1/*
2 * Copyright 2020 Actyx AG
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16//! A mutual exclusion primitive that relies on static type information only
17//!
18//! This library is inspired by [this discussion](https://internals.rust-lang.org/t/what-shall-sync-mean-across-an-await/12020/2).
19#![doc(html_logo_url = "https://developer.actyx.com/img/logo.svg")]
20#![doc(html_favicon_url = "https://developer.actyx.com/img/favicon.ico")]
21#![no_std]
22
23use core::{
24 fmt::{self, Debug, Formatter},
25 pin::Pin,
26 future::Future,
27 task::{Context, Poll},
28};
29
30/// A mutual exclusion primitive that relies on static type information only
31///
32/// In some cases synchronization can be proven statically: whenever you hold an exclusive `&mut`
33/// reference, the Rust type system ensures that no other part of the program can hold another
34/// reference to the data. Therefore it is safe to access it even if the current thread obtained
35/// this reference via a channel. Whenever this is the case, the overhead of allocating and locking
36/// a [`Mutex`] can be avoided by using this static version.
37///
38/// One example where this is often applicable is [`Future`], which requires an exclusive reference
39/// for its [`poll`] method: While a given `Future` implementation may not be safe to access by
40/// multiple threads concurrently, the executor can only run the `Future` on one thread at any
41/// given time, making it [`Sync`] in practice as long as the implementation is `Send`. You can
42/// therefore use the static mutex to prove that your data structure is `Sync` even though it
43/// contains such a `Future`.
44///
45/// # Example
46///
47/// ```
48/// use sync_wrapper::SyncWrapper;
49/// use std::future::Future;
50///
51/// struct MyThing {
52/// future: SyncWrapper<Box<dyn Future<Output = String> + Send>>,
53/// }
54///
55/// impl MyThing {
56/// // all accesses to `self.future` now require an exclusive reference or ownership
57/// }
58///
59/// fn assert_sync<T: Sync>() {}
60///
61/// assert_sync::<MyThing>();
62/// ```
63///
64/// [`Mutex`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html
65/// [`Future`]: https://doc.rust-lang.org/std/future/trait.Future.html
66/// [`poll`]: https://doc.rust-lang.org/std/future/trait.Future.html#method.poll
67/// [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
68#[repr(transparent)]
69pub struct SyncWrapper<T>(T);
70
71impl<T> SyncWrapper<T> {
72 /// Creates a new static mutex containing the given value.
73 ///
74 /// # Examples
75 ///
76 /// ```
77 /// use sync_wrapper::SyncWrapper;
78 ///
79 /// let mutex = SyncWrapper::new(42);
80 /// ```
81 pub const fn new(value: T) -> Self {
82 Self(value)
83 }
84
85 /// Acquires a reference to the protected value.
86 ///
87 /// This is safe because it requires an exclusive reference to the mutex. Therefore this method
88 /// neither panics nor does it return an error. This is in contrast to [`Mutex::get_mut`] which
89 /// returns an error if another thread panicked while holding the lock. It is not recommended
90 /// to send an exclusive reference to a potentially damaged value to another thread for further
91 /// processing.
92 ///
93 /// [`Mutex::get_mut`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html#method.get_mut
94 ///
95 /// # Examples
96 ///
97 /// ```
98 /// use sync_wrapper::SyncWrapper;
99 ///
100 /// let mut mutex = SyncWrapper::new(42);
101 /// let value = mutex.get_mut();
102 /// *value = 0;
103 /// assert_eq!(*mutex.get_mut(), 0);
104 /// ```
105 pub fn get_mut(&mut self) -> &mut T {
106 &mut self.0
107 }
108
109 /// Acquires a pinned reference to the protected value.
110 ///
111 /// See [`Self::get_mut`] for why this method is safe.
