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util: add ReusableBoxFuture utility (#3464)
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Original file line number | Diff line number | Diff line change |
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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}; | ||
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/// 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>, | ||
} | ||
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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 boxed: Box<dyn Future<Output = T> + Send> = Box::new(future); | ||
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let boxed = Box::into_raw(boxed); | ||
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// SAFETY: Box::into_raw does not return null pointers. | ||
let boxed = unsafe { NonNull::new_unchecked(boxed) }; | ||
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Self { boxed } | ||
} | ||
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/// 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, | ||
{ | ||
if let Err(future) = self.try_set(future) { | ||
*self = Self::new(future); | ||
} | ||
} | ||
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/// 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, | ||
{ | ||
// 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) | ||
}; | ||
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if Layout::new::<F>() == self_layout { | ||
// SAFETY: We just checked that the layout of F is correct. | ||
unsafe { | ||
self.set_same_layout(future); | ||
} | ||
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Ok(()) | ||
} else { | ||
Err(future) | ||
} | ||
} | ||
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/// 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()); | ||
})); | ||
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// 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); | ||
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// 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); | ||
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// If the old future's destructor panicked, resume unwinding. | ||
match result { | ||
Ok(()) => {} | ||
Err(payload) => { | ||
panic::resume_unwind(payload); | ||
} | ||
} | ||
} | ||
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/// Get a pinned reference to the underlying future. | ||
pub 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()) } | ||
} | ||
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/// Poll the future stored inside this box. | ||
pub fn poll(&mut self, cx: &mut Context<'_>) -> Poll<T> { | ||
self.get_pin().poll(cx) | ||
} | ||
} | ||
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impl<T> Future for ReusableBoxFuture<T> { | ||
type Output = T; | ||
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/// 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) | ||
} | ||
} | ||
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// The future stored inside ReusableBoxFuture<T> must be Send. | ||
unsafe impl<T> Send for ReusableBoxFuture<T> {} | ||
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// 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> {} | ||
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// Just like a Pin<Box<dyn Future>> is always Unpin, so is this type. | ||
impl<T> Unpin for ReusableBoxFuture<T> {} | ||
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impl<T> Drop for ReusableBoxFuture<T> { | ||
fn drop(&mut self) { | ||
unsafe { | ||
drop(Box::from_raw(self.boxed.as_ptr())); | ||
} | ||
} | ||
} | ||
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impl<T> fmt::Debug for ReusableBoxFuture<T> { | ||
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | ||
f.debug_struct("ReusableBoxFuture").finish() | ||
} | ||
} |
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Original file line number | Diff line number | Diff line change |
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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; | ||
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#[test] | ||
fn test_different_futures() { | ||
let fut = async move { 10 }; | ||
// Not zero sized! | ||
assert_eq!(Layout::for_value(&fut).size(), 1); | ||
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let mut b = ReusableBoxFuture::new(fut); | ||
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assert_eq!(b.get_pin().now_or_never(), Some(10)); | ||
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b.try_set(async move { 20 }) | ||
.unwrap_or_else(|_| panic!("incorrect size")); | ||
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assert_eq!(b.get_pin().now_or_never(), Some(20)); | ||
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b.try_set(async move { 30 }) | ||
.unwrap_or_else(|_| panic!("incorrect size")); | ||
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assert_eq!(b.get_pin().now_or_never(), Some(30)); | ||
} | ||
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#[test] | ||
fn test_different_sizes() { | ||
let fut1 = async move { 10 }; | ||
let val = [0u32; 1000]; | ||
let fut2 = async move { val[0] }; | ||
let fut3 = ZeroSizedFuture {}; | ||
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assert_eq!(Layout::for_value(&fut1).size(), 1); | ||
assert_eq!(Layout::for_value(&fut2).size(), 4004); | ||
assert_eq!(Layout::for_value(&fut3).size(), 0); | ||
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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)); | ||
} | ||
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struct ZeroSizedFuture {} | ||
impl Future for ZeroSizedFuture { | ||
type Output = u32; | ||
fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<u32> { | ||
Poll::Ready(5) | ||
} | ||
} | ||
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#[test] | ||
fn test_zero_sized() { | ||
let fut = ZeroSizedFuture {}; | ||
// Zero sized! | ||
assert_eq!(Layout::for_value(&fut).size(), 0); | ||
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let mut b = ReusableBoxFuture::new(fut); | ||
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assert_eq!(b.get_pin().now_or_never(), Some(5)); | ||
assert_eq!(b.get_pin().now_or_never(), Some(5)); | ||
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b.try_set(ZeroSizedFuture {}) | ||
.unwrap_or_else(|_| panic!("incorrect size")); | ||
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assert_eq!(b.get_pin().now_or_never(), Some(5)); | ||
assert_eq!(b.get_pin().now_or_never(), Some(5)); | ||
} |