Consider supporting polling and io_uring at the same time

[nazar-pc]

Would be great to have something like https://docs.rs/monoio/latest/monoio/struct.FusionDriver.html that can make a decision on whether to use io_uring or polling on Linux depending on availability.

In the app I'm working on I don't have control over what users run it on, so io_uring may or may not be available or configured with proper limits or have modern enough kernel. FusionDriver in monoio helps with that.

[George-Miao]

Do you think a manual config entry (e.g. a builder with with_iouring(bool)) is necessary? Currently all runtime api's are behind thread local, which makes it hard to configure and run like what tokio and monoio does. We might able to only add some feature and tweak the conditional compilation to add runtime detection without changing the public API.

[Berrysoft]

To my opinion, we can make it easier: if both polling and io-uring is enabled, a fusion driver is chosen as the underlying driver, and the fusion driver determines if io-uring should be used.

[George-Miao]

That does make life a whole lot easier.

[nazar-pc]

That is the magical "just works" I would certainly prefer as well