CodecEmulator provides a client-like api and a ServerCodec interface.
 // Can be used to test ServeRequest.
 type CodecEmulator struct {