Method poll

crates/wasi/src/cli/locked_async.rs:261–323

(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        op: impl FnOnce(&mut S) -> p2::StreamResult<T>,
    )

    S: SharedHandleReady,
{
    fn poll<T>(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        op: impl FnOnce(&mut S) -> p2::StreamResult<T>,
    ) -> Poll<Option<io::Result<T>>> {
        // If we don't currently have the lock on this handle, initiate the
        // lock acquisition.
        if let StdioHandle::Ready(lock) = &*self {
            self.set(StdioHandle::Locking(Box::new(lock.clone().lock_owned())));
        }

        // If we're in the process of locking this handle, wait for that to
        // finish.
        if let Some(lock) = self.as_mut().as_locking() {
            let guard = ready!(lock.poll(cx));
            self.set(StdioHandle::Locked(guard));
        }

        let mut guard = match self.as_mut().take_guard() {
            Some(guard) => guard,
            // If the guard can't be acquired that means that this stream is
            // closed, so return that we're ready without filling in data.
            None => return Poll::Ready(None),
        };

        // Wait for our locked stream to be ready, resetting to the "locked"
        // state if it's not quite ready yet.
        match guard.poll_ready(cx) {
            Poll::Ready(()) => {}

            // If the read isn't ready yet then restore our "locked" state
            // since we haven't finished, then return pending.
            Poll::Pending => {
                self.set(StdioHandle::Locked(guard));
                return Poll::Pending;
            }
        }

        // Perform the I/O and delegate on the result.
        match op(&mut guard) {
            // The I/O succeeded so relinquish the lock on this stream by
            // transitioning back to the "Ready" state.
            Ok(result) => {
                self.set(StdioHandle::Ready(OwnedMutexGuard::mutex(&guard).clone()));
                Poll::Ready(Some(Ok(result)))
            }

            // The stream is closed, and `take_guard` above already set the
            // closed state, so return nothing indicating the closure.
            Err(p2::StreamError::Closed) => Poll::Ready(None),

            // The stream failed so propagate the error. Errors should only
            // come from the underlying I/O object and thus should cast
            // successfully. Additionally `take_guard` replaced our state
            // with "closed" above which is the desired state at this point.
            Err(p2::StreamError::LastOperationFailed(e)) => {
                Poll::Ready(Some(Err(e.downcast().unwrap())))
            }