Type cast panic when using RPC

[daniloarcidiacono]

Hello,
I'd like to develop a custom RPC backend but the ListFlags method in remote.go unmarshals the response as a generic []interface{}. This causes problems when the callers of ListFlags try to downcast the result to either []string or []store.BlockedUser.
The memory_store example backend has the same problem.

I tried to fix that method with:

func unmarshalString(data[] byte) (list []interface{}, err error) {
	var typedList []string
	err = json.Unmarshal(data, &typedList)
	if err == nil {
		// https://go101.org/article/interface.html
		list := make([]interface{}, 0, len(typedList))
		for _, w := range typedList {
			list = append(list, w)
		}

		return list, err
	}

	return nil, err
}

func unmarshalBlockedUser(data[] byte) (list []interface{}, err error) {
	var typedList []store.BlockedUser
	err = json.Unmarshal(data, &typedList)
	if err == nil {
		// https://go101.org/article/interface.html
		list := make([]interface{}, 0, len(typedList))
		for _, w := range typedList {
			list = append(list, w)
		}

		return list, err
	}

	return nil, err
}

// ListFlags get list of flagged keys, like blocked & verified user
func (r *RPC) ListFlags(req FlagRequest) (list []interface{}, err error) {
	resp, err := r.Call("store.list_flags", req)
	if err != nil {
		return nil, err
	}

	// Try store.BlockedUser
	list, err = unmarshalBlockedUser(*resp.Result)
	if err == nil {
		return list, err
	}

	// Try strings
	list, err = unmarshalString(*resp.Result)
	if err == nil {
		return list, err
	}
	
	// Generic (but it won't work)
	err = json.Unmarshal(*resp.Result, &list)
	return list, err
}

Method generics help a bit but they require Go 1.18

func polyUnmarshal[C any](data[] byte) (list []any, err error) {
	var typedList []C
	err = json.Unmarshal(data, &typedList)
	if err == nil {
		// https://go101.org/article/interface.html
		list := make([]interface{}, 0, len(typedList))
		for _, w := range typedList {
			list = append(list, w)
		}

		return list, err
	}

	return nil, err
}

Probably there is a better approach to solve this issue, I'm not a Go developer.
Thanks