Firebase Login and Login with Apple not linking to same user account - firebase

Question

Using SwiftUI, Xcode12.5.1, Swift5.4.2, iOS14.7.1,
My Firebase-Login page shall be extended with other Login possibilities such as Apple-Login (eventually Google-login, Facebook-login etc).
I have an implementation of Firebase-Login that works well.
I extended the LoginView with the Sign in with Apple Button.
And this new Apple Login in its basic implementation also works.
Now the problem:
If I log in with Apple, I need to access the corresponding Firebase-user in order to query the correct user-data. Right now, login in with Apple works but the retrieved data is not the user-data of the corresponding Firebase-user.
What I want to achieve:
From a logout-state, I want to
a) Being able to log in with Firebase Email/Password and sometimes later want to log-out and log in again with Apple.
--> and for both cases, I would like to get the same user-data
b) Being able to log in with Apple and sometimes later want to log-out and log in again with Firebase Email/Password
--> and for both cases, I would like to get the same user-data
--- THE IDEA ----------
I learned from the Firebase documentation that there is a way to link two login-accounts that we are able to know that these two accounts are corresponding.
--- THE IMPLEMENTATION -----------
Below is my current implementation for the Apple login:
I learned that you can get userInformation of the corresponding other account in the error of the link-callback. But in my case, I get the wrong linkError:
My linkError:
The email address is already in use by another account.
Instead of:
AuthErrorCode.credentialAlreadyInUse
For me this doesn't make sense. Especially since I know that I already did log in before with Firebase-Email/Password. Then I logged out and now I tried to log in with Apple.
Shouldn't the link method recognise that I am allowed to have been logged in via Firebase-Email/Password before and shouldn't it be ok to have that email being used before ?? I don't understand this linkError.
Questions:
In the link-callback, why do I get the linkError The email address is already in use by another account. instead of AuthErrorCode.credentialAlreadyInUse ??
What do I need to change in order to make a) work ??
How does the implementation look for the b) workflow (i.e. if user logs in to Apple, then logs-out and logs in again with Firebase-Email/Password ??). How do I link the two accounts then ??
Here my code:
switch state {
case .signIn:
Auth.auth().signIn(with: credential) { (authResult, error) in
if let error = error {
print("Error authenticating: \(error.localizedDescription)")
return
}
do {
if let email = try THKeychain.getEmail(),
let password = try THKeychain.getPassword() {
let credential = EmailAuthProvider.credential(withEmail: email, password: password)
if let user = authResult?.user {
user.link(with: credential) { (result, linkError) in
if let linkError = linkError, (linkError as NSError).code == AuthErrorCode.credentialAlreadyInUse.rawValue {
print("The user you're signing in with has already been linked, signing in to the new user and migrating the anonymous users [\(user.uid)] tasks.")
if let updatedCredential = (linkError as NSError).userInfo[AuthErrorUserInfoUpdatedCredentialKey] as? OAuthCredential {
print("Signing in using the updated credentials")
Auth.auth().signIn(with: updatedCredential) { (result, error) in
if let user = result?.user {
// eventually do a data-migration
user.getIDToken { (token, error) in
if let _ = token {
// do data migration here with the token....
self.doSignIn(appleIDCredential: appleIDCredential, user: user)
}
}
}
}
}
}
else if let linkError = linkError {
// I END UP HERE !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
// WHY WHY WHY WHY WHY WHY WHY WHY ????????????????????????
print("Error trying to link user: \(linkError.localizedDescription)")
}
else {
if let user = result?.user {
self.doSignIn(appleIDCredential: appleIDCredential, user: user)
}
}
}
}
}
} catch {
print(error.localizedDescription)
}
if let user = authResult?.user {
if let onSignedInHandler = self.onSignedInHandler {
onSignedInHandler(user)
}
}
}
case .link:
// t.b.d.
case .reauth:
// t.b.d.
}