io: remove Poll from the AsyncSeek::start_seek return value

[zaharidichev]

Motivation

To remove the Poll from start_seek and rely on poll_ready instead

Fix: #1994

Signed-off-by: Zahari Dichev zaharidichev@gmail.com

[carllerche]

Thanks for getting this going 👍 Looking at this, i'm wondering it would make more sense to do something like:

trait AsyncSeek {
    fn start_seek(self: Pin<&mut Self>, position: SeekFrom) -> io::Result<()>;

   fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>>;
}

poll_complete would be the equivalent of Sink::poll_flush. The reason I am thinking this is it doesn't matter if any existing start_seek is in flight before starting another one... you just want to make sure all the seeks are applied.

What do you think?

[Darksonn]

It's a bit dangerous because you have to handle e.g. two seeks of Current(i64::MAX_VALUE), which cannot be implemented as a single seek.

[zaharidichev]

@carllerche @Darksonn Just so I am sure I get this correct. Would that mean that start_seek will simply accumulate seek ops to perform and then we try to batch them and wait for them to all be applied, returning the final result in poll_complete ?

[carllerche]

@Darksonn @zaharidichev How about we split the difference. start_seek returns Err if there is an in-flight seek. To ensure the in-flight is complete, you call poll_complete until Ready to know the operation completed. This way we avoid the problem of handling a queue of seek requests.

In practice, this limits the trait to 1 in-flight seek, but I think that is OK.

[Darksonn]

I think that's good.

[carllerche]

I think this panic message is out of date?

[carllerche]

LGTM thanks 👍 I believe there is an out of date panic message.