Firebase Authentication Link Facebook to Google - ios

Question

After many tests I decided to create a new xCode project to better understand Firebase authentication with multiple providers.
I set up in Firebase -> SignIn Methods -> An account per email address
An account per email address
Prevents users from creating multiple
accounts using the same email address with different authentication
providers
At this point I have implemented, carefully following the Firebase guide, the login with Facebook and with Google .. Everything seems to work perfectly but I always find myself with the same error that I can't manage:
When my user creates a Firebase account via Google he is no longer able to log in if he decides to use Facebook.
Facebook returns its error when it completes its authentication flow with Firebase:
Firebase Error With Facebook Provider: An account already exists with the same email address but different sign-in credentials. Sign in using a provider associated with this email address.
Continuing to follow the documentation step by step I stopped here (firebase explains how to handle this error)
I have also implemented error handling but after calling Auth.auth().fetchSignInMethods Firebase says I should authenticate the user with the existing provider, at this point how do I get the credentials for authentication with the existing provider?
I wouldn't want to reopen the existing provider controller to get new credentials
Am I obliged to ask the user to log in with the existing provider and show another access controller again (in this case that of Google)?
How should I handle this situation?
override func viewDidLoad() {
super.viewDidLoad()
facebookSetup()
}
func facebookSetup() {
let loginButton = FBLoginButton(permissions: [ .publicProfile, .email ])
loginButton.center = view.center
loginButton.delegate = self
view.addSubview(loginButton)
}
//MARK: - FACEBOOK Delegate
func loginButton(_ loginButton: FBLoginButton, didCompleteWith result: LoginManagerLoginResult?, error: Error?) {
if let error = error {
print(error.localizedDescription)
return
}
let credential = FacebookAuthProvider.credential(withAccessToken: AccessToken.current!.tokenString)
Auth.auth().signIn(with: credential) { (authResult, error) in
if let error = error {
print("\n FIREBASE: ",error.localizedDescription)
// An account with the same email already exists.
if (error as NSError?)?.code == AuthErrorCode.accountExistsWithDifferentCredential.rawValue {
// Get pending credential and email of existing account.
let existingAcctEmail = (error as NSError).userInfo[AuthErrorUserInfoEmailKey] as! String
let pendingCred = (error as NSError).userInfo[AuthErrorUserInfoUpdatedCredentialKey] as! AuthCredential
// Lookup existing account identifier by the email.
Auth.auth().fetchSignInMethods(forEmail: existingAcctEmail) { providers, error in
if (providers?.contains(GoogleAuthProviderID))! {
// Sign in with existing account.
Auth.auth().signIn(with: "? ? ? ?") { user, error in
// Successfully signed in.
if user != nil {
// Link pending credential to account.
Auth.auth().currentUser?.link(with: pendingCred) { result, error in
// Link Facebook to Google Account
}
}
}
}
}
}
}
}