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//! A "once initialization" primitive //! //! This primitive is meant to be used to run one-time initialization. An //! example use case would be for initializing an FFI library. // A "once" is a relatively simple primitive, and it's also typically provided // by the OS as well (see `pthread_once` or `InitOnceExecuteOnce`). The OS // primitives, however, tend to have surprising restrictions, such as the Unix // one doesn't allow an argument to be passed to the function. // // As a result, we end up implementing it ourselves in the standard library. // This also gives us the opportunity to optimize the implementation a bit which // should help the fast path on call sites. Consequently, let's explain how this // primitive works now! // // So to recap, the guarantees of a Once are that it will call the // initialization closure at most once, and it will never return until the one // that's running has finished running. This means that we need some form of // blocking here while the custom callback is running at the very least. // Additionally, we add on the restriction of **poisoning**. Whenever an // initialization closure panics, the Once enters a "poisoned" state which means // that all future calls will immediately panic as well. // // So to implement this, one might first reach for a `Mutex`, but those cannot // be put into a `static`. It also gets a lot harder with poisoning to figure // out when the mutex needs to be deallocated because it's not after the closure // finishes, but after the first successful closure finishes. // // All in all, this is instead implemented with atomics and lock-free // operations! Whee! Each `Once` has one word of atomic state, and this state is // CAS'd on to determine what to do. There are four possible state of a `Once`: // // * Incomplete - no initialization has run yet, and no thread is currently // using the Once. // * Poisoned - some thread has previously attempted to initialize the Once, but // it panicked, so the Once is now poisoned. There are no other // threads currently accessing this Once. // * Running - some thread is currently attempting to run initialization. It may // succeed, so all future threads need to wait for it to finish. // Note that this state is accompanied with a payload, described // below. // * Complete - initialization has completed and all future calls should finish // immediately. // // With 4 states we need 2 bits to encode this, and we use the remaining bits // in the word we have allocated as a queue of threads waiting for the thread // responsible for entering the RUNNING state. This queue is just a linked list // of Waiter nodes which is monotonically increasing in size. Each node is // allocated on the stack, and whenever the running closure finishes it will // consume the entire queue and notify all waiters they should try again. // // You'll find a few more details in the implementation, but that's the gist of // it! use crate::fmt; use crate::marker; use crate::ptr; use crate::sync::atomic::{AtomicUsize, AtomicBool, Ordering}; use crate::thread::{self, Thread}; /// A synchronization primitive which can be used to run a one-time global /// initialization. Useful for one-time initialization for FFI or related /// functionality. This type can only be constructed with the [`ONCE_INIT`] /// value or the equivalent [`Once::new`] constructor. /// /// [`ONCE_INIT`]: constant.ONCE_INIT.html /// [`Once::new`]: struct.Once.html#method.new /// /// # Examples /// /// ``` /// use std::sync::Once; /// /// static START: Once = Once::new(); /// /// START.call_once(|| { /// // run initialization here /// }); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub struct Once { // This `state` word is actually an encoded version of just a pointer to a // `Waiter`, so we add the `PhantomData` appropriately. state: AtomicUsize, _marker: marker::PhantomData<*mut Waiter>, } // The `PhantomData` of a raw pointer removes these two auto traits, but we // enforce both below in the implementation so this should be safe to add. #[stable(feature = "rust1", since = "1.0.0")] unsafe impl Sync for Once {} #[stable(feature = "rust1", since = "1.0.0")] unsafe impl Send for Once {} /// State yielded to [`call_once_force`]’s closure parameter. The state can be /// used to query the poison status of the [`Once`]. /// /// [`call_once_force`]: struct.Once.html#method.call_once_force /// [`Once`]: struct.Once.html #[unstable(feature = "once_poison", issue = "33577")] #[derive(Debug)] pub struct OnceState { poisoned: bool, } /// Initialization value for static [`Once`] values. /// /// [`Once`]: struct.Once.html /// /// # Examples /// /// ``` /// use std::sync::{Once, ONCE_INIT}; /// /// static START: Once = ONCE_INIT; /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[rustc_deprecated( since = "1.38.0", reason = "the `new` function is now preferred", suggestion = "Once::new()", )] pub const ONCE_INIT: Once = Once::new(); // Four states that a Once can be in, encoded into the lower bits of `state` in // the Once structure. const INCOMPLETE: usize = 0x0; const POISONED: usize = 0x1; const RUNNING: usize = 0x2; const COMPLETE: usize = 0x3; // Mask to learn about the state. All other bits are the queue of waiters if // this is in the RUNNING state. const STATE_MASK: usize = 0x3; // Representation of a node in the linked list of waiters in the RUNNING state. struct Waiter { thread: Option<Thread>, signaled: AtomicBool, next: *mut Waiter, } // Helper struct used to clean up after a closure call with a `Drop` // implementation to also run on panic. struct Finish<'a> { panicked: bool, me: &'a Once, } impl Once { /// Creates a new `Once` value. #[stable(feature = "once_new", since = "1.2.0")] pub const fn new() -> Once { Once { state: AtomicUsize::new(INCOMPLETE), _marker: marker::PhantomData, } } /// Performs an initialization routine once and only once. The given closure /// will be executed if this is the first time `call_once` has been called, /// and otherwise the routine will *not* be invoked. /// /// This method will block the calling thread if another initialization /// routine is currently running. /// /// When this function returns, it is guaranteed that some initialization /// has run and completed (it may not be the closure specified). It is also /// guaranteed that any memory writes performed by the executed closure can /// be reliably observed by other threads at this point (there is a /// happens-before relation between the closure and code executing after the /// return). /// /// If the given closure recursively invokes `call_once` on the same `Once` /// instance the exact behavior is not specified, allowed outcomes are /// a panic or a deadlock. /// /// # Examples /// /// ``` /// use std::sync::Once; /// /// static mut VAL: usize = 0; /// static INIT: Once = Once::new(); /// /// // Accessing a `static mut` is unsafe much of the time, but if we do so /// // in a synchronized fashion (e.g., write once or read all) then we're /// // good to go! /// // /// // This function will only call `expensive_computation` once, and will /// // otherwise always return the value returned from the first invocation. /// fn get_cached_val() -> usize { /// unsafe { /// INIT.call_once(|| { /// VAL = expensive_computation(); /// }); /// VAL /// } /// } /// /// fn expensive_computation() -> usize { /// // ... /// # 2 /// } /// ``` /// /// # Panics /// /// The closure `f` will only be executed once if this is called /// concurrently amongst many threads. If that closure panics, however, then /// it will *poison* this `Once` instance, causing all future invocations of /// `call_once` to also panic. /// /// This is similar to [poisoning with mutexes][poison]. /// /// [poison]: struct.Mutex.html#poisoning #[stable(feature = "rust1", since = "1.0.0")] pub fn call_once<F>(&self, f: F) where F: FnOnce() { // Fast path check if self.is_completed() { return; } let mut f = Some(f); self.call_inner(false, &mut |_| f.take().unwrap()()); } /// Performs the same function as [`call_once`] except ignores poisoning. /// /// Unlike [`call_once`], if this `Once` has been poisoned (i.e., a previous /// call to `call_once` or `call_once_force` caused a panic), calling /// `call_once_force` will still invoke the closure `f` and will _not_ /// result in an immediate panic. If `f` panics, the `Once` will remain /// in a poison state. If `f` does _not_ panic, the `Once` will no /// longer be in a poison state and all future calls to `call_once` or /// `call_one_force` will be no-ops. /// /// The closure `f` is yielded a [`OnceState`] structure which can be used /// to query the poison status of the `Once`. /// /// [`call_once`]: struct.Once.html#method.call_once /// [`OnceState`]: struct.OnceState.html /// /// # Examples /// /// ``` /// #![feature(once_poison)] /// /// use std::sync::Once; /// use std::thread; /// /// static INIT: Once = Once::new(); /// /// // poison the once /// let handle = thread::spawn(|| { /// INIT.call_once(|| panic!