| //! 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()); |
| |
| } |
| } |