Method first_poll

crates/wasmtime/src/runtime/component/concurrent.rs:2995–3112

Poll the specified future once on behalf of a guest->host call using an async-lowered import. If it returns `Ready`, return `Ok(None)`. Otherwise, if it returns `Pending`, add it to the set of futures to be polled as part of this instance's event loop until it completes, and then return `Ok(Some(handle))` where `handle` is the waitable handle to return. Whether the future returns `Ready` immedi

(
        self,
        mut store: StoreContextMut<'_, T>,
        future: impl Future<Output = Result<R>> + Send + 'static,
        lower: impl FnOnce(StoreContextMut<T>, Option<R>) -> Result<()> + S

    /// stack and linear memory. The `lower` function is invoked with `None` if
    /// the future is cancelled.
    pub(crate) fn first_poll<T: 'static, R: Send + 'static>(
        self,
        mut store: StoreContextMut<'_, T>,
        future: impl Future<Output = Result<R>> + Send + 'static,
        lower: impl FnOnce(StoreContextMut<T>, Option<R>) -> Result<()> + Send + 'static,
    ) -> Result<Option<u32>> {
        let token = StoreToken::new(store.as_context_mut());
        let state = store.0.concurrent_state_mut();
        let task = state.current_host_thread()?;

        // Create an abortable future which hooks calls to poll and manages call
        // context state for the future.
        let (join_handle, future) = JoinHandle::run(future);
        {
            let state = &mut state.get_mut(task)?.state;
            assert!(matches!(state, HostTaskState::CalleeStarted));
            *state = HostTaskState::CalleeRunning(join_handle);
        }

        let mut future = Box::pin(future);

        // Finally, poll the future.  We can use a dummy `Waker` here because
        // we'll add the future to `ConcurrentState::futures` and poll it
        // automatically from the event loop if it doesn't complete immediately
        // here.
        let poll = tls::set(store.0, || {
            future
                .as_mut()
                .poll(&mut Context::from_waker(&Waker::noop()))
        });

        match poll {
            // It finished immediately; lower the result and delete the task.
            Poll::Ready(result) => {
                let result = result.transpose()?;
                lower(store.as_context_mut(), result)?;
                return Ok(None);
            }

            // Future isn't ready yet, so fall through.
            Poll::Pending => {}
        }

        // It hasn't finished yet; add the future to
        // `ConcurrentState::futures` so it will be polled by the event
        // loop and allocate a waitable handle to return to the guest.

        // Wrap the future in a closure responsible for lowering the result into
        // the guest's stack and memory, as well as notifying any waiters that
        // the task returned.
        let future = Box::pin(async move {
            let result = match future.await {
                Some(result) => Some(result?),
                None => None,
            };
            let on_complete = move |store: &mut dyn VMStore| {
                // Restore the `current_thread` to be the host so `lower` knows
                // how to manipulate borrows and knows which scope of borrows