112 ///
113 /// # Examples
114 ///
115 /// ```
116 /// use std::future::Future;
117 /// use std::pin::Pin;
118 /// use std::task::{Context, Poll};
119 ///
120 /// use pin_project_lite::pin_project;
121 /// use sync_wrapper::SyncWrapper;
122 ///
123 /// pin_project! {
124 /// struct FutureWrapper<F> {
125 /// #[pin]
126 /// inner: SyncWrapper<F>,
127 /// }
128 /// }
129 ///
130 /// impl<F: Future> Future for FutureWrapper<F> {
131 /// type Output = F::Output;
132 ///
133 /// fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
134 /// self.project().inner.get_pin_mut().poll(cx)
135 /// }
136 /// }
137 /// ```
138 pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T> {
139 unsafe { Pin::map_unchecked_mut(self, |this| &mut this.0) }
140 }
141
142 /// Consumes this mutex, returning the underlying data.
143 ///
144 /// This is safe because it requires ownership of the mutex, therefore this method will neither
145 /// panic nor does it return an error. This is in contrast to [`Mutex::into_inner`] which
146 /// returns an error if another thread panicked while holding the lock. It is not recommended
147 /// to send an exclusive reference to a potentially damaged value to another thread for further
148 /// processing.
149 ///
150 /// [`Mutex::into_inner`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html#method.into_inner
151 ///
152 /// # Examples
153 ///
154 /// ```
155 /// use sync_wrapper::SyncWrapper;
156 ///
157 /// let mut mutex = SyncWrapper::new(42);
158 /// assert_eq!(mutex.into_inner(), 42);
159 /// ```
160 pub fn into_inner(self) -> T {
161 self.0
162 }
163}
164
165// this is safe because the only operations permitted on this data structure require exclusive
166// access or ownership
167unsafe impl<T> Sync for SyncWrapper<T> {}
168
169impl<T> Debug for SyncWrapper<T> {
170 fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
171 f.pad("SyncWrapper")
172 }
173}
174
175impl<T: Default> Default for SyncWrapper<T> {
176 fn default() -> Self {
177 Self::new(T::default())
178 }
179}
180
181impl<T> From<T> for SyncWrapper<T> {
182 fn from(value: T) -> Self {
183 Self::new(value)
184 }
185}
186
187/// `Future` which is `Sync`.
188///
189/// # Examples
190///
191/// ```
192/// use sync_wrapper::{SyncWrapper, SyncFuture};
193///
194/// let fut = async { 1 };
195/// let fut = SyncFuture::new(fut);
196/// ```
197pub struct SyncFuture<F> {
198 inner: SyncWrapper<F>
199}
200impl <F: Future> SyncFuture<F> {
201 pub fn new(inner: F) -> Self {
202 Self { inner: SyncWrapper::new(inner) }
203 }
204 pub fn into_inner(self) -> F {
205 self.inner.into_inner()
206 }
207}
208impl <F: Future> Future for SyncFuture<F> {
209 type Output = F::Output;
210 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
211 let inner: Pin<&mut F> = unsafe { self.map_unchecked_mut(|x: &mut SyncFuture| x.inner.get_mut()) };
212 inner.poll(cx)
213 }
214}
215impl<T> Debug for SyncFuture<T> {
216 fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
217 f.pad("SyncFuture")
218 }
219}
220
221/// `Stream` which is `Sync`.
222///
223/// # Examples
224///
225/// ```
226/// use sync_wrapper::SyncStream;
227/// use futures::stream;
228///
229/// let st = stream::iter(vec![1]);
230/// let st = SyncStream::new(st);
231/// ```
232#[cfg(feature = "futures")]
233pub struct SyncStream<S> {
234 inner: SyncWrapper<S>
235}
236#[cfg(feature = "futures")]
237impl <S: futures_core::Stream> SyncStream<S> {
238 pub fn new(inner: S) -> Self {
239 Self { inner: SyncWrapper::new(inner) }
240 }
241 pub fn into_inner(self) -> S {
242 self.inner.into_inner()
243 }
244}
245#[cfg(feature = "futures")]
246impl <S: futures_core::Stream> futures_core::Stream for SyncStream<S> {
247 type Item = S::Item;
248 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
249 let inner: Pin<&mut S> = unsafe { self.map_unchecked_mut(|x: &mut SyncStream| x.inner.get_mut()) };
250 inner.poll_next(cx)
251 }
252}
253#[cfg(feature = "futures")]
254impl<T> Debug for SyncStream<T> {
255 fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
256 f.pad("SyncStream")
257 }
258}
259