Allow requiring `oneof` values

[alexander-yevsyukov]

In some cases, it is needed to require that one of the fields coming under oneof is set. For example, if we create a user account it can be either person or service field must be set. In order to arrange such a requirement, we can use the following construct:

option (required_field) = "person|service";

// ...

oneof type {
    // The user is a human.
    PersonProfile person = 2;

    // The user is a computer program.
    ServiceProfile service = 3;
}

The problem with this approach is that it's fragile and breaks when we:

1. rename fields;
2. add fields under oneof.

We cannot annotate oneof itself, but we cannot reduce the dependency on the content of it by telling the compiler: “We do want one of the things coming under that oneof.” Here's how:

// One of the fields coming under `type` `oneof` must be populated.
option (required).field = "type";

This is still fragile for renaming the oneof itself, but they tend to be more stable, and it's still safer as we refer only one element.

[dmdashenkov]

We can mark oneofs with custom options. I suggest we add a (required_fields) option for oneofs and allow using oneof names in the (required_field) option.

[armiol]

@dmdashenkov I would have both.

But... Can we have a (required) option for oneof, not the required_fields? Or maybe I don't understand how it's going to look from your perspective. Could you post a sample?

[dmdashenkov]

@armiol, we need a distinct name because in Java all the extension options are represented as static fields in a single class.

Example:

message User {
    // ...
    oneof type {
        option (required_fields) = true;

        PersonProfile person = 2;

        ServiceProfile service = 3;
    }
}

[alexander-yevsyukov]

The problem with plural here is that it sounds as if all of them are required, which is not the case. It's a bit misleading.

[alexander-yevsyukov]

How about is_required:

message User {
    // ...
    oneof type {
        option (is_required) = true;

        PersonProfile person = 2;

        ServiceProfile service = 3;
    }
}