JSON Unmarshal Irregular JSON field - json

Question

I have this code:
type Response struct {
ID string `json:"id"`
Tags Tags `json:"tags,omitempty"`
}
type Tags struct {
Geo []string `json:"geo,omitempty"`
Keyword []string `json:"keyword,omitempty"`
Storm []string `json:"storm,omitempty"`
}
func (t *Tags) UnmarshalJSON(b []byte) (err error) {
str := string(b)
if str == "" {
t = &Tags{}
return nil
}
err = json.Unmarshal(b, t)
if err != nil {
return err
}
return nil
}
Now, my JSON response looks like this:
[{
"id": "/cms/v4/assets/en_US",
"doc": [{
"id": "af02b41d-c2c5-48ec-9dbc-ceed693bdbac",
"tags": {
"geo": [
"DMA:US.740:US"
]
}
},
{
"id": "6a90d9ed-7978-4c18-8e36-c01cf4260492",
"tags": ""
},
{
"id": "32cfd045-98ac-408c-b464-c74e02466339",
"tags": {
"storm": [
"HARVEY - AL092017"
],
"keyword": [
"hurrcane",
"wunderground"
]
}
}
]
}]
Preferably, I'd change the JSON response to be done correctly, but I cannot. Unmarshaling continues to error out (goroutine stack exceeds 1000000000-byte limit). Preferably, I'd rather do this using easyjson or ffjson but doubt it is possible. Suggestions?

Answer 1

Your UnmarshalJSON function calls itself recursively, which will cause the stack to explode in size.
func (t *Tags) UnmarshalJSON(b []byte) (err error) {
str := string(b)
if str == "" {
t = &Tags{}
return nil
}
err = json.Unmarshal(b, t) <--- here it calls itself again
if err != nil {
return err
}
return nil
}
If you have a reason to call json.Unmarshal from within a UnmarshalJSON function, it must be on a different type. A common way to do this is to use a local alias:
type tagsAlias Tags
var ta = &tagsAlias
err = json.Unmarshal(b, ta)
if err != nil {
return err
}
*t = Tags(ta)
Also note that t = &Tags{} does nothing in your function; it assigns a new value to t, but that value is lost as soon as the function exits. If you really want to assign to t, you need *t; but you also don't need that at all, unless you're trying to unsset a previously set instance of *Tags.