encoding/json: fix decoding of null into Unmarshaler, TextUnmarshaler

1. Define behavior for Unmarshal of JSON null into Unmarshaler and
TextUnmarshaler. Specifically, an Unmarshaler will be given the
literal null and can decide what to do (because otherwise
json.RawMessage is impossible to implement), and a TextUnmarshaler
will be skipped over (because there is no text to unmarshal), like
most other inappropriate types. Document this in Unmarshal, with a
reminder in UnmarshalJSON about handling null.

2. Test all this.

3. Fix the TextUnmarshaler case, which was returning an unmarshalling
error, to match the definition.

4. Fix the error that had been used for the TextUnmarshaler, since it
was claiming that there was a JSON string when in fact the problem was
NOT having a string.

5. Adjust time.Time and big.Int's UnmarshalJSON to ignore null, as is
conventional.

Fixes #9037.

Change-Id: If78350414eb8dda712867dc8f4ca35a9db041b0c
Reviewed-on: https://go-review.googlesource.com/30944
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 193 insertions, 32 deletions

diff --git a/src/encoding/json/decode_test.go b/src/encoding/json/decode_test.go
index b1c4658e1b..af84b1b527 100644
--- a/src/encoding/json/decode_test.go
+++ b/src/encoding/json/decode_test.go
@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"image"
 	"math"
+	"math/big"
 	"net"
 	"reflect"
 	"strconv"
@@ -1524,40 +1525,148 @@ func TestInterfaceSet(t *testing.T) {
 	}
 }
 
+type NullTest struct {
+	Bool      bool
+	Int       int
+	Int8      int8
+	Int16     int16
+	Int32     int32
+	Int64     int64
+	Uint      uint
+	Uint8     uint8
+	Uint16    uint16
+	Uint32    uint32
+	Uint64    uint64
+	Float32   float32
+	Float64   float64
+	String    string
+	PBool     *bool
+	Map       map[string]string
+	Slice     []string
+	Interface interface{}
+
+	PRaw    *RawMessage
+	PTime   *time.Time
+	PBigInt *big.Int
+	PText   *MustNotUnmarshalText
+	PBuffer *bytes.Buffer // has methods, just not relevant ones
+	PStruct *struct{}
+
+	Raw    RawMessage
+	Time   time.Time
+	BigInt big.Int
+	Text   MustNotUnmarshalText
+	Buffer bytes.Buffer
+	Struct struct{}
+}
+
+type NullTestStrings struct {
+	Bool      bool              `json:",string"`
+	Int       int               `json:",string"`
+	Int8      int8              `json:",string"`
+	Int16     int16             `json:",string"`
+	Int32     int32             `json:",string"`
+	Int64     int64             `json:",string"`
+	Uint      uint              `json:",string"`
+	Uint8     uint8             `json:",string"`
+	Uint16    uint16            `json:",string"`
+	Uint32    uint32            `json:",string"`
+	Uint64    uint64            `json:",string"`
+	Float32   float32           `json:",string"`
+	Float64   float64           `json:",string"`
+	String    string            `json:",string"`
+	PBool     *bool             `json:",string"`
+	Map       map[string]string `json:",string"`
+	Slice     []string          `json:",string"`
+	Interface interface{}       `json:",string"`
+
+	PRaw    *RawMessage           `json:",string"`
+	PTime   *time.Time            `json:",string"`
+	PBigInt *big.Int              `json:",string"`
+	PText   *MustNotUnmarshalText `json:",string"`
+	PBuffer *bytes.Buffer         `json:",string"`
+	PStruct *struct{}             `json:",string"`
+
+	Raw    RawMessage           `json:",string"`
+	Time   time.Time            `json:",string"`
+	BigInt big.Int              `json:",string"`
+	Text   MustNotUnmarshalText `json:",string"`
+	Buffer bytes.Buffer         `json:",string"`
+	Struct struct{}             `json:",string"`
+}
+
 // JSON null values should be ignored for primitives and string values instead of resulting in an error.
 // Issue 2540
 func TestUnmarshalNulls(t *testing.T) {
+	// Unmarshal docs:
+	// The JSON null value unmarshals into an interface, map, pointer, or slice
+	// by setting that Go value to nil. Because null is often used in JSON to mean
+	// ``not present,'' unmarshaling a JSON null into any other Go type has no effect
+	// on the value and produces no error.
+
 	jsonData := []byte(`{
-		"Bool"    : null,
-		"Int"     : null,
-		"Int8"    : null,
-		"Int16"   : null,
-		"Int32"   : null,
-		"Int64"   : null,
-		"Uint"    : null,
-		"Uint8"   : null,
-		"Uint16"  : null,
-		"Uint32"  : null,
-		"Uint64"  : null,
-		"Float32" : null,
-		"Float64" : null,
-		"String"  : null}`)
-
-	nulls := All{
-		Bool:    true,
-		Int:     2,
-		Int8:    3,
-		Int16:   4,
-		Int32:   5,
-		Int64:   6,
-		Uint:    7,
-		Uint8:   8,
-		Uint16:  9,
-		Uint32:  10,
-		Uint64:  11,
-		Float32: 12.1,
-		Float64: 13.1,
-		String:  "14"}
+				"Bool"    : null,
+				"Int"     : null,
+				"Int8"    : null,
+				"Int16"   : null,
+				"Int32"   : null,
+				"Int64"   : null,
+				"Uint"    : null,
+				"Uint8"   : null,
+				"Uint16"  : null,
+				"Uint32"  : null,
+				"Uint64"  : null,
+				"Float32" : null,
+				"Float64" : null,
+				"String"  : null,
+				"PBool": null,
+				"Map": null,
+				"Slice": null,
+				"Interface": null,
+				"PRaw": null,
+				"PTime": null,
+				"PBigInt": null,
+				"PText": null,
+				"PBuffer": null,
+				"PStruct": null,
+				"Raw": null,
+				"Time": null,
+				"BigInt": null,
+				"Text": null,
+				"Buffer": null,
+				"Struct": null
+			}`)
+	nulls := NullTest{
+		Bool:      true,
+		Int:       2,
+		Int8:      3,
+		Int16:     4,
+		Int32:     5,
+		Int64:     6,
+		Uint:      7,
+		Uint8:     8,
+		Uint16:    9,
+		Uint32:    10,
+		Uint64:    11,
+		Float32:   12.1,
+		Float64:   13.1,
+		String:    "14",
+		PBool:     new(bool),
+		Map:       map[string]string{},
+		Slice:     []string{},
+		Interface: new(MustNotUnmarshalJSON),
+		PRaw:      new(RawMessage),
+		PTime:     new(time.Time),
+		PBigInt:   new(big.Int),
+		PText:     new(MustNotUnmarshalText),
+		PStruct:   new(struct{}),
+		PBuffer:   new(bytes.Buffer),
+		Raw:       RawMessage("123"),
+		Time:      time.Unix(123456789, 0),
+		BigInt:    *big.NewInt(123),
+	}
+
+	before := nulls.Time.String()
 
