#
actix_server/
join_all.rs

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    use std::{
        future::Future,
        pin::Pin,
        task::{Context, Poll},
    };

    use futures_core::future::BoxFuture;

    // a poor man's join future. joined future is only used when starting/stopping the server.
    // pin_project and pinned futures are overkill for this task.
    pub(crate) struct JoinAll<T> {
        fut: Vec<JoinFuture<T>>,
    }

    pub(crate) fn join_all<T>(fut: Vec<impl Future<Output = T> + Send + 'static>) -> JoinAll<T> {
        let fut = fut
            .into_iter()
            .map(|f| JoinFuture::Future(Box::pin(f)))
            .collect();

        JoinAll { fut }
    }

    enum JoinFuture<T> {
        Future(BoxFuture<'static, T>),
        Result(Option<T>),
    }

    impl<T> Unpin for JoinAll<T> {}

    impl<T> Future for JoinAll<T> {
        type Output = Vec<T>;

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

            let this = self.get_mut();
            for fut in this.fut.iter_mut() {
                if let JoinFuture::Future(f) = fut {
                    match f.as_mut().poll(cx) {
                        Poll::Ready(t) => {
                            *fut = JoinFuture::Result(Some(t));
                        }
                        Poll::Pending => ready = false,
                    }
                }
            }

            if ready {
                let mut res = Vec::new();
                for fut in this.fut.iter_mut() {
                    if let JoinFuture::Result(f) = fut {
                        res.push(f.take().unwrap());
                    }
                }

                Poll::Ready(res)
            } else {
                Poll::Pending
            }
        }
    }

    #[cfg(test)]
    mod test {
        use actix_utils::future::ready;

        use super::*;

        #[actix_rt::test]
        async fn test_join_all() {
            let futs = vec![ready(Ok(1)), ready(Err(3)), ready(Ok(9))];
            let mut res = join_all(futs).await.into_iter();
            assert_eq!(Ok(1), res.next().unwrap());
            assert_eq!(Err(3), res.next().unwrap());
            assert_eq!(Ok(9), res.next().unwrap());
        }
    }