User identifier attribute option as single value is inadequate with heterogenous IdPs scenarios

[cicnavi]

Config option for designating user identifier

simplesamlphp-module-oidc/config/module_oidc.php.dist

Line 79 in 0080cf2

ModuleConfig::OPTION_AUTH_USER_IDENTIFIER_ATTRIBUTE => 'uid',

is a single value. This means that it is expected that this attribute is always available in the set of user attributes after user authentication.

In scenarios with multiple different IdPs, like for example in eduGAIN inter-federation, not every IdP will be able (or willing) to provide designated identifier. For this reason, this config option (and all relevant code that uses it) should be refactored in a way to be able to accept multiple prioritized values (array of prioritized values), or be left as single value.