Unmarshal embedded field pointer panics when field implements UnmarshalJSON - go

Question

I have a struct that embeds an embedded pointer to another struct. When I use the default json.Unmarshal behavior, it works perfectly. But when I implement UnmarshalJSON for the embedded struct's type but not the outer struct, then go panics with null pointer dereference.
If I implement UnmarshalJSON for the outer struct type as well, then it works. However, the outer struct has many fields that I would rather not have to manually unmarshal.
Why does implementing UnmarshalJSON on one and not the other cause a panic?
Is there a way to get it work without implemented UnmarshalJSON for the outer type?
If not, is there a simpler/less manual way to implement UnmarshalJSON for the outer type?
Note: There is a question with a similar title, "json.Unmarshal fails when embedded type has UnmarshalJSON
", but the issue there is different from mine.
tl;dr: The rest of this question is just a lengthy example of the above.
Base Example
(play.golang.org version of example)
The two structs, one with an embedded field pointer to the other:
(Simplified for example -- this doesn't really need its own UnmarshalJSON but it demonstrates the problem.)
type Obj struct {
X int `json:"x"`
}
type Container struct {
*Obj
Y int `json:"y"`
}
Invoking unmarshal:
func main() {
b := []byte(`{"x": 5, "y": 3}`)
c := &Container{}
err := json.Unmarshal(b, c)
if err != nil {
fmt.Printf("error ummarshalling json: %+v\n", err)
return
}
fmt.Printf("unmarshalled: %+v --> %+v\n", c, c.Obj)
}
Without implementing any UnmarshalJSON funcs, this works fine:
unmarshalled: &{Obj:0x416080 Y:3} --> &{X:5}
Panic
But, if I add UnmarshalJSON to the embedded Obj type only, then the program panics, as the json.Unmarshal call passes a nil pointer when it tries to unmarshal *Obj.
func (o *Obj) UnmarshalJSON(b []byte) (err error) {
m := make(map[string]int)
err = json.Unmarshal(b, &m)
if err != nil {
return nil
}
o.X = m["x"] // the line indicated by panic
return nil
}
Output:
panic: runtime error: invalid memory address or nil pointer dereference
[...]
main.(*Obj).UnmarshalJSON(0x0, 0x416030, 0x10, 0x10, 0x0, 0x0)
/tmp/sandbox185809294/main.go:18 +0x130
[...]
Question: Why does it panic here but not with the default unmarshal behavior? I'd think that if a nil *Obj is being passed here, then the default behavior also passes around a nil pointer...
Fixing the panic
It no longer panics when I implement UnmarshalJSON for the outer Container type:
func (c *Container) UnmarshalJSON(b []byte) (err error) {
m := make(map[string]int)
err = json.Unmarshal(b, &m)
if err != nil {
return err
}
c.Obj = &Obj{X: m["x"]}
c.Y = m["y"]
return nil
}
But unmarshalling Container manually this way gets tedious if both the real Container and real Obj have more fields than this, each with different types.
Question: Is there a simpler way to prevent this panic?

Answer 1

Because the default behavior checks for nil and your custom unmarshaller does not. You need some logic in your UnmarshalJSON to check if o is nil and behave appropriately, instead of assuming o is not nil (by trying to access one of its fields), thereby triggering a panic.
func (o *Obj) UnmarshalJSON(b []byte) (err error) {
if o == nil {
return nil // maybe? What do you want to happen in this case?
}
m := make(map[string]int)
err = json.Unmarshal(b, &m)
if err != nil {
return nil
}
o.X = m["x"] // the line indicated by panic
return nil
}
Also just for future reference, your *Obj field is not an "anonymous field", it is an embedded field: https://golang.org/ref/spec#Struct_types