tokio_util/sync/
reusable_box.rs

1use std::alloc::Layout;
2use std::fmt;
3use std::future::{self, Future};
4use std::mem::{self, ManuallyDrop};
5use std::pin::Pin;
6use std::ptr;
7use std::task::{Context, Poll};
8
9/// A reusable `Pin<Box<dyn Future<Output = T> + Send + 'a>>`.
10///
11/// This type lets you replace the future stored in the box without
12/// reallocating when the size and alignment permits this.
13pub struct ReusableBoxFuture<'a, T> {
14    boxed: Pin<Box<dyn Future<Output = T> + Send + 'a>>,
15}
16
17impl<'a, T> ReusableBoxFuture<'a, T> {
18    /// Create a new `ReusableBoxFuture<T>` containing the provided future.
19    pub fn new<F>(future: F) -> Self
20    where
21        F: Future<Output = T> + Send + 'a,
22    {
23        Self {
24            boxed: Box::pin(future),
25        }
26    }
27
28    /// Replace the future currently stored in this box.
29    ///
30    /// This reallocates if and only if the layout of the provided future is
31    /// different from the layout of the currently stored future.
32    pub fn set<F>(&mut self, future: F)
33    where
34        F: Future<Output = T> + Send + 'a,
35    {
36        if let Err(future) = self.try_set(future) {
37            *self = Self::new(future);
38        }
39    }
40
41    /// Replace the future currently stored in this box.
42    ///
43    /// This function never reallocates, but returns an error if the provided
44    /// future has a different size or alignment from the currently stored
45    /// future.
46    pub fn try_set<F>(&mut self, future: F) -> Result<(), F>
47    where
48        F: Future<Output = T> + Send + 'a,
49    {
50        // If we try to inline the contents of this function, the type checker complains because
51        // the bound `T: 'a` is not satisfied in the call to `pending()`. But by putting it in an
52        // inner function that doesn't have `T` as a generic parameter, we implicitly get the bound
53        // `F::Output: 'a` transitively through `F: 'a`, allowing us to call `pending()`.
54        #[inline(always)]
55        fn real_try_set<'a, F>(
56            this: &mut ReusableBoxFuture<'a, F::Output>,
57            future: F,
58        ) -> Result<(), F>
59        where
60            F: Future + Send + 'a,
61        {
62            // future::Pending<T> is a ZST so this never allocates.
63            let boxed = mem::replace(&mut this.boxed, Box::pin(future::pending()));
64            reuse_pin_box(boxed, future, |boxed| this.boxed = Pin::from(boxed))
65        }
66
67        real_try_set(self, future)
68    }
69
70    /// Get a pinned reference to the underlying future.
71    pub fn get_pin(&mut self) -> Pin<&mut (dyn Future<Output = T> + Send)> {
72        self.boxed.as_mut()
73    }
74
75    /// Poll the future stored inside this box.
76    pub fn poll(&mut self, cx: &mut Context<'_>) -> Poll<T> {
77        self.get_pin().poll(cx)
78    }
79}
80
81impl<T> Future for ReusableBoxFuture<'_, T> {
82    type Output = T;
83
84    /// Poll the future stored inside this box.
85    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
86        Pin::into_inner(self).get_pin().poll(cx)
87    }
88}
89
90// The only method called on self.boxed is poll, which takes &mut self, so this
91// struct being Sync does not permit any invalid access to the Future, even if
92// the future is not Sync.
93unsafe impl<T> Sync for ReusableBoxFuture<'_, T> {}
94
95impl<T> fmt::Debug for ReusableBoxFuture<'_, T> {
96    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
97        f.debug_struct("ReusableBoxFuture").finish()
98    }
99}
100
101fn reuse_pin_box<T: ?Sized, U, O, F>(boxed: Pin<Box<T>>, new_value: U, callback: F) -> Result<O, U>
102where
103    F: FnOnce(Box<U>) -> O,
104{
105    let layout = Layout::for_value::<T>(&*boxed);
106    if layout != Layout::new::<U>() {
107        return Err(new_value);
108    }
109
110    // SAFETY: We don't ever construct a non-pinned reference to the old `T` from now on, and we
111    // always drop the `T`.
112    let raw: *mut T = Box::into_raw(unsafe { Pin::into_inner_unchecked(boxed) });
113
114    // When dropping the old value panics, we still want to call `callback` — so move the rest of
115    // the code into a guard type.
116    let guard = CallOnDrop::new(|| {
117        let raw: *mut U = raw.cast::<U>();
118        unsafe { raw.write(new_value) };
119
120        // SAFETY:
121        // - `T` and `U` have the same layout.
122        // - `raw` comes from a `Box` that uses the same allocator as this one.
123        // - `raw` points to a valid instance of `U` (we just wrote it in).
124        let boxed = unsafe { Box::from_raw(raw) };
125
126        callback(boxed)
127    });
128
129    // Drop the old value.
130    unsafe { ptr::drop_in_place(raw) };
131
132    // Run the rest of the code.
133    Ok(guard.call())
134}
135
136struct CallOnDrop<O, F: FnOnce() -> O> {
137    f: ManuallyDrop<F>,
138}
139
140impl<O, F: FnOnce() -> O> CallOnDrop<O, F> {
141    fn new(f: F) -> Self {
142        let f = ManuallyDrop::new(f);
143        Self { f }
144    }
145    fn call(self) -> O {
146        let mut this = ManuallyDrop::new(self);
147        let f = unsafe { ManuallyDrop::take(&mut this.f) };
148        f()
149    }
150}
151
152impl<O, F: FnOnce() -> O> Drop for CallOnDrop<O, F> {
153    fn drop(&mut self) {
154        let f = unsafe { ManuallyDrop::take(&mut self.f) };
155        f();
156    }
157}
tokio_util/sync/reusable_box.rs

tokio_util/sync/
reusable_box.rs
