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//! [![crates.io version](https://img.shields.io/crates/v/safe-lock.svg)](https://crates.io/crates/safe-lock) //! [![license: Apache 2.0](https://gitlab.com/leonhard-llc/ops/-/raw/main/license-apache-2.0.svg)](https://gitlab.com/leonhard-llc/ops/-/raw/main/safe-lock/LICENSE) //! [![unsafe forbidden](https://gitlab.com/leonhard-llc/ops/-/raw/main/unsafe-forbidden.svg)](https://github.com/rust-secure-code/safety-dance/) //! [![pipeline status](https://gitlab.com/leonhard-llc/ops/badges/main/pipeline.svg)](https://gitlab.com/leonhard-llc/ops/-/pipelines) //! //! # safe-lock //! //! A simple `SafeLock` struct. //! //! ## Use Cases //! - Run tests sequentially //! - Prevent concurrent operations on atomic values //! - Prevent concurrent operations on data and systems outside the Rust runtime //! //! ## Features //! - Const constructor //! - Depends only on `std` //! - `forbid(unsafe_code)` //! - 100% test coverage //! //! ## Limitations //! - Not a `Mutex<T>`. Does not contain a value. //! - Unoptimized. Uses //! [`AtomicBool`](https://doc.rust-lang.org/core/sync/atomic/struct.AtomicBool.html) //! in a spinlock, not fast OS locks. //! - Not a fair lock. If multiple threads acquire the lock in loops, //! some may never acquire it. //! //! ## Alternatives //! - [`rusty-fork`](https://crates.io/crates/rusty-fork) //! - Run tests in separate processes //! - [`std::sync::Mutex`](https://doc.rust-lang.org/std/sync/struct.Mutex.html) //! - Part of the Rust standard library: well reviewed, well tested, and well maintained. //! - Uses fast OS locks //! - Has no const constructor. See [rust#66806](https://github.com/rust-lang/rust/issues/66806) //! and [const-eval#3](https://github.com/rust-lang/const-eval/issues/3). //! You can work around this with unstable //! [`core::lazy::OnceCell`](https://doc.rust-lang.org/core/lazy/struct.OnceCell.html) //! or various `unsafe` crates: //! [`lazy_static`](https://crates.io/crates/lazy_static), //! [`once_cell`](https://crates.io/crates/once_cell), //! [`lazycell`](https://crates.io/crates/lazycell), and //! [`conquer-once`](https://crates.io/crates/conquer-once). //! - [`parking_lot`](https://crates.io/crates/parking_lot) //! - Well written code. //! Many hope that it will end up in the Rust standard library someday. //! - Contains plenty of `unsafe` //! - [`try-lock`](https://crates.io/crates/try-lock) //! - Popular //! - No dependencies, `no_std` //! - Uses `unsafe` //! - [`ruspiro-lock`](https://crates.io/crates/ruspiro-lock) //! - Sync and async locks //! - No dependencies, `no_std` //! - Uses `unsafe` //! - [`flexible-locks`](https://crates.io/crates/flexible-locks) //! - Lots of `unsafe` //! - Uses fast OS locks //! - Unmaintained //! //! ## Related Crates //! - [`safina-sync`](https://crates.io/crates/safina-sync) //! provides a safe async `Mutex` //! //! ## Example //! //! Make some tests run sequentially so they don't interfere with each other: //! ```unknown //! use safe_lock::SafeLock; //! static LOCK: SafeLock = SafeLock::new(); //! //! [#test] //! fn test1() { //! let _guard = LOCK.lock(); //! // ... //! } //! //! [#test] //! fn test2() { //! let _guard = LOCK.lock(); //! // ... //! } //! ``` //! //! ## Cargo Geiger Safety Report //! //! ## Changelog //! - v0.1.3 - Increase test coverage //! - v0.1.2 - Use `Acquire` and `Release` ordering //! - v0.1.1 - Update docs //! - v0.1.0 - Initial version //! //! ## Happy Contributors 🙂 //! Fixing bugs and adding features is easy and fast. //! Send us a pull request and we intend to: //! - Always respond within 24 hours //! - Provide clear & concrete feedback //! - Immediately make a new release for your accepted change #![forbid(unsafe_code)] use core::sync::atomic::{AtomicBool, Ordering}; /// A handle to the acquired lock. Drop this to release the lock. pub struct SafeLockGuard<'x> { inner: &'x SafeLock, } impl<'x> Drop for SafeLockGuard<'x> { fn drop(&mut self) { if !self.inner.locked.swap(false, Ordering::Release) { unreachable!() } } } #[cfg(test)] #[test] fn test_unreachable() { let lock: SafeLock = Default::default(); let guard1 = lock.lock(); let guard2 = SafeLockGuard { inner: &lock }; drop(guard1); match std::panic::catch_unwind(move || drop(guard2)) { Ok(_) => panic!("expected panic"), Err(any) => { assert_eq!( "internal error: entered unreachable code", *any.downcast::<&'static str>().unwrap() ); } } } /// A lock. /// /// See [`lock`](#method.lock). /// /// This is not a fair lock. /// If multiple threads acquire the lock in loops, /// some may never acquire it. /// /// # Example /// /// Make some tests run sequentially so they don't interfere with each other: /// ```unknown /// use safe_lock::SafeLock; /// static LOCK: SafeLock = SafeLock::new(); /// /// [#test] /// fn test1() { /// let _guard = LOCK.lock(); /// // ... /// } /// /// [#test] /// fn test2() { /// let _guard = LOCK.lock(); /// // ... /// } /// ``` pub struct SafeLock { locked: AtomicBool, } impl SafeLock { #[must_use] pub const fn new() -> SafeLock { SafeLock { locked: AtomicBool::new(false), } } /// Waits until the lock is free, then acquires the lock. /// /// Multiple threads can call `lock` but only one will acquire the lock /// and return. /// /// Drop the returned `SafeLockGuard` to release the lock. /// /// This is not a fair lock. /// If multiple threads acquire the lock in loops, /// some may never acquire it. /// /// Uses /// [`Ordering::Acquire`](https://doc.rust-lang.org/stable/std/sync/atomic/enum.Ordering.html#variant.Acquire) /// to acquire the lock and /// [`Ordering::Release`](https://doc.rust-lang.org/stable/std/sync/atomic/enum.Ordering.html#variant.Release) /// to release it, so the lock orders operations on other atomic values. #[must_use] pub fn lock(&self) -> Option<SafeLockGuard> { loop { // We could use `Ordering::Relaxed` here: // "Typical use for relaxed memory ordering is incrementing // counters, such as the reference counters of `std::shared_ptr`, // since this only requires atomicity, but not ordering or // synchronization (note that decrementing the `shared_ptr` // counters requires acquire-release synchronization with the // destructor)" // https://en.cppreference.com/w/cpp/atomic/memory_order#Relaxed_ordering // // But one use-case for `SafeLock` is to prevent concurrent // operations on atomic values. So we need ordering between // acquiring the lock and seeing changes to other atomic values. // Therefore we use Ordering::Acquire and Ordering::Release. if self .locked .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed) .is_ok() { return Some(SafeLockGuard { inner: &self }); } std::thread::yield_now(); } } } impl Default for SafeLock { fn default() -> Self { Self::new() } }