From 29bc4f12581d836a96139c924f16a4987324edd1 Mon Sep 17 00:00:00 2001
From: LE Manh Cuong <cuong.manhle.vn@gmail.com>
Date: Sun, 28 Oct 2018 01:44:09 +0700
Subject: [PATCH] encoding/json: add Path to UnmarshalTypeError
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

When parsing nested object, UnmarshalTypeError does not contain actual
path to nested field in original JSON.

This commit change Field to contain the full path to that field. One
can get the Field name by stripping all the leading path elements.

Fixes #22369

Change-Id: I6969cc08abe8387a351e3fb2944adfaa0dccad2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/145218
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---
 src/encoding/json/decode.go      |  8 ++++++--
 src/encoding/json/decode_test.go | 24 ++++++++++++++++++++++--
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index 731553dca6b..3900bcc165c 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -125,7 +125,7 @@ type UnmarshalTypeError struct {
 	Type   reflect.Type // type of Go value it could not be assigned to
 	Offset int64        // error occurred after reading Offset bytes
 	Struct string       // name of the struct type containing the field
-	Field  string       // name of the field holding the Go value
+	Field  string       // the full path from root node to the field
 }
 
 func (e *UnmarshalTypeError) Error() string {
@@ -730,7 +730,11 @@ func (d *decodeState) object(v reflect.Value) error {
 					}
 					subv = subv.Field(i)
 				}
-				d.errorContext.Field = f.name
+				if originalErrorContext.Field == "" {
+					d.errorContext.Field = f.name
+				} else {
+					d.errorContext.Field = originalErrorContext.Field + "." + f.name
+				}
 				d.errorContext.Struct = t
 			} else if d.disallowUnknownFields {
 				d.saveError(fmt.Errorf("json: unknown field %q", key))
diff --git a/src/encoding/json/decode_test.go b/src/encoding/json/decode_test.go
index 60454c60583..d99d65d7634 100644
--- a/src/encoding/json/decode_test.go
+++ b/src/encoding/json/decode_test.go
@@ -45,6 +45,14 @@ type W struct {
 	S SS
 }
 
+type P struct {
+	PP PP
+}
+
+type PP struct {
+	T T
+}
+
 type SS string
 
 func (*SS) UnmarshalJSON(data []byte) error {
@@ -816,7 +824,7 @@ var unmarshalTests = []unmarshalTest{
 		err: &UnmarshalTypeError{
 			Value:  "string",
 			Struct: "V",
-			Field:  "F2",
+			Field:  "V.F2",
 			Type:   reflect.TypeOf(int32(0)),
 			Offset: 20,
 		},
@@ -827,7 +835,7 @@ var unmarshalTests = []unmarshalTest{
 		err: &UnmarshalTypeError{
 			Value:  "string",
 			Struct: "V",
-			Field:  "F2",
+			Field:  "V.F2",
 			Type:   reflect.TypeOf(int32(0)),
 			Offset: 30,
 		},
@@ -923,6 +931,18 @@ var unmarshalTests = []unmarshalTest{
 		ptr: new(MustNotUnmarshalText),
 		err: &UnmarshalTypeError{Value: "object", Type: reflect.TypeOf(&MustNotUnmarshalText{}), Offset: 1},
 	},
+	// #22369
+	{
+		in:  `{"PP": {"T": {"Y": "bad-type"}}}`,
+		ptr: new(P),
+		err: &UnmarshalTypeError{
+			Value:  "string",
+			Struct: "T",
+			Field:  "PP.T.Y",
+			Type:   reflect.TypeOf(int(0)),
+			Offset: 29,
+		},
+	},
 }
 
 func TestMarshal(t *testing.T) {