optjson.go better handling of zero value

[ksykulev]

💥  Actual behavior

Given a struct such as

type testStruct struct {
  SomeID optjson.Any[uint] `json:"some_id"`
}

If you were to initialize this struct without specifying SomeID

ts := testStruct{}

Marshaling this object will product

{ "some_id": null }

🧑‍💻  Steps to reproduce

Discovered during #24658
You can reproduce by creating a patch request to the policies endpoint with a struct that does not define ScriptID or SoftwareTitleID

modifyGlobalPolicyRequest{
		ModifyPolicyPayload: fleet.ModifyPolicyPayload{
			Name:        ptr.String("TestQuery4"),
			Query:       ptr.String("select * from osquery_info;"),
			Description: ptr.String("Some description updated"),
			Resolution:  ptr.String("some global resolution updated"),
		},
	}

Notice how the json that is generated for this struct looks like:

{
		"PolicyID": 0,
		"name": "TestQuery4",
		"query": "select * from osquery_info;",
		"description": "Some description updated",
		"resolution": "some global resolution updated",
		"platform": null,
		"critical": null,
		"calendar_events_enabled": null,
                "script_id": null,
                "software_title_id": null
	}

🕯️ More info (optional)

https://victoronsoftware.com/posts/go-json-unmarshal/

🛠️ To fix

Is there a way to change optjson.go to not output the key if the Set is equal to false. Can we use the omitempty tag to get the intended behavior.

SomeID optjson.Any[uint] `json:"some_id,omitempty"`

[mna]

@ksykulev The problem is Go's support for omitempty is limited to non-struct, so when you have a struct type that implements a custom marshaler (MarshalJSON), as is the case for the optjson types, MarshalJSON will be called, and once it's called you have to return some valid JSON representation (hence, null). You cannot make Go skip the current field at this point, the key is already written out and it calls the type only to get the value.

The good thing is that this was raised a while ago as a bug on Go's tracker: golang/go#10648, which then created a proposal to add an omitzero tag option that would skip calling the marshaler if the struct was the zero-value: golang/go#45669 and it was accepted just recently (!) so it will be in Go's next version (1.24, as noted in the changelog). golang/go#45669 (comment)

[sharon-fdm]

Probably MDM has better context