Add AllUsers and AllGroups

[OLEGSHA]

AllUsers and AllGroups are traits that provide all_users() and all_groups() similar to how Users and Groups provide other library functions. Also included is UsersSnapshot, a pre-loaded version of UsersCache that can iterate over its contents safely. MockUsers now also implements AllUsers and AllGroups.

Resolves #10.

Example usage

fn old_consumer<U: Users>(u: &U) {
    println!("{:?}", u.get_current_username());
}

fn new_consumer<U: Users + AllUsers>(u: &U) {
    for user in u.get_all_users() { // This call is safe
        println!("{user:?}");
    }
    old_consumer(u);
}

fn main() {
    // The unsafe `all_users` call happens here
    let snapshot = unsafe { UsersSnapshot::new() };
    new_consumer(&snapshot);

    let mut mock = MockUsers::with_current_uid(1234);
    mock.add_user(User::new(1234, "tom", 1234));
    new_consumer(&mock);
}

---

Checklist:

• AllUsers
• AllGroups
• UsersSnapshot
• Implement new traits in MockUsers
• Update docs
• Add tests

[OLEGSHA]

I have stopped short of writing the groups code and updating docs because I'm not entirely sure this is the correct approach.

An alternative implementation exists in make-iters-mockable, where AllUsers includes Arc-less versions of all Users methods (get_user_by_uid -> Option<Arc<User>> becomes user_by_uid -> Option<&User>). Using AllUsers then reduces dynamic memory allocations significantly. However, this is not backwards-compatible with Users; attempting to implement Users in UsersSnapshot results in memory allocations in getters, which is definitely bad.

So I decided to design UsersSnapshot around the Users trait instead. This means Arcs are stored in the hashmap for no real reason.

Finally, I don't really like the name collision with the private return type of all_users() function.

Please let me know what you think, and I'll finish the PR then.

[gierens]

Please let me know what you think, and I'll finish the PR then.

Sorry, for the late response, have been busy lately and I knew this would take a moment to read through properly ... especially with the two options.

Anyway, this look very promising to me in general, thank you!

I have stopped short of writing the groups code and updating docs because I'm not entirely sure this is the correct approach.

An alternative implementation exists in make-iters-mockable

Hm, comparing both I gotta say the "dryer" version here looks a lot cleaner to me or much easier to understand at the very least. I'm not sure the potential performance benefits of the other version are worth the over-complication it introduces in the AllUsers trait. So I would continue with the version here.

[OLEGSHA]

UsersCache::with_all_users() becomes confusing and largely unnecessary with this PR. A GitHub-wide search shows there is barely any use of this method, certainly none where the transition to UsersSnapshot should be bad. I think it's safe to deprecate it.

[gierens]

I'm very sorry it took me so long to get back to you! ... Again, thanks! LGTM! ... I added a deprecation warning to UsersCache::with_all_users() for the upcoming minor version and reworded one of the commit messages a little. With that it should be ready for merge now!