encoding/json: encode nil encoding.TextMarshaler instance as "null"

Fixes #34235.

Change-Id: Ia3795fd18860530fa6a4b171545f525e784ffdcb
GitHub-Last-Rev: 1a319c452857818f7aaf22ef46823b43ca9b2276
GitHub-Pull-Request: golang/go#34238
Reviewed-on: https://go-review.googlesource.com/c/go/+/194642Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/encode.go

@@ -481,7 +481,11 @@ func textMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 		e.WriteString("null")
 		return
 	}
-	m := v.Interface().(encoding.TextMarshaler)
+	m, ok := v.Interface().(encoding.TextMarshaler)
+	if !ok {
+		e.WriteString("null")
+		return
+	}
 	b, err := m.MarshalText()
 	if err != nil {
 		e.error(&MarshalerError{v.Type(), err})

src/encoding/json/encode_test.go

@@ -6,6 +6,7 @@ package json
 
 import (
 	"bytes"
+	"encoding"
 	"fmt"
 	"log"
 	"math"
@@ -453,18 +454,31 @@ type BugX struct {
 	BugB
 }
 
-// Issue 16042. Even if a nil interface value is passed in
-// as long as it implements MarshalJSON, it should be marshaled.
-type nilMarshaler string
+// golang.org/issue/16042.
+// Even if a nil interface value is passed in, as long as
+// it implements Marshaler, it should be marshaled.
+type nilJSONMarshaler string
 
-func (nm *nilMarshaler) MarshalJSON() ([]byte, error) {
+func (nm *nilJSONMarshaler) MarshalJSON() ([]byte, error) {
 	if nm == nil {
 		return Marshal("0zenil0")
 	}
 	return Marshal("zenil:" + string(*nm))
 }
 
-// Issue 16042.
+// golang.org/issue/34235.
+// Even if a nil interface value is passed in, as long as
+// it implements encoding.TextMarshaler, it should be marshaled.
+type nilTextMarshaler string
+
+func (nm *nilTextMarshaler) MarshalText() ([]byte, error) {
+	if nm == nil {
+		return []byte("0zenil0"), nil
+	}
+	return []byte("zenil:" + string(*nm)), nil
+}
+
+// See golang.org/issue/16042 and golang.org/issue/34235.
 func TestNilMarshal(t *testing.T) {
 	testCases := []struct {
 		v    interface{}
@@ -478,8 +492,11 @@ func TestNilMarshal(t *testing.T) {
 		{v: []byte(nil), want: `null`},
 		{v: struct{ M string }{"gopher"}, want: `{"M":"gopher"}`},
 		{v: struct{ M Marshaler }{}, want: `{"M":null}`},
-		{v: struct{ M Marshaler }{(*nilMarshaler)(nil)}, want: `{"M":"0zenil0"}`},
-		{v: struct{ M interface{} }{(*nilMarshaler)(nil)}, want: `{"M":null}`},
+		{v: struct{ M Marshaler }{(*nilJSONMarshaler)(nil)}, want: `{"M":"0zenil0"}`},
+		{v: struct{ M interface{} }{(*nilJSONMarshaler)(nil)}, want: `{"M":null}`},
+		{v: struct{ M encoding.TextMarshaler }{}, want: `{"M":null}`},
+		{v: struct{ M encoding.TextMarshaler }{(*nilTextMarshaler)(nil)}, want: `{"M":"0zenil0"}`},
+		{v: struct{ M interface{} }{(*nilTextMarshaler)(nil)}, want: `{"M":null}`},
 	}
 
 	for _, tt := range testCases {