runtime: add Handle::try_current

[tomaka]

Motivation

There is currently no way to conditonnally determine whether tokio::spawn will succeed.
Since this is a niche situation, I went for putting a method on the lower-level component.

[LucioFranco]

I wonder since you just need to check if it makes sense to expose a bool over Option<Self>? like Handle::is_current() -> bool

[carllerche]

is_current implies a check for a specific handle, so you would have: Handle::is_current(&self), which wouldn't help here.

I'm ok with this PR. We can always add a boolean. At the ultra bikeshed level, I wonder if try_current should return a Result... but that isn't critical. I also don't think there are error cases.

[hawkw]

I definitely have a slight preference for @carllerche's suggestion that this return a Result...the error type could just format with the message "not currently running on the Tokio runtime.", like the expect in current(). Not a big deal, though, IMO.

[LucioFranco]

👍 on Result over Option same thing for thread_locals https://doc.rust-lang.org/stable/std/thread/struct.LocalKey.html#method.try_with

[carllerche]

It seems like there is consensus that try_current should return a Result. To follow other patterns, the error probably should be named.... TryCurrentError 🤷. It probably can be an empty struct: struct TryCurrentError(()) as there is only one failure case today.

Thanks for getting this going 👍

[tomaka]

Changed to an error

[tomaka]

Any update on that?

[LucioFranco]

Sorry for the deal, LGTM! Thanks.

stale