jsonrpc: use `error: null` for success, not `error: ""`

handle missing id in server.

Fixes #1017.
Fixes #1018.

R=r
CC=golang-dev
https://golang.org/cl/1986044

src/pkg/rpc/jsonrpc/all_test.go

@@ -53,7 +53,7 @@ func TestServer(t *testing.T) {
 	type addResp struct {
 		Id     interface{} "id"
 		Result Reply       "result"
-		Error  string      "error"
+		Error  interface{} "error"
 	}
 
 	cli, srv := net.Pipe()
@@ -69,7 +69,7 @@ func TestServer(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Decode: %s", err)
 		}
-		if resp.Error != "" {
+		if resp.Error != nil {
 			t.Fatalf("resp.Error: %s", resp.Error)
 		}
 		if resp.Id.(string) != string(i) {
@@ -79,6 +79,15 @@ func TestServer(t *testing.T) {
 			t.Fatalf("resp: bad result: %d+%d=%d", i, i+1, resp.Result.C)
 		}
 	}
+
+	fmt.Fprintf(cli, "{}\n")
+	var resp addResp
+	if err := dec.Decode(&resp); err != nil {
+		t.Fatalf("Decode after empty: %s", err)
+	}
+	if resp.Error == nil {
+		t.Fatalf("Expected error, got nil")
+	}
 }
 
 func TestClient(t *testing.T) {

src/pkg/rpc/jsonrpc/client.go

@@ -7,6 +7,7 @@
 package jsonrpc
 
 import (
+	"fmt"
 	"io"
 	"json"
 	"net"
@@ -61,13 +62,13 @@ func (c *clientCodec) WriteRequest(r *rpc.Request, param interface{}) os.Error {
 type clientResponse struct {
 	Id     uint64           "id"
 	Result *json.RawMessage "result"
-	Error  string           "error"
+	Error  interface{}      "error"
 }
 
 func (r *clientResponse) reset() {
 	r.Id = 0
 	r.Result = nil
-	r.Error = ""
+	r.Error = nil
 }
 
 func (c *clientCodec) ReadResponseHeader(r *rpc.Response) os.Error {
@@ -81,8 +82,18 @@ func (c *clientCodec) ReadResponseHeader(r *rpc.Response) os.Error {
 	c.pending[c.resp.Id] = "", false
 	c.mutex.Unlock()
 
+	r.Error = ""
 	r.Seq = c.resp.Id
-	r.Error = c.resp.Error
+	if c.resp.Error != nil {
+		x, ok := c.resp.Error.(string)
+		if !ok {
+			return os.NewError(fmt.Sprintf("invalid error %v", c.resp.Error))
+		}
+		if x == "" {
+			x = "unspecified error"
+		}
+		r.Error = x
+	}
 	return nil
 }
 

src/pkg/rpc/jsonrpc/server.go

@@ -61,7 +61,7 @@ func (r *serverRequest) reset() {
 type serverResponse struct {
 	Id     *json.RawMessage "id"
 	Result interface{}      "result"
-	Error  string           "error"
+	Error  interface{}      "error"
 }
 
 func (c *serverCodec) ReadRequestHeader(r *rpc.Request) os.Error {
@@ -94,6 +94,8 @@ func (c *serverCodec) ReadRequestBody(x interface{}) os.Error {
 	return json.Unmarshal(*c.req.Params, &params)
 }
 
+var null = json.RawMessage([]byte("null"))
+
 func (c *serverCodec) WriteResponse(r *rpc.Response, x interface{}) os.Error {
 	var resp serverResponse
 	c.mutex.Lock()
@@ -105,9 +107,17 @@ func (c *serverCodec) WriteResponse(r *rpc.Response, x interface{}) os.Error {
 	c.pending[r.Seq] = nil, false
 	c.mutex.Unlock()
 
+	if b == nil {
+		// Invalid request so no id.  Use JSON null.
+		b = &null
+	}
 	resp.Id = b
 	resp.Result = x
-	resp.Error = r.Error
+	if r.Error == "" {
+		resp.Error = nil
+	} else {
+		resp.Error = r.Error
+	}
 	return c.enc.Encode(resp)
 }