rpc: fix double reference in CallContext unmarshal

rpc/client.go

@@ -345,7 +345,10 @@ func (c *Client) CallContext(ctx context.Context, result interface{}, method str
 	case len(resp.Result) == 0:
 		return ErrNoResult
 	default:
-		return json.Unmarshal(resp.Result, &result)
+		if result == nil {
+			return nil
+		}
+		return json.Unmarshal(resp.Result, result)
 	}
 }
 

rpc/client_test.go

@@ -37,6 +37,40 @@ import (
 	"github.com/ethereum/go-ethereum/log"
 )
 
+type nullTestService struct{}
+
+func (s *nullTestService) ReturnNull() json.RawMessage {
+	// An example where null results are returned is calling eth_getTransactionReceipt on a non-existent
+	// transaction. The result is null, but the call is not an error.
+	return json.RawMessage("null")
+}
+
+func TestNullResponse(t *testing.T) {
+	server := newTestServer()
+	defer server.Stop()
+	err := server.RegisterName("test", new(nullTestService))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	client := DialInProc(server)
+	defer client.Close()
+	result := &jsonrpcMessage{}
+
+	err = client.Call(&result.Result, "test_returnNull")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if result.Result == nil {
+		t.Fatal("Expected non-nil result")
+	}
+
+	if !reflect.DeepEqual(result.Result, json.RawMessage("null")) {
+		t.Errorf("Expected null, got %s", result.Result)
+	}
+}
+
 func TestClientRequest(t *testing.T) {
 	server := newTestServer()
 	defer server.Stop()