private async Task <(IdentityUser user, string provider, string providerUserId, IEnumerable <Claim> claims)> FindUserFromExternalProviderAsync(AuthenticateResult result)
{
var externalUser = result.Principal;
// try to determine the unique id of the external user (issued by the provider)
// the most common claim type for that are the sub claim and the NameIdentifier
// depending on the external provider, some other claim type might be used
var userIdClaim = externalUser.FindFirst("sub") ??
externalUser.FindFirst(ClaimTypes.NameIdentifier) ??
throw new Exception("Unknown userid");
// remove the user id claim so we don't include it as an extra claim if/when we provision the user
var claims = externalUser.Claims.ToList();
claims.Remove(userIdClaim);
var provider = result.Properties.Items["scheme"];
var providerUserId = userIdClaim.Value;
// find external user
var user = await _userManager.FindByLoginAsync(provider, providerUserId);
return(user, provider, providerUserId, claims);
}