io: make copy cooperative

[Rustin170506]

Motivation

close #6250

Solution

Add poll_proceed_and_make_progress to check if it exceeds the budget.

[Rustin170506]

🔢 Self-check

[Darksonn]

You should only make progress if it actually copied data.

See the mpsc channel for an example:

tokio/tokio/src/sync/mpsc/chan.rs

Lines 250 to 251 in 7c606ab

	// Keep track of task budget
	let coop = ready!(crate::runtime::coop::poll_proceed(cx));

tokio/tokio/src/sync/mpsc/chan.rs

Lines 259 to 263 in 7c606ab

	Some(Read::Value(value)) => {
	self.inner.semaphore.add_permit();
	coop.made_progress();
	return Ready(Some(value));
	}

[Rustin170506]

Thank you for your review.
How about now? After filling, writing, and flushing data, I called make_progress.

[Darksonn]

Like in the examples I linked, you need to call crate::runtime::coop::poll_proceed at the start of the function, and then call coop.made_progress() in the places where you are currently calling poll_proceed_and_make_progress.

It's not enough to just call it when it exits. The poll_proceed_and_make_progress function can only be used for functions that never return Poll::Pending.

[Rustin170506]

Like in the examples I linked, you need to call crate::runtime::coop::poll_proceed at the start of the function, and then call coop.made_progress()

But the crate::runtime::coop::poll_proceed only exists if we enable the coop feature. What should we do if we don't have the coop feature?

[Darksonn]

Then do nothing.

[Rustin170506]

Updated. I just used #[cfg(any( feature = "fs", feature = "io-std", feature = "net", feature = "process", feature = "rt", feature = "signal", feature = "sync", feature = "time", ))] to control it. I am not sure if we have a better way to do it. Thanks for your help. Sorry for those stupid questions. 😢

[Darksonn]

I believe we have a macro for doing that. You should be able to write

cfg_coop! {
    let coop = ready!(crate::runtime::coop::poll_proceed(cx));
}

And similar for the calls to make_progress. The macro does not actually introduce a scope, so I believe this should work.

Your questions are not stupid :). You're asking questions about internal Tokio APIs that don't exist elsewhere. It's no surprise that you are not familiar with them. If we did not have the cfg_coop! macro, then what you did would be the only way to do it.

[Rustin170506]

Thanks for your reply.

Actually, I tried it. But it seemed this macro only supports starting with an item keyword.

Do you think I should extend it to support it? (I will try, but I am unsure if it is doable).

[Darksonn]

Thank you. If the cfg_coop! macro can't do this today, then this is good enough.

[Rustin170506]

Thanks for your review! 💚 💙 💜 💛 ❤️