()); /// }); /// assert!(handle.join().is_err()); /// /// // poisoning propagates /// let handle = thread::spawn(|| { /// INIT.call_once(|| {}); /// }); /// assert!(handle.join().is_err()); /// /// // call_once_force will still run and reset the poisoned state /// INIT.call_once_force(|state| { /// assert!(state.poisoned()); /// }); /// /// // once any success happens, we stop propagating the poison /// INIT.call_once(|| {}); /// ``` #[unstable(feature = "once_poison", issue = "33577")] pub fn call_once_force<F>(&self, f: F) where F: FnOnce(&OnceState) { // Fast path check if self.is_completed() { return; } let mut f = Some(f); self.call_inner(true, &mut |p| { f.take().unwrap()(&OnceState { poisoned: p }) }); } /// Returns `true` if some `call_once` call has completed /// successfully. Specifically, `is_completed` will return false in /// the following situations: /// * `call_once` was not called at all, /// * `call_once` was called, but has not yet completed, /// * the `Once` instance is poisoned /// /// It is also possible that immediately after `is_completed` /// returns false, some other thread finishes executing /// `call_once`. /// /// # Examples /// /// ``` /// #![feature(once_is_completed)] /// use std::sync::Once; /// /// static INIT: Once = Once::new(); /// /// assert_eq!(INIT.is_completed(), false); /// INIT.call_once(|| { /// assert_eq!(INIT.is_completed(), false); /// }); /// assert_eq!(INIT.is_completed(), true); /// ``` /// /// ``` /// #![feature(once_is_completed)] /// use std::sync::Once; /// use std::thread; /// /// static INIT: Once = Once::new(); /// /// assert_eq!(INIT.is_completed(), false); /// let handle = thread::spawn(|| { /// INIT.call_once(|| panic!()); /// }); /// assert!(handle.join().is_err()); /// assert_eq!(INIT.is_completed(), false); /// ``` #[unstable(feature = "once_is_completed", issue = "54890")] #[inline] pub fn is_completed(&self) -> bool { // An `Acquire` load is enough because that makes all the initialization // operations visible to us, and, this being a fast path, weaker // ordering helps with performance. This `Acquire` synchronizes with // `SeqCst` operations on the slow path. self.state.load(Ordering::Acquire) == COMPLETE } // This is a non-generic function to reduce the monomorphization cost of // using `call_once` (this isn't exactly a trivial or small implementation). // // Additionally, this is tagged with `#[cold]` as it should indeed be cold // and it helps let LLVM know that calls to this function should be off the // fast path. Essentially, this should help generate more straight line code // in LLVM. // // Finally, this takes an `FnMut` instead of a `FnOnce` because there's // currently no way to take an `FnOnce` and call it via virtual dispatch // without some allocation overhead. #[cold] fn call_inner(&self, ignore_poisoning: bool, init: &mut dyn FnMut(bool)) { // This cold path uses SeqCst consistently because the // performance difference really does not matter there, and // SeqCst minimizes the chances of something going wrong. let mut state = self.state.load(Ordering::SeqCst); 'outer: loop { match state { // If we're complete, then there's nothing to do, we just // jettison out as we shouldn't run the closure. COMPLETE => return, // If we're poisoned and we're not in a mode to ignore // poisoning, then we panic here to propagate the poison. POISONED if !ignore_poisoning => { panic!("Once instance has previously been poisoned"); } // Otherwise if we see a poisoned or otherwise incomplete state // we will attempt to move ourselves into the RUNNING state. If // we succeed, then the queue of waiters starts at null (all 0 // bits). POISONED | INCOMPLETE => { let old = self.state.compare_and_swap(state, RUNNING, Ordering::SeqCst); if old != state { state = old; continue } // Run the initialization routine, letting it know if we're // poisoned or not. The `Finish` struct is then dropped, and // the `Drop` implementation here is responsible for waking // up other waiters both in the normal return and panicking // case. let mut complete = Finish { panicked: true, me: self, }; init(state == POISONED); complete.panicked = false; return } // All other values we find should correspond to the RUNNING // state with an encoded waiter list in the more significant // bits. We attempt to enqueue ourselves by moving us to the // head of the list and bail out if we ever see a state that's // not RUNNING. _ => { assert!