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// Copyright 2019 Google LLC // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // https://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. // //! # Arc Guard //! //! This crate[^1] provides the [`ArcGuard`] class, which is a container for a single object //! with lifetime that is bound to that of an `Arc`. This is useful for passing around boxed //! futures that have a lifetime that is limited to that of the object that created it. //! //! For example, the following does not compile: //! //! ```compile_fail //! # use async_coap::arc_guard; // Remove if spun off into own crate //! use futures::{future::ready,future::BoxFuture,prelude::*}; //! use std::sync::{Arc,Weak}; //! use arc_guard::{ArcGuard,ArcGuardExt}; //! //! trait PropertyFetcher { //! fn fetch( //! &self, //! key: &str, //! ) -> BoxFuture<Option<String>>; //! } //! //! struct WeakFetcher { //! sub_obj: Weak<Box<PropertyFetcher>>, //! } //! //! impl PropertyFetcher for WeakFetcher { //! fn fetch(&self, key: &str) -> BoxFuture<Option<String>> { //! if let Some(arc) = self.sub_obj.upgrade() { //! // error[E0515]: cannot return value referencing local variable `arc` //! arc.fetch(key).boxed() //! } else { //! ready(None).boxed() //! } //! } //! } //! ``` //! //! If you think about it, the fact that `rustc` doesn't like this code makes perfect sense: //! because `sub_obj` is a weak reference, it could be dropped at any moment, violating the //! lifetime guarantee for the return value of `fetch()`. To fix this, we need to ensure that //! the value we return internally keeps an `Arc` reference to the object that created it. That's //! where `ArcGuard` comes in: //! //! ``` //! # use async_coap::arc_guard; // Remove if spun off into own crate //! # use futures::{future::ready,future::BoxFuture,prelude::*}; //! # use std::sync::{Arc,Weak}; //! # use arc_guard::{ArcGuard,ArcGuardExt}; //! # //! # trait PropertyFetcher { //! # fn fetch( //! # &self, //! # key: &str, //! # ) -> BoxFuture<Option<String>>; //! # } //! # //! # struct WeakFetcher { //! # sub_obj: Weak<Box<PropertyFetcher>>, //! # } //! //! impl PropertyFetcher for WeakFetcher { //! fn fetch(&self, key: &str) -> BoxFuture<Option<String>> { //! if let Some(arc) = self.sub_obj.upgrade() { //! // Compiles and works! //! arc.guard(|x|x.fetch(key)).boxed() //! } else { //! ready(None).boxed() //! } //! } //! } //! ``` //! //! ## Additional Examples //! //! ``` //! # use async_coap::arc_guard; // Remove if spun off into own crate //! # use std::sync::{Arc,Weak}; //! # use arc_guard::{ArcGuard,ArcGuardExt}; //! //! let mut arc = Arc::new("foobar".to_string()); //! //! let guarded = arc.guard(|s| &s.as_str()[3..]); //! //! assert_eq!(guarded, "bar"); //! //! // We can't get a mutable instance to the //! // string while `guarded` is still around. //! assert_eq!(Arc::get_mut(&mut arc), None); //! //! core::mem::drop(guarded); //! //! assert!(Arc::get_mut(&mut arc).is_some()); //! ``` //! //! [^1]: I would have loved to call this crate `lifeguard`, because it is a "guard" on the //! lifetime of the contained "head" instance, but sadly that name was //! [already taken](https://crates.io/crates/lifeguard). //! #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)] #![warn(clippy::all)] use futures::prelude::*; use pin_utils::unsafe_pinned; use std::cmp::Ordering; use std::ops::Deref; use std::pin::Pin; use std::sync::Arc; /// A container for a single object with lifetime that is bound to that of an `Arc`. /// /// See [Module Documentation](index.html) for more information. #[derive(Debug, Clone)] pub struct ArcGuard<RC, T> { inner: T, head: Arc<RC>, } impl<RC, T> ArcGuard<RC, T> { unsafe_pinned!(inner: T); /// Constructs a new `ArcGuard<>` instance using the given `Arc<>` and getter closure. /// The use of a closure for the getter allows for a more convenient syntax while ensuring /// the lifetimes are properly accounted for. /// /// See the main documentation for `ArcGuard<>` for a usage example. pub fn new<'head, F>(head: Arc<RC>, getter: F) -> ArcGuard<RC, T> where F: FnOnce(&'head RC) -> T, RC: 'head, T: 'head, { // SAFETY: This is safe because we are only using this reference to create our object, // and, by holding a reference to `head`, this class ensures that it does not live longer // than the contained reference. ArcGuard { inner: getter(unsafe { std::mem::transmute::<&RC, &RC>(&head) }), head, } } /// Borrows a reference to the `Arc` that is being held to preserve the underlying value. pub fn head(&self) -> &Arc<RC> { &self.head } } unsafe impl<RC, T: Send> Send for ArcGuard<RC, T> {} unsafe impl<RC, T: Sync> Sync for ArcGuard<RC, T> {} impl<RC, T> Deref for ArcGuard<RC, T> { type Target = T; fn deref(&self) -> &Self::Target { &self.inner } } impl<RC, T: std::fmt::Display> std::fmt::Display for ArcGuard<RC, T> { fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> { self.inner.fmt(f) } } impl<RC, T: AsRef<R>, R> AsRef<R> for ArcGuard<RC, T> { fn as_ref(&self) -> &R { self.inner.as_ref() } } impl<RC, T> std::borrow::Borrow<T> for ArcGuard<RC, T> { fn borrow(&self) -> &T { &self.inner } } impl<RC, T: PartialEq<R>, R> PartialEq<R> for ArcGuard<RC, T> { fn eq(&self, other: &R) -> bool { self.inner.eq(other) } } impl<RC, T: PartialOrd<R>, R> PartialOrd<R> for ArcGuard<RC, T> { fn partial_cmp(&self, other: &R) -> Option<Ordering> { self.inner.partial_cmp(other) } } impl<RC, T: Future> Future for ArcGuard<RC, T> { type Output = T::Output; fn poll( mut self: Pin<&mut Self>, cx: &mut futures::task::Context<'_>, ) -> futures::task::Poll<Self::Output> { self.as_mut().inner().poll(cx) } } impl<RC, T: Stream> Stream for ArcGuard<RC, T> { type Item = T::Item; fn poll_next( mut self: Pin<&mut Self>, cx: &mut futures::task::Context<'_>, ) -> futures::task::Poll<Option<Self::Item>> { self.as_mut().inner().poll_next(cx) } } /// A convenience trait for `Arc<>` that makes it easier to construct `ArcGuard<>` instances. /// /// See [Module Documentation](index.html) for more information. pub trait ArcGuardExt<RC> { /// Convenience method for constructing `ArcGuard<>` instances. /// /// See [Module Documentation](index.html) for more information. fn guard<'head, F, T>(&self, getter: F) -> ArcGuard<RC, T> where F: FnOnce(&'head RC) -> T, RC: 'head, T: 'head; } impl<RC> ArcGuardExt<RC> for Arc<RC> { fn guard<'head, F, T>(&self, getter: F) -> ArcGuard<RC, T> where F: FnOnce(&'head RC) -> T, RC: 'head, T: 'head, { ArcGuard::new(self.clone(), getter) } }