encoding/json: marshaling nil value stored in interface value doesn't omitempty

[jnross]

1. What version of Go are you using (go version)?
   1.7, also verified with 1.6.3
2. What operating system and processor architecture are you using (go env)?
   darwin/amd64
3. What did you do?
   I have a struct "result" defined with one of the members "Err" as an interface type "error". The json annotation specifies "omitempty". I define another struct type concreteError which implements this interface on the pointer receiver (i.e. *concreteError implements the interface).

If I assign a nil pointer variable to result.Err, the field is not omitted from the marshaled JSON as expected. Instead the field is marshaled as a JSON null.

If I assign the nil literal to result.Err, the field is omitted from the marshaled JSON as expected

This issue is clearly demonstrated here: https://play.golang.org/p/uw6jvKN-qX

1. What did you expect to see?
   A nil interface value should be omitted from the marshaled JSON when the field is annotated with "omitempty"
2. What did you see instead?
   The nil interface value was not omitted.

[dgryski]

This looks like fallout from https://golang.org/doc/faq#nil_error

[jnross]

Yes, that must be the case. Perhaps the JSON marshaller should use reflect.ValueOf(res.Err).IsNil() to determine whether an interface is empty and should be omitted per the annotation.

[jnross]

This appears to resolve the issue: jnross@36aa259

I'll see about contributing this patch directly per the instructions here: https://golang.org/doc/contribute.html

[rsc]

Working as intended. If you have

I int `json:",omitempty"
S string `json:",omitempty"`
J interface{} `json:",omitempty"

Then if I = 0 it gets omitted, or if S is "" it gets omitted, or if J is nil it gets omitted. But if J is 0 or "", it does not get omitted. That's not considered "empty" for an interface.

[jnross]

No, this issue still occurs as originally reported. See my example: https://play.golang.org/p/uw6jvKN-qX

[freeformz]

@jnross The example you provide is what both @dgryski and @rsc described.

FWIW: I would like an interface of a nil concrete value to also omitempty, but I would be surprised if that change happens.