Support for generic OIDC

[ktkk]

We use Firebase auth in conjunction with our own OIDC backend and this works somewhat ok with the firebase_auth package on Android and iOS. We are looking into this package now because our requirements changed and we need a Windows application for our users too.

I've tried implementing this myself by extending from ProviderArgs, but I can't get it to work.

class OAuthSignInArgs extends ProviderArgs {
  OAuthSignInArgs({
    required this.host,
    required this.path,
    required this.redirectUri,
    required this.clientId,
    this.scopes = const [],
    this.responseType = 'code',
    String? nonce,
  }) : nonce = nonce ?? generateNonce();

  @override
  final String host;

  @override
  final String path;

  @override
  final String redirectUri;

  final String clientId;
  final List<String> scopes;
  final String responseType;
  final String nonce;

  @override
  Map<String, String> buildQueryParameters() => {
        'response_type': responseType,
        'client_id': clientId,
        'scope': scopes.join(' '),
        'redirect_uri': redirectUri,
        'nonce': sha256.convert(utf8.encode(nonce)).toString(),
      };
}

I'd love to know what I'm doing wrong here. The callback url I get back from Firebase directs me to a page saying the following:

Unable to process request due to missing initial state. This may happen if browser sessionStorage is inaccessible or accidentally cleared. Some specific scenarios are - 1) Using IDP-Initiated SAML SSO. 2) Using signInWithRedirect in a storage-partitioned browser environment.

Any feedback would be greatly appreciated!