util: add ReusableBoxFuture utility

tokio-util/src/sync/mod.rs

@@ -7,3 +7,6 @@ mod intrusive_double_linked_list;
 
 mod poll_semaphore;
 pub use poll_semaphore::PollSemaphore;
+
+mod reusable_box;
+pub use reusable_box::ReusableBoxFuture;

tokio-util/src/sync/reusable_box.rs

@@ -0,0 +1,150 @@
+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 struct ReusableBoxFuture<T> {
+    boxed: NonNull<dyn Future<Output = T> + Send>,
+    layout: Layout,
+}
+
+impl<T> ReusableBoxFuture<T> {
+    /// Create a new `ReusableBoxFuture<T>` containing the provided future.
+    pub fn new<F>(future: F) -> Self
+    where
+        F: Future<Output = T> + Send + 'static,
+    {
+        let layout = Layout::new::<F>();
+        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, layout }
+    }
+
+    /// 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 fn set<F>(&mut self, future: F)
+    where
+        F: Future<Output = T> + Send + 'static,
+    {
+        match self.try_set(future) {
+            Ok(()) => {}
+            Err(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 fn try_set<F>(&mut self, future: F) -> Result<(), F>
+    where
+        F: Future<Output = T> + Send + 'static,
+    {
+        let layout = Layout::new::<F>();
+
+        if layout == 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 fn get_pin(&mut self) -> Pin<&mut (dyn Future<Output = T> + Send)> {
+        unsafe { Pin::new_unchecked(self.boxed.as_mut()) }
+    }
+
+    /// Poll the future stored inside this box.
+    pub 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()
+    }
+}

tokio-util/tests/reusable_box.rs

@@ -0,0 +1,72 @@
+use futures::future::FutureExt;
+use std::alloc::Layout;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use tokio_util::sync::ReusableBoxFuture;
+
+#[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));
+}