# reusable_box.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.4.0/src/signal/reusable_box.rs`.
Keywords: rust, rustlang, rust-lang

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    use std::alloc::Layout;
    use std::future::Future;
    use std::panic::AssertUnwindSafe;
    use std::pin::Pin;
    use std::ptr::{self, NonNull};
    use std::task::{Context, Poll};
    use std::{fmt, panic};

    /// A reusable `Pin<Box<dyn Future<Output = T> + Send>>`.
    ///
    /// This type lets you replace the future stored in the box without
    /// reallocating when the size and alignment permits this.
    pub(crate) struct ReusableBoxFuture<T> {
        boxed: NonNull<dyn Future<Output = T> + Send>,
    }

    impl<T> ReusableBoxFuture<T> {
        /// Create a new `ReusableBoxFuture<T>` containing the provided future.
        pub(crate) fn new<F>(future: F) -> Self
        where
            F: Future<Output = T> + Send + 'static,
        {
            let boxed: Box<dyn Future<Output = T> + Send> = Box::new(future);

            let boxed = Box::into_raw(boxed);

            // SAFETY: Box::into_raw does not return null pointers.
            let boxed = unsafe { NonNull::new_unchecked(boxed) };

            Self { boxed }
        }

        /// Replace the future currently stored in this box.
        ///
        /// This reallocates if and only if the layout of the provided future is
        /// different from the layout of the currently stored future.
        pub(crate) fn set<F>(&mut self, future: F)
        where
            F: Future<Output = T> + Send + 'static,
        {
            if let Err(future) = self.try_set(future) {
                *self = Self::new(future);
            }
        }

        /// Replace the future currently stored in this box.
        ///
        /// This function never reallocates, but returns an error if the provided
        /// future has a different size or alignment from the currently stored
        /// future.
        pub(crate) fn try_set<F>(&mut self, future: F) -> Result<(), F>
        where
            F: Future<Output = T> + Send + 'static,
        {
            // SAFETY: The pointer is not dangling.
            let self_layout = {
                let dyn_future: &(dyn Future<Output = T> + Send) = unsafe { self.boxed.as_ref() };
                Layout::for_value(dyn_future)
            };

            if Layout::new::<F>() == self_layout {
                // SAFETY: We just checked that the layout of F is correct.
                unsafe {
                    self.set_same_layout(future);
                }

                Ok(())
            } else {
                Err(future)
            }
        }

        /// Set the current future.
        ///
        /// # Safety
        ///
        /// This function requires that the layout of the provided future is the
        /// same as `self.layout`.
        unsafe fn set_same_layout<F>(&mut self, future: F)
        where
            F: Future<Output = T> + Send + 'static,
        {
            // Drop the existing future, catching any panics.
            let result = panic::catch_unwind(AssertUnwindSafe(|| {
                ptr::drop_in_place(self.boxed.as_ptr());
            }));

            // Overwrite the future behind the pointer. This is safe because the
            // allocation was allocated with the same size and alignment as the type F.
            let self_ptr: *mut F = self.boxed.as_ptr() as *mut F;
            ptr::write(self_ptr, future);

            // Update the vtable of self.boxed. The pointer is not null because we
            // just got it from self.boxed, which is not null.
            self.boxed = NonNull::new_unchecked(self_ptr);

            // If the old future's destructor panicked, resume unwinding.
            match result {
                Ok(()) => {}
                Err(payload) => {
                    panic::resume_unwind(payload);
                }
            }
        }

        /// Get a pinned reference to the underlying future.
        pub(crate) fn get_pin(&mut self) -> Pin<&mut (dyn Future<Output = T> + Send)> {
            // SAFETY: The user of this box cannot move the box, and we do not move it
            // either.
            unsafe { Pin::new_unchecked(self.boxed.as_mut()) }
        }

        /// Poll the future stored inside this box.
        pub(crate) fn poll(&mut self, cx: &mut Context<'_>) -> Poll<T> {
            self.get_pin().poll(cx)
        }
    }

    impl<T> Future for ReusableBoxFuture<T> {
        type Output = T;

        /// Poll the future stored inside this box.
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
            Pin::into_inner(self).get_pin().poll(cx)
        }
    }

    // The future stored inside ReusableBoxFuture<T> must be Send.
    unsafe impl<T> Send for ReusableBoxFuture<T> {}

    // The only method called on self.boxed is poll, which takes &mut self, so this
    // struct being Sync does not permit any invalid access to the Future, even if
    // the future is not Sync.
    unsafe impl<T> Sync for ReusableBoxFuture<T> {}

    // Just like a Pin<Box<dyn Future>> is always Unpin, so is this type.
    impl<T> Unpin for ReusableBoxFuture<T> {}

    impl<T> Drop for ReusableBoxFuture<T> {
        fn drop(&mut self) {
            unsafe {
                drop(Box::from_raw(self.boxed.as_ptr()));
            }
        }
    }

    impl<T> fmt::Debug for ReusableBoxFuture<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("ReusableBoxFuture").finish()
        }
    }

    #[cfg(test)]
    mod test {
        use super::ReusableBoxFuture;
        use futures::future::FutureExt;
        use std::alloc::Layout;
        use std::future::Future;
        use std::pin::Pin;
        use std::task::{Context, Poll};

        #[test]
        fn test_different_futures() {
            let fut = async move { 10 };
            // Not zero sized!
            assert_eq!(Layout::for_value(&fut).size(), 1);

            let mut b = ReusableBoxFuture::new(fut);

            assert_eq!(b.get_pin().now_or_never(), Some(10));

            b.try_set(async move { 20 })
                .unwrap_or_else(|_| panic!("incorrect size"));

            assert_eq!(b.get_pin().now_or_never(), Some(20));

            b.try_set(async move { 30 })
                .unwrap_or_else(|_| panic!("incorrect size"));

            assert_eq!(b.get_pin().now_or_never(), Some(30));
        }

        #[test]
        fn test_different_sizes() {
            let fut1 = async move { 10 };
            let val = [0u32; 1000];
            let fut2 = async move { val[0] };
            let fut3 = ZeroSizedFuture {};

            assert_eq!(Layout::for_value(&fut1).size(), 1);
            assert_eq!(Layout::for_value(&fut2).size(), 4004);
            assert_eq!(Layout::for_value(&fut3).size(), 0);

            let mut b = ReusableBoxFuture::new(fut1);
            assert_eq!(b.get_pin().now_or_never(), Some(10));
            b.set(fut2);
            assert_eq!(b.get_pin().now_or_never(), Some(0));
            b.set(fut3);
            assert_eq!(b.get_pin().now_or_never(), Some(5));
        }

        struct ZeroSizedFuture {}
        impl Future for ZeroSizedFuture {
            type Output = u32;
            fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<u32> {
                Poll::Ready(5)
            }
        }

        #[test]
        fn test_zero_sized() {
            let fut = ZeroSizedFuture {};
            // Zero sized!
            assert_eq!(Layout::for_value(&fut).size(), 0);

            let mut b = ReusableBoxFuture::new(fut);

            assert_eq!(b.get_pin().now_or_never(), Some(5));
            assert_eq!(b.get_pin().now_or_never(), Some(5));

            b.try_set(ZeroSizedFuture {})
                .unwrap_or_else(|_| panic!("incorrect size"));

            assert_eq!(b.get_pin().now_or_never(), Some(5));
            assert_eq!(b.get_pin().now_or_never(), Some(5));
        }
    }