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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!
//
// Atomic orderings:
// When running `Once` we deal with multiple atomics:
// `Once.state_and_queue` and an unknown number of `Waiter.signaled`.
// * `state_and_queue` is used (1) as a state flag, (2) for synchronizing the
// result of the `Once`, and (3) for synchronizing `Waiter` nodes.
// - At the end of the `call_inner` function we have to make sure the result
// of the `Once` is acquired. So every load which can be the only one to
// load COMPLETED must have at least Acquire ordering, which means all
// three of them.
// - `WaiterQueue::Drop` is the only place that may store COMPLETED, and
// must do so with Release ordering to make the result available.
// - `wait` inserts `Waiter` nodes as a pointer in `state_and_queue`, and
// needs to make the nodes available with Release ordering. The load in
// its `compare_exchange` can be Relaxed because it only has to compare
// the atomic, not to read other data.
// - `WaiterQueue::Drop` must see the `Waiter` nodes, so it must load
// `state_and_queue` with Acquire ordering.
// - There is just one store where `state_and_queue` is used only as a
// state flag, without having to synchronize data: switching the state
// from INCOMPLETE to RUNNING in `call_inner`. This store can be Relaxed,