Wasm32-wasi target support

[rjzak]

I'm working on getting Tokio working on Rust, and some changes were needed for Mio.

Companion to tokio-rs/tokio#4716

Signed-off-by: Richard Zak richard@profian.com

[Thomasdezeeuw]

@rjzak we're not going to accept this as is. Poll needs to have mutable access on Windows as it can only handle a single poll call at a time. Furthermore we don't want to force everyone to use an Arc.

Could you please explain why you need this change? "Wasm improvements" are not really clear.

[Thomasdezeeuw]

We're not going to accept a different public API based on the compile target.

[rjzak]

It's a work-around for try_clone() not being available to the wasi registry here https://github.com/tokio-rs/tokio/blob/2d90ef5861029a9cfc8e89c05ddc65ed754bb77b/tokio/src/io/driver/mod.rs#L133 requires https://github.com/tokio-rs/mio/blob/master/src/poll.rs#L638, but try_clone() isn't working on the underlying object with Wasi, that's a compiler error.

[rjzak]

@Thomasdezeeuw I certainly wouldn't want this PR accepted as-is, as the test application here https://github.com/rjzak/tokio-echo-test compiles to Wasm but doesn't work. I thought that by sharing the code I have so far, I could receive some feedback and pointers.

[Thomasdezeeuw]

Ping @rjzak.

[rjzak]

@Thomasdezeeuw I'm still working on it!

[rjzak]

@Thomasdezeeuw I have Axum & Tokio working on WebAssembly.
Still baffled at the moment with that to do with Arc<Registry>. This is where it's used in Tokio: https://github.com/rjzak/tokio/blob/wasi_wip/tokio/src/io/driver/mod.rs#L145. I could use some assistance.

[Thomasdezeeuw]

@Thomasdezeeuw I have Axum & Tokio working on WebAssembly. Still baffled at the moment with that to do with Arc<Registry>. This is where it's used in Tokio: https://github.com/rjzak/tokio/blob/wasi_wip/tokio/src/io/driver/mod.rs#L145. I could use some assistance.

Maybe you can put the Selector in an Arc?

[rjzak]

@Thomasdezeeuw @carllerche I moved the Arc<> to Registry::selector, and all seems to be okay, except for the tests on Windows. On Windows, the Poll::poll() function is failing with an error stating that a mutable reference is needed for Arc<Selector>, however, the function Selector::select() doesn't take a mutable reference to itself, so this shouldn't be a problem, unless I'm misunderstanding the error.

Line in question:

mio/src/poll.rs

Line 378 in 6312041

self.registry.selector.select(events.sys(), timeout)

Error:

error[E0596]: cannot borrow data in an `Arc` as mutable
   --> src\poll.rs:378:9
    |
378 |         self.registry.selector.select(events.sys(), timeout)
    |         ^^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable
    |
    = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `std::sync::Arc<sys::windows::selector::Selector>`

Otherwise, formatting issues and the previously API difference between platforms has been resolved.

[Thomasdezeeuw]

Two small nits, LGTM otherwise.

[Thomasdezeeuw]

Thanks @rjzak.

[rjzak]

Thank you!