Make entry point synchronous and switch to global runtime instead.

[ThomasFrans]

An example to illustrate what I mean in issue #993. This isn't meant to be merged (therefore a draft PR), but more as a way to show the proposed changes in the issue and potentially get input on this approach. Not a lot would have to change. From what I understand about async, this wouldn't have negative performance implications, but I might be wrong about this. I tested this and it works like before.

[hrkfdn]

So I did some more code (re-)reading and it looks like it makes sense to proceed with this one, as there's barely any async usage starting from main (as you also pointed out). We can still revert if that changes. Do you agree or do you want to make more adjustments (as this is still a draft PR)?

[ThomasFrans]

I think I noticed two more futures::block_on that I missed as this was just a draft. They also need to be switched. I'll quickly do that, or you can change them. Then I think it's good to merge.

[ThomasFrans]

There are still two futures::future::pending() calls, I think somewhere in the spotify_worker.rs file. I'm still not too familiar with the difference between futures::* stuff and just the Tokio stuff (like futures::executor vs tokio::runtime). I think it's ok, as I tried without credentials which worked fine, and the pending happens in a bit of code that is responsible for fetching the token, so I think it should work.

[hrkfdn]

Those should be fine. futures::pending() just returns a never ending async task that is non-blocking (i.e. doesn't block a thread using sleep or similar). These are used as a placeholder for when there is no token task instead of an Option. Thanks for the adjustments, hope I'll be able to review and merge this later today.