I have a suggestion for improving org.apache.karaf.jaas.modules.BackingEngine
Currently, the only way to lookup an individual user (as far as I can tell) is to iterate through the result of BackingEngine#listUsers() and find the required user. This is fine if the number of users is reasonably low. However, when JAAS infrastructure is used for public systems (say, a SaaS), it quickly becomes a bottleneck.
I suggest adding something like `public UserPrincipal lookupUser(String name)` to avoid this problem (also, it should return `null` if no such user found , or throw an exception)
I am happy to contribute a PR promptly if this change will be approved.