 	err := Unmarshal(jsonData, &nulls)
 	if err != nil {
@@ -1566,9 +1675,61 @@ func TestUnmarshalNulls(t *testing.T) {
 	if !nulls.Bool || nulls.Int != 2 || nulls.Int8 != 3 || nulls.Int16 != 4 || nulls.Int32 != 5 || nulls.Int64 != 6 ||
 		nulls.Uint != 7 || nulls.Uint8 != 8 || nulls.Uint16 != 9 || nulls.Uint32 != 10 || nulls.Uint64 != 11 ||
 		nulls.Float32 != 12.1 || nulls.Float64 != 13.1 || nulls.String != "14" {
-
 		t.Errorf("Unmarshal of null values affected primitives")
 	}
+
+	if nulls.PBool != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PBool")
+	}
+	if nulls.Map != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.Map")
+	}
+	if nulls.Slice != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.Slice")
+	}
+	if nulls.Interface != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.Interface")
+	}
+	if nulls.PRaw != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PRaw")
+	}
+	if nulls.PTime != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PTime")
+	}
+	if nulls.PBigInt != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PBigInt")
+	}
+	if nulls.PText != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PText")
+	}
+	if nulls.PBuffer != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PBuffer")
+	}
+	if nulls.PStruct != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PStruct")
+	}
+
+	if string(nulls.Raw) != "null" {
+		t.Errorf("Unmarshal of RawMessage null did not record null: %v", string(nulls.Raw))
+	}
+	if nulls.Time.String() != before {
+		t.Errorf("Unmarshal of time.Time null set time to %v", nulls.Time.String())
+	}
+	if nulls.BigInt.String() != "123" {
+		t.Errorf("Unmarshal of big.Int null set int to %v", nulls.BigInt.String())
+	}
+}
+
+type MustNotUnmarshalJSON struct{}
+
+func (x MustNotUnmarshalJSON) UnmarshalJSON(data []byte) error {
+	return errors.New("MustNotUnmarshalJSON was used")
+}
+
+type MustNotUnmarshalText struct{}
+
+func (x MustNotUnmarshalText) UnmarshalText(text []byte) error {
+	return errors.New("MustNotUnmarshalText was used")
 }
 
 func TestStringKind(t *testing.T) {
@@ -1807,7 +1968,7 @@ var invalidUnmarshalTextTests = []struct {
 	{nil, "json: Unmarshal(nil)"},
 	{struct{}{}, "json: Unmarshal(non-pointer struct {})"},
 	{(*int)(nil), "json: Unmarshal(nil *int)"},
-	{new(net.IP), "json: cannot unmarshal string into Go value of type *net.IP"},
+	{new(net.IP), "json: cannot unmarshal number into Go value of type *net.IP"},
 }
 
 func TestInvalidUnmarshalText(t *testing.T) {