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, 26 insertions, 5 deletions

diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index ee3585f3e6..4a40752dc2 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -34,6 +34,13 @@ import (
 // the value pointed at by the pointer. If the pointer is nil, Unmarshal
 // allocates a new value for it to point to.
 //
+// To unmarshal JSON into a value implementing the Unmarshaler interface,
+// Unmarshal calls that value's UnmarshalJSON method, including
+// when the input is a JSON null.
+// Otherwise, if the value implements encoding.TextUnmarshaler
+// and the input is a JSON quoted string, Unmarshal calls that value's
+// UnmarshalText method with the unquoted form of the string.
+//
 // To unmarshal JSON into a struct, Unmarshal matches incoming object
 // keys to the keys used by Marshal (either the struct field name or its tag),
 // preferring an exact match but also accepting a case-insensitive match.
@@ -102,6 +109,9 @@ func Unmarshal(data []byte, v interface{}) error {
 // The input can be assumed to be a valid encoding of
 // a JSON value. UnmarshalJSON must copy the JSON data
 // if it wishes to retain the data after returning.
+//
+// By convention, to approximate the behavior of Unmarshal itself,
+// Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.
 type Unmarshaler interface {
 	UnmarshalJSON([]byte) error
 }
@@ -458,8 +468,10 @@ func (d *decodeState) indirect(v reflect.Value, decodingNull bool) (Unmarshaler,
 			if u, ok := v.Interface().(Unmarshaler); ok {
 				return u, nil, reflect.Value{}
 			}
-			if u, ok := v.Interface().(encoding.TextUnmarshaler); ok {
-				return nil, u, reflect.Value{}
+			if !decodingNull {
+				if u, ok := v.Interface().(encoding.TextUnmarshaler); ok {
+					return nil, u, reflect.Value{}
+				}
 			}
 		}
 		v = v.Elem()
@@ -814,8 +826,8 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
 		return
 	}
-	wantptr := item[0] == 'n' // null
-	u, ut, pv := d.indirect(v, wantptr)
+	isNull := item[0] == 'n' // null
+	u, ut, pv := d.indirect(v, isNull)
 	if u != nil {
 		err := u.UnmarshalJSON(item)
 		if err != nil {
@@ -828,7 +840,16 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 			if fromQuoted {
 				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
 			} else {
-				d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.off)})
+				var val string
+				switch item[0] {
+				case 'n':
+					val = "null"
+				case 't', 'f':
+					val = "bool"
+				default:
+					val = "number"
+				}
+				d.saveError(&UnmarshalTypeError{Value: val, Type: v.Type(), Offset: int64(d.off)})
 			}
 			return
 		}