PresentationDefinition fetches the remote PD endpoint twice per request

[stevenvegt]

Summary

HTTPClient.PresentationDefinition issues the same GET request twice. Every presentation definition resolution therefore makes two identical calls to the remote authorization server's PD endpoint.

Where

auth/client/iam/client.go:100-118:

func (hb HTTPClient) PresentationDefinition(ctx context.Context, presentationDefinitionURL url.URL) (*pe.PresentationDefinition, error) {
	// create a GET request with scope query param
	request, err := http.NewRequestWithContext(ctx, http.MethodGet, presentationDefinitionURL.String(), nil)
	if err != nil {
		return nil, err
	}
	var presentationDefinition pe.PresentationDefinition
	err = hb.doRequest(ctx, request, &presentationDefinition)
	if err != nil {
		// any OAuth error should be passed
		// any other error should result in a 502 Bad Gateway
		if errors.As(err, new(oauth.OAuth2Error)) {
			return nil, err
		}
		return nil, errors.Join(ErrBadGateway, err)
	}

	return &presentationDefinition, hb.doRequest(ctx, request, &presentationDefinition)
}

The final line calls doRequest a second time, using it as the returned error expression. The first call has already fetched and unmarshalled the document, and its error has already been handled above.

Because the request is a GET with a nil body it is safely reusable, so both calls succeed and the duplication is invisible in normal operation.

Impact

Two consequences, neither security-relevant.

Outbound load on every peer's presentation definition endpoint is doubled. The call path is PresentationDefinitionResolver.resolveRemote (auth/client/iam/pd_resolver.go:64-79), reached from requestVPTokenAccessToken (auth/client/iam/openid4vp.go:302), so it runs on every RFC021 service access token request that resolves its PD remotely.

The returned error is the second call's error rather than the first's. A transient failure on the redundant request makes the function return a non-nil error alongside a valid *pe.PresentationDefinition. Callers check the error first, so a presentation definition that was fetched successfully gets discarded and the token request fails for no real reason.

Fix

Return the already-populated value:

	return &presentationDefinition, nil

A test asserting that a single call reaches the endpoint once would prevent a recurrence. The existing tests pass with the duplicate because both requests succeed.