I was using that as an example of a library that uses multiple file descriptors and has its own timeouts. And yes, it is already possible, my issue is that it's just so clunky.
For example, ADNS has two calls to fiddle with your select()/poll() parameters before and after, so if you use something else, you have to hack a bit to know its file descriptors. In particular, it doesn't match well with an API where you register the interests only once, which is the case of every single new API (because it's fundamentally more efficient than starting from scratch every time).
C-Ares cuts on the hacking a small bit, since it gives out the file descriptors it's interested in up-front, rather than having to go through some an array of struct pollfd. But it's still oriented toward a "from scratch every time" API like select()/poll(), so you have to remember what it said last time, and tweak your interest set accordingly. Not my idea of fun and painless, but I've done it, and I've lived through it.
UDNS restricts itself to using a single file descriptor, in an attempt to make this integration easier, but this comes at the cost of not being able to do TCP queries (which are required when a response is too large, which is actually fairly common for MX queries of large sites). So, it arguably crippled itself functionally in order to do what I said, still leaving timeout management to deal with (but that part is easy, at least).
With timerfd, one can use epoll_create() in a library, return that to the application and tell them quite simply "when this fd is readable, call this function here", and that's it. The main application can use select(), poll(), or whatever it feels like using, it doesn't have to deal with anything in its timeout management, it's all reduced to a single bit of information: is this fd readable?
Xlib is also like that (through ConnectionNumber()), which makes it very easy to deal with, but that' a bit easier, since it doesn't have timeouts and really just has the one file descriptor to deal with. Hence my using asynchronous DNS as an example with multiple descriptors (if you don't punt on the TCP queries) and timeouts.
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