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//! Future types for the `Batch` middleware.

use super::{error::Closed, message};
use futures_core::ready;
use pin_project::pin_project;
use std::{
    future::Future,
    pin::Pin,
    task::{Context, Poll},
};

/// Future that completes when the batch processing is complete.
#[pin_project]
#[derive(Debug)]
pub struct ResponseFuture<T> {
    #[pin]
    state: ResponseState<T>,
}

#[pin_project(project = ResponseStateProj)]
#[derive(Debug)]
enum ResponseState<T> {
    Failed(Option<crate::BoxError>),
    Rx(#[pin] message::Rx<T>),
    Poll(#[pin] T),
}

impl<T> ResponseFuture<T> {
    pub(crate) fn new(rx: message::Rx<T>) -> Self {
        ResponseFuture {
            state: ResponseState::Rx(rx),
        }
    }

    pub(crate) fn failed(err: crate::BoxError) -> Self {
        ResponseFuture {
            state: ResponseState::Failed(Some(err)),
        }
    }
}

impl<F, T, E> Future for ResponseFuture<F>
where
    F: Future<Output = Result<T, E>>,
    E: Into<crate::BoxError>,
{
    type Output = Result<T, crate::BoxError>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let mut this = self.project();

        // CORRECTNESS
        //
        // The current task must be scheduled for wakeup every time we return
        // `Poll::Pending`.
        //
        // This loop ensures that the task is scheduled as required, because it
        // only returns Pending when another future returns Pending.
        loop {
            match this.state.as_mut().project() {
                ResponseStateProj::Failed(e) => {
                    return Poll::Ready(Err(e.take().expect("polled after error")));
                }
                ResponseStateProj::Rx(rx) => match ready!(rx.poll(cx)) {
                    Ok(Ok(f)) => this.state.set(ResponseState::Poll(f)),
                    Ok(Err(e)) => return Poll::Ready(Err(e.into())),
                    Err(_) => return Poll::Ready(Err(Closed::new().into())),
                },
                ResponseStateProj::Poll(fut) => return fut.poll(cx).map_err(Into::into),
            }
        }
    }
}