tokio/signal/
reusable_box.rs

1use std::alloc::Layout;
2use std::future::Future;
3use std::panic::AssertUnwindSafe;
4use std::pin::Pin;
5use std::ptr::{self, NonNull};
6use std::task::{Context, Poll};
7use std::{fmt, panic};
8
9/// A reusable `Pin<Box<dyn Future<Output = T> + Send>>`.
10///
11/// This type lets you replace the future stored in the box without
12/// reallocating when the size and alignment permits this.
13pub(crate) struct ReusableBoxFuture<T> {
14    boxed: NonNull<dyn Future<Output = T> + Send>,
15}
16
17impl<T> ReusableBoxFuture<T> {
18    /// Create a new `ReusableBoxFuture<T>` containing the provided future.
19    pub(crate) fn new<F>(future: F) -> Self
20    where
21        F: Future<Output = T> + Send + 'static,
22    {
23        let boxed: Box<dyn Future<Output = T> + Send> = Box::new(future);
24
25        let boxed = Box::into_raw(boxed);
26
27        // SAFETY: Box::into_raw does not return null pointers.
28        let boxed = unsafe { NonNull::new_unchecked(boxed) };
29
30        Self { boxed }
31    }
32
33    /// Replaces the future currently stored in this box.
34    ///
35    /// This reallocates if and only if the layout of the provided future is
36    /// different from the layout of the currently stored future.
37    pub(crate) fn set<F>(&mut self, future: F)
38    where
39        F: Future<Output = T> + Send + 'static,
40    {
41        if let Err(future) = self.try_set(future) {
42            *self = Self::new(future);
43        }
44    }
45
46    /// Replaces the future currently stored in this box.
47    ///
48    /// This function never reallocates, but returns an error if the provided
49    /// future has a different size or alignment from the currently stored
50    /// future.
51    pub(crate) fn try_set<F>(&mut self, future: F) -> Result<(), F>
52    where
53        F: Future<Output = T> + Send + 'static,
54    {
55        // SAFETY: The pointer is not dangling.
56        let self_layout = {
57            let dyn_future: &(dyn Future<Output = T> + Send) = unsafe { self.boxed.as_ref() };
58            Layout::for_value(dyn_future)
59        };
60
61        if Layout::new::<F>() == self_layout {
62            // SAFETY: We just checked that the layout of F is correct.
63            unsafe {
64                self.set_same_layout(future);
65            }
66
67            Ok(())
68        } else {
69            Err(future)
70        }
71    }
72
73    /// Sets the current future.
74    ///
75    /// # Safety
76    ///
77    /// This function requires that the layout of the provided future is the
78    /// same as `self.layout`.
79    unsafe fn set_same_layout<F>(&mut self, future: F)
80    where
81        F: Future<Output = T> + Send + 'static,
82    {
83        // Drop the existing future, catching any panics.
84        let result = panic::catch_unwind(AssertUnwindSafe(|| {
85            ptr::drop_in_place(self.boxed.as_ptr());
86        }));
87
88        // Overwrite the future behind the pointer. This is safe because the
89        // allocation was allocated with the same size and alignment as the type F.
90        let self_ptr: *mut F = self.boxed.as_ptr() as *mut F;
91        ptr::write(self_ptr, future);
92
93        // Update the vtable of self.boxed. The pointer is not null because we
94        // just got it from self.boxed, which is not null.
95        self.boxed = NonNull::new_unchecked(self_ptr);
96
97        // If the old future's destructor panicked, resume unwinding.
98        match result {
99            Ok(()) => {}
100            Err(payload) => {
101                panic::resume_unwind(payload);
102            }
103        }
104    }
105
106    /// Gets a pinned reference to the underlying future.
107    pub(crate) fn get_pin(&mut self) -> Pin<&mut (dyn Future<Output = T> + Send)> {
108        // SAFETY: The user of this box cannot move the box, and we do not move it
109        // either.
110        unsafe { Pin::new_unchecked(self.boxed.as_mut()) }
111    }
112
113    /// Polls the future stored inside this box.
