make wait_readable and wait_writable public

[stakach]

restoring the changes made in #7366
Change was originally made in this release: https://github.com/crystal-lang/crystal/blob/3aa1a04ca666dcf12a625b58deccbf0f209e1e4d/CHANGELOG.md#0280-2019-04-17

[RX14]

reluctantly approved

[stakach]

Thanks @RX14
I feel that access to functions like this are what make a language powerful.

Agree that they shouldn't be used day to day but useful when doing something a little more complex that lies outside of the typical.

I'd put this in a similar category to the uninitialized keyword

[straight-shoota]

@ysbaddaden Is there any particular reason this was reverted in #7505?

[ysbaddaden]

Because it's internal API detail, and there aren't any need nor incentive for them to be publicly available, and nobody complained in over a year. Be aware that using this will break, most likely when adding support for Windows, or moving from libevent to a custom solution.

I'd love to see a real use-case. If it's just a "feel that it should be accessible", then let's close this PR.

[RX14]

@ysbaddaden the real usecase is binding C libraries: https://github.com/spider-gazelle/ssh2.cr/blob/a93eb957420b9e9f31bb8cd46e50ff862c5bc54b/src/session.cr#L39-L49

libgit2 returns EAGAIN from function calls when it needs to read more data, just like read and write, then expects the caller to handle them like read and write. So it needs this API.

I suspect when we abstract the event loop, we will have Crystal::System::EventLoop.read(fd) which will call Crystal::System::EventLoop.wait_readable(fd) as part of it's implementation. The latter will not exist on Windows. We could make the latter public (though undocumented and unstable) instead of private to support this kind of implementation on unix at least.

Or, we could force users to use threads, which i'm not super against anyway.

[ysbaddaden]

@RX14 ok.

[bcardiff]

The refactors in #7505 is where the changes in #7366 got dropped. (this is maybe a note for future self to write the changelog 🙈 )