Why doesn't time::timeout work with io::copy?

[Jimmy-Z]

The following code will run indefinitely instead of stopping after 1s, but why?

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1418b22bb3afc5a36a368f80cbcefc40

the document of timeout says:

if the future does not yield during execution then it is possible for the future to complete and exceed the timeout without returning an error.

I'm not sure what exactly does "yield" mean here, but I'm quite sure copy is not blocking.

[Jimmy-Z]

A little more test shows if I use tokio::fs::File::open("/dev/zero") instead of repeat, it will stop, I'm guessing this is because Repeat::poll_read() always returns Ready, never Pending.

https://docs.rs/tokio/latest/src/tokio/io/util/repeat.rs.html#51-61

I'd appreciate if someone could confirm my guessing though.

[Darksonn]

It's because neither repeat nor sink ever yield, and copy has no protection against that case. I've opened #6249 and #6250 to track this issue.

[Jimmy-Z]

https://tokio.rs/tokio/glossary#yielding

a future yields whenever it returns from the poll method.

I think the document need to clarify on the term "yield", clearly that glossary entry doesn't apply here.

[Darksonn]

It's accurate for futures, but repeat and sink are not futures.

(But updating it would be still be reasonable, of course.)

[Jimmy-Z]

Well I mean the difference between

returns from the poll method

and

returns Pending from the poll method