114    pub(crate) fn poll(&mut self, cx: &mut Context<'_>) -> Poll<T> {
115        self.get_pin().poll(cx)
116    }
117}
118
119impl<T> Future for ReusableBoxFuture<T> {
120    type Output = T;
121
122    /// Polls the future stored inside this box.
123    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
124        Pin::into_inner(self).get_pin().poll(cx)
125    }
126}
127
128// The future stored inside ReusableBoxFuture<T> must be Send.
129unsafe impl<T> Send for ReusableBoxFuture<T> {}
130
131// The only method called on self.boxed is poll, which takes &mut self, so this
132// struct being Sync does not permit any invalid access to the Future, even if
133// the future is not Sync.
134unsafe impl<T> Sync for ReusableBoxFuture<T> {}
135
136// Just like a Pin<Box<dyn Future>> is always Unpin, so is this type.
137impl<T> Unpin for ReusableBoxFuture<T> {}
138
139impl<T> Drop for ReusableBoxFuture<T> {
140    fn drop(&mut self) {
141        unsafe {
142            drop(Box::from_raw(self.boxed.as_ptr()));
143        }
144    }
145}
146
147impl<T> fmt::Debug for ReusableBoxFuture<T> {
148    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
149        f.debug_struct("ReusableBoxFuture").finish()
150    }
151}
152
153#[cfg(test)]
154mod test {
155    use super::ReusableBoxFuture;
156    use futures::future::FutureExt;
157    use std::alloc::Layout;
158    use std::future::Future;
159    use std::pin::Pin;
160    use std::task::{Context, Poll};
161
162    #[test]
163    fn test_different_futures() {
164        let fut = async move { 10 };
165        // Not zero sized!
166        assert_eq!(Layout::for_value(&fut).size(), 1);
167
168        let mut b = ReusableBoxFuture::new(fut);
169
170        assert_eq!(b.get_pin().now_or_never(), Some(10));
171
172        b.try_set(async move { 20 })
173            .unwrap_or_else(|_| panic!("incorrect size"));
174
175        assert_eq!(b.get_pin().now_or_never(), Some(20));
176
177        b.try_set(async move { 30 })
178            .unwrap_or_else(|_| panic!("incorrect size"));
179
180        assert_eq!(b.get_pin().now_or_never(), Some(30));
181    }
182
183    #[test]
184    fn test_different_sizes() {
185        let fut1 = async move { 10 };
186        let val = [0u32; 1000];
187        let fut2 = async move { val[0] };
188        let fut3 = ZeroSizedFuture {};
189
190        assert_eq!(Layout::for_value(&fut1).size(), 1);
191        assert_eq!(Layout::for_value(&fut2).size(), 4004);
192        assert_eq!(Layout::for_value(&fut3).size(), 0);
193
194        let mut b = ReusableBoxFuture::new(fut1);
195        assert_eq!(b.get_pin().now_or_never(), Some(10));
196        b.set(fut2);
197        assert_eq!(b.get_pin().now_or_never(), Some(0));
198        b.set(fut3);
199        assert_eq!(b.get_pin().now_or_never(), Some(5));
200    }
201
202    struct ZeroSizedFuture {}
203    impl Future for ZeroSizedFuture {
204        type Output = u32;
205        fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<u32> {
206            Poll::Ready(5)
207        }
208    }
209
210    #[test]
211    fn test_zero_sized() {
212        let fut = ZeroSizedFuture {};
213        // Zero sized!
214        assert_eq!(Layout::for_value(&fut).size(), 0);
215
216        let mut b = ReusableBoxFuture::new(fut);
217
218        assert_eq!(b.get_pin().now_or_never(), Some(5));
219        assert_eq!(b.get_pin().now_or_never(), Some(5));
220
221        b.try_set(ZeroSizedFuture {})
222            .unwrap_or_else(|_| panic!("incorrect size"));
223
224        assert_eq!(b.get_pin().now_or_never(), Some(5));
225        assert_eq!(b.get_pin().now_or_never(), Some(5));
226    }
227}
tokio/signal/reusable_box.rs

tokio/signal/
reusable_box.rs
