Update async.md

[ibeckermayer]

Fixes an edge case bug in the Delay implementation wherein the waker could be awoken after having returned Poll::Ready if the Delay was short enough (e.g. 0).

[Darksonn]

There's a compilation error:

---- target/debug/build/doc-test-8c33abc07f170500/out/doctests.rs - tokio::tutorial::async_md (line 685) stdout ----
error[E0053]: method `poll` has an incompatible type for trait
  --> target/debug/build/doc-test-8c33abc07f170500/out/doctests.rs:702:64
   |
19 |     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<&'static str> {
   |                                                                ^^^^^^^^^^^^^^^^^^
   |                                                                |
   |                                                                expected `()`, found `&'static str`
   |                                                                help: change the output type to match the trait: `Poll<()>`
   |
   = note: expected signature `fn(Pin<&mut Delay>, &mut Context<'_>) -> Poll<()>`
              found signature `fn(Pin<&mut Delay>, &mut Context<'_>) -> Poll<&'static str>`

error[E0308]: mismatched types
  --> target/debug/build/doc-test-8c33abc07f170500/out/doctests.rs:706:32
   |
23 |             return Poll::Ready(());
   |                    ----------- ^^ expected `&str`, found `()`
   |                    |
   |                    arguments to this enum variant are incorrect
   |
help: the type constructed contains `()` due to the type of the argument passed
  --> target/debug/build/doc-test-8c33abc07f170500/out/doctests.rs:706:20
   |
23 |             return Poll::Ready(());
   |                    ^^^^^^^^^^^^--^
   |                                |
   |                                this argument influences the type of `Poll`
note: tuple variant defined here
  --> /rustc/a28077b28a02b92985b3a3faecf92813[155](https://github.com/tokio-rs/website/actions/runs/7178385919/job/19558862391?pr=739#step:5:156)f1ea1/library/core/src/task/poll.rs:17:5

error: aborting due to 2 previous errors

[ibeckermayer]

There's a compilation error:

Oops my bad, see the latest commit.

Waking the waker after returning Poll::Ready is not actually incorrect. Spurious wakeups are always allowed.

Interesting. In that case there may technically be a bug in the Mini Tokio implementation itself, at least the one I pieced together from the tutorial (which is admittedly substantively different from the full codebase the tutorial links to).

When I run it with Delay set to 0, I get

$ RUST_BACKTRACE=1 cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/mini_tokio`
Running mini_tokio...
delay 0 started!
delay 0 completed; out = "done"
thread 'main' panicked at src/main.rs:9:22:
`async fn` resumed after completion
stack backtrace:
   0: rust_begin_unwind
             at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/std/src/panicking.rs:595:5
   1: core::panicking::panic_fmt
             at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/panicking.rs:67:14
   2: core::panicking::panic
             at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/panicking.rs:117:5
   3: mini_tokio::main::{{closure}}
             at ./src/main.rs:9:22
   4: mini_tokio::Task::poll
             at ./src/lib.rs:125:17
   5: mini_tokio::MiniTokio::run
             at ./src/lib.rs:157:13
   6: mini_tokio::main
             at ./src/main.rs:21:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

(playground here, repo here)

[Darksonn]

Ah, yes, that's very possible.