(state & STATE_MASK == RUNNING); let mut node = Waiter { thread: Some(thread::current()), signaled: AtomicBool::new(false), next: ptr::null_mut(), }; let me = &mut node as *mut Waiter as usize; assert!(me & STATE_MASK == 0); while state & STATE_MASK == RUNNING { node.next = (state & !STATE_MASK) as *mut Waiter; let old = self.state.compare_and_swap(state, me | RUNNING, Ordering::SeqCst); if old != state { state = old; continue } // Once we've enqueued ourselves, wait in a loop. // Afterwards reload the state and continue with what we // were doing from before. while !node.signaled.load(Ordering::SeqCst) { thread::park(); } state = self.state.load(Ordering::SeqCst); continue 'outer } } } } } } #[stable(feature = "std_debug", since = "1.16.0")] impl fmt::Debug for Once { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("Once { .. }") } } impl Drop for Finish<'_> { fn drop(&mut self) { // Swap out our state with however we finished. We should only ever see // an old state which was RUNNING. let queue = if self.panicked { self.me.state.swap(POISONED, Ordering::SeqCst) } else { self.me.state.swap(COMPLETE, Ordering::SeqCst) }; assert_eq!(queue & STATE_MASK, RUNNING); // Decode the RUNNING to a list of waiters, then walk that entire list // and wake them up. Note that it is crucial that after we store `true` // in the node it can be free'd! As a result we load the `thread` to // signal ahead of time and then unpark it after the store. unsafe { let mut queue = (queue & !STATE_MASK) as *mut Waiter; while !queue.is_null() { let next = (*queue).next; let thread = (*queue).thread.take().unwrap(); (*queue).signaled.store(true, Ordering::SeqCst); thread.unpark(); queue = next; } } } } impl OnceState { /// Returns `true` if the associated [`Once`] was poisoned prior to the /// invocation of the closure passed to [`call_once_force`]. /// /// [`call_once_force`]: struct.Once.html#method.call_once_force /// [`Once`]: struct.Once.html /// /// # Examples /// /// A poisoned `Once`: /// /// ``` /// #![feature(once_poison)] /// /// use std::sync::Once; /// use std::thread; /// /// static INIT: Once = Once::new(); /// /// // poison the once /// let handle = thread::spawn(|| { /// INIT.call_once(|| panic!()); /// }); /// assert!(handle.join().is_err()); /// /// INIT.call_once_force(|state| { /// assert!(state.poisoned()); /// }); /// ``` /// /// An unpoisoned `Once`: /// /// ``` /// #![feature(once_poison)] /// /// use std::sync::Once; /// /// static INIT: Once = Once::new(); /// /// INIT.call_once_force(|state| { /// assert!(!state.poisoned()); /// }); #[unstable(feature = "once_poison", issue = "33577")] pub fn poisoned(&self) -> bool { self.poisoned } } #[cfg(all(test, not(target_os = "emscripten")))] mod tests { use crate::panic; use crate::sync::mpsc::channel; use crate::thread; use super::Once; #[test] fn smoke_once() { static O: Once = Once::new(); let mut a = 0; O.call_once(|| a += 1); assert_eq!(a, 1); O.call_once(|| a += 1); assert_eq!(a, 1); } #[test] fn stampede_once() { static O: Once = Once::new(); static mut RUN: bool = false; let (tx, rx) = channel(); for _ in 0..10 { let tx = tx.clone(); thread::spawn(move|| { for _ in 0..4 { thread::yield_now() } unsafe { O.call_once(|| { assert!(!RUN); RUN = true; }); assert!(RUN); } tx.send(()).unwrap(); }); } unsafe { O.call_once(|| { assert!(!RUN); RUN = true; }); assert!(RUN); } for _ in 0..10 { rx.recv().unwrap(); } } #[test] fn poison_bad() { static O: Once = Once::new(); // poison the once let t = panic::catch_unwind(|| { O.call_once(|| panic!()); }); assert!(t.is_err()); // poisoning propagates let t = panic::catch_unwind(|| { O.call_once(|| {}); }); assert!(t.is_err()); // we can subvert poisoning, however let mut called = false; O.call_once_force(|p| { called = true; assert!(p.poisoned()) }); assert!(called); // once any success happens, we stop propagating the poison O.call_once(|| {}); } #[test] fn wait_for_force_to_finish() { static O: Once = Once::new(); // poison the once let t = panic::catch_unwind(|| { O.call_once(|| panic!()); }); assert!(t.is_err()); // make sure someone's waiting inside the once via a force let (tx1, rx1) = channel(); let (tx2, rx2) = channel(); let t1 = thread::spawn(move || { O.call_once_force(|p| { assert!(p.poisoned()); tx1.send(()).unwrap(); rx2.recv().unwrap(); }); }); rx1.recv().unwrap(); // put another waiter on the once let t2 = thread::spawn(|| { let mut called = false; O.call_once(|| { called = true; }); assert!(!called); }); tx2.send(()).unwrap(); assert!(t1.join().is_ok()); assert!(t2.join().is_ok()); } }