base/registry: introduce empty() const method

[skalk]

For several times I tumpled over the Genode::Registry having no method to check whether it is empty or not. The typical workaround is to use do something like:

bool empty = true;
my_registry.for_each([&] (auto&) { empty = false; });

This is not nice to read, nor efficient. Of course, I can derive from Registry and implement empty, but here again I can only iterate over all items to check for it, because the internally used List is completely hidden for derivates too.
Therefore, I would like to add something like the following to base/include/base/registry.h to the Genode::Registry class:

bool empty() const
{
    return !_elements.first();
}

@nfeske do you agree?

[nfeske]

I agree that an empty method would be handy to have in some situations.

The registry does not have such a probing method as a precaution against potential race conditions. The current interface is safe in this respect. The user has to opt-out of the safety by manually doing the for_each. In contrast, your suggestion of empty is not thread safe, even if it protected the access via a mutex. (time of check, time of use)

Back when the Registry was designed, thread safety was one of its most important aspects. In the meantime, multi-threaded components have become a scarcity. In most cases where the registry is used today, the inter-thread synchronization is actually unneeded. The later introduced Id_space does not even address thread safety.

In short, I'm divided. The most natural direction would be to have two Registry-like data structures, one that's thread safe (e.g., named Synced_registry with the current interface), and one that's not thread-safe (much simpler, less overhead, featuring niceties like empty).