encoding/json: fix handling of nil anonymous structs

Given the following types:

	type S2 struct{ Field string }
	type S  struct{ *S2 }

Marshalling a value of type T1 should result in "{}", as there's no way
to access any value of T2.Field. This is how Go 1.10 and earlier
versions behave.

However, in the recent refactor golang.org/cl/125417 I broke this logic.
When the encoder found an anonymous struct pointer field that was nil,
it no longer skipped the embedded fields underneath it. This can be seen
in the added test:

	--- FAIL: TestAnonymousFields/EmbeddedFieldBehindNilPointer (0.00s)
	    encode_test.go:430: Marshal() = "{\"Field\":\"\\u003c*json.S2 Value\\u003e\"}", want "{}"

The human error was a misplaced label, meaning we weren't actually
skipping the right loop iteration. Fix that.

Change-Id: Iba8a4a77d358dac73dcba4018498fe4f81afa263
Reviewed-on: https://go-review.googlesource.com/131376
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 7e5e209b4f2b6a15de51488cea3598ab906a2590..f10124e67d13c8f1348efb3d6b55008f59636ef4 100644 (file)
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -629,12 +629,12 @@ type structEncoder struct {
 
 func (se structEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
        next := byte('{')
+FieldLoop:
        for i := range se.fields {
                f := &se.fields[i]
 
                // Find the nested struct field by following f.index.
                fv := v
-       FieldLoop:
                for _, i := range f.index {
                        if fv.Kind() == reflect.Ptr {
                                if fv.IsNil() {

diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index 1b7838c895d119dd283245bb45080f02993ca943..cd5eadf3c1cd2afab2c6da75b93eb8a367271299 100644 (file)
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -405,6 +405,19 @@ func TestAnonymousFields(t *testing.T) {
                        return S{s1{1, 2, s2{3, 4}}, 6}
                },
                want: `{"MyInt1":1,"MyInt2":3}`,
+       }, {
+               // If an anonymous struct pointer field is nil, we should ignore
+               // the embedded fields behind it. Not properly doing so may
+               // result in the wrong output or reflect panics.
+               label: "EmbeddedFieldBehindNilPointer",
+               makeInput: func() interface{} {
+                       type (
+                               S2 struct{ Field string }
+                               S  struct{ *S2 }
+                       )
+                       return S{}
+               },
+               want: `{}`,
        }}
 
        for _, tt := range tests {