Fix null result field bug for JSON-RPC raw response (#362)

geth/api/backend_accounts_test.go

@@ -376,3 +376,18 @@ func (s *BackendTestSuite) TestRPCEthAccountsWithUpstream() {
       }`)
 	require.Equal(expectedResponse, resp)
 }
+
+// regression test: eth_getTransactionReceipt with invalid transaction hash should return null
+func (s *BackendTestSuite) TestRegressionGetTransactionReceipt() {
+	require := s.Require()
+
+	s.StartTestBackend(params.RopstenNetworkID)
+	defer s.StopTestBackend()
+
+	rpcClient := s.backend.NodeManager().RPCClient()
+
+	// note: transaction hash is assumed to be invalid
+	got := rpcClient.CallRaw(`{"jsonrpc":"2.0","method":"eth_getTransactionReceipt","params":["0xbbebf28d0a3a3cbb38e6053a5b21f08f82c62b0c145a17b1c4313cac3f68ae7c"],"id":7}`)
+	expected := `{"jsonrpc":"2.0","id":7,"result":null}`
+	require.Equal(expected, got)
+}

geth/rpc/call_raw.go

@@ -157,6 +157,13 @@ func newSuccessResponse(result json.RawMessage, id json.RawMessage) string {
 		id = defaultMsgID
 	}
 
+	// original response with `null` is lost due to geth RPC client
+	// conversion (unmarshalling). We'll have nil result object in this case,
+	// so handle it specially. `result` is required field in response.
+	if result == nil {
+		result = json.RawMessage("null")
+	}
+
 	msg := &jsonrpcMessage{
 		ID:      id,
 		Version: jsonrpcVersion,

geth/rpc/call_raw_test.go

@@ -18,6 +18,8 @@ func TestNewSuccessResponse(t *testing.T) {
 		{"string", json.RawMessage(`"3434=done"`), nil, `{"jsonrpc":"2.0","id":0,"result":"3434=done"}`},
 		{"struct_nil_id", json.RawMessage(`{"field": "value"}`), nil, `{"jsonrpc":"2.0","id":0,"result":{"field":"value"}}`},
 		{"struct_non_nil_id", json.RawMessage(`{"field": "value"}`), json.RawMessage(`42`), `{"jsonrpc":"2.0","id":42,"result":{"field":"value"}}`},
+		{"null", json.RawMessage(`null`), json.RawMessage(`7`), `{"jsonrpc":"2.0","id":7,"result":null}`},
+		{"null_nil", nil, json.RawMessage(`7`), `{"jsonrpc":"2.0","id":7,"result":null}`},
 	}
 
 	for _, test := range cases {