encoding/json: don't indirect pointers when decoding null

The indirect method checked the type of the child when indirecting a
pointer. If the current value is a pointer and we are decoding null, we
can skip this entirely and return early, avoiding the whole descent.

Fixes #31776

Change-Id: Ib8b2a2357572c41f56fceac59b5a858980f3f65e
Reviewed-on: https://go-review.googlesource.com/c/go/+/174699
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/encoding/json/decode.go

@@ -416,8 +416,9 @@ func (d *decodeState) valueQuoted() interface{} {
 
 // indirect walks down v allocating pointers as needed,
 // until it gets to a non-pointer.
-// if it encounters an Unmarshaler, indirect stops and returns that.
-// if decodingNull is true, indirect stops at the last pointer so it can be set to nil.
+// If it encounters an Unmarshaler, indirect stops and returns that.
+// If decodingNull is true, indirect stops at the first settable pointer so it
+// can be set to nil.
 func indirect(v reflect.Value, decodingNull bool) (Unmarshaler, encoding.TextUnmarshaler, reflect.Value) {
 	// Issue #24153 indicates that it is generally not a guaranteed property
 	// that you may round-trip a reflect.Value by calling Value.Addr().Elem()
@@ -456,7 +457,7 @@ func indirect(v reflect.Value, decodingNull bool) (Unmarshaler, encoding.TextUnm
 			break
 		}
 
-		if v.Elem().Kind() != reflect.Ptr && decodingNull && v.CanSet() {
+		if decodingNull && v.CanSet() {
 			break
 		}
 

src/encoding/json/decode_test.go

@@ -401,6 +401,11 @@ type B struct {
 	B bool `json:",string"`
 }
 
+type DoublePtr struct {
+	I **int
+	J **int
+}
+
 var unmarshalTests = []unmarshalTest{
 	// basic types
 	{in: `true`, ptr: new(bool), out: true},
@@ -656,6 +661,11 @@ var unmarshalTests = []unmarshalTest{
 		err:                   fmt.Errorf("json: unknown field \"X\""),
 		disallowUnknownFields: true,
 	},
+	{
+		in:  `{"I": 0, "I": null, "J": null}`,
+		ptr: new(DoublePtr),
+		out: DoublePtr{I: nil, J: nil},
+	},
 
 	// invalid UTF-8 is coerced to valid UTF-8.
 	{