feat(idtoken): Allow format options

[eisandbar]

Added field CustomFormat to DialSettings in internal/settings.go
Added field format to computerIDTokenSource in idtoken/compute.go
Function computeTokenSource now sets field format to full, and if ds.CustomFormat != "" overwrites the field
Method Token now uses c.format instead of string literal "full"

Fixes #542

[codyoss]

Thanks for the contribution. I think I am going to hold off on this for now as we would need to figure out a way to limit the scope of these changes.

[codyoss]

This changes the default behaviour which is not desired.

[eisandbar]

The default format was already full, so default behavior should be the same.

[codyoss]

I don't think we want this plumbed to the DialSettings as this is a package things and not an over all client library thing.

Also, there is no mechanism to set this field. I think I have a pattern laying around somewhere for doing something like this but it involves exposing custom client options.

[eisandbar]

Right now DialSettings seems to already have a field called CustomClaims that only package idtoken uses

google-api-go-client/internal/settings.go

Line 45 in d6ee425

CustomClaims map[string]interface{}

And the way it is set

google-api-go-client/idtoken/idtoken.go

Lines 146 to 154 in 62364a2

	func WithCustomClaims(customClaims map[string]interface{}) ClientOption {
	return withCustomClaims(customClaims)
	}
	type withCustomClaims map[string]interface{}
	func (w withCustomClaims) Apply(o *internal.DialSettings) {
	o.CustomClaims = w
	}

[codyoss]

You are right, but I don't think it should. This works but only because these two packages are in the same repo today, but in the future they may not be. DialSetting was used here out of convenience, but it muddies the waters of what DialSetting should be used for I think.

There are patterns we could adopt to allow any package to declare its own options without needing to touch DialOptions at all. I think we CustomClaims could be re-written in such a manner -- as well as format.

[eisandbar]

Do you have a pattern in mind?

My first thought is have opts be of type interface{} and then type switch each opt

func NewClient(ctx context.Context, audience string, opts ...interface{}) (*http.Client, error) {
	var ds internal.DialSettings
	for _, opt := range opts {
                switch x := opt.(type) {
                case ClientOption: 
                        x.Apply(&ds)
                }
	}

Are there any immediate drawbacks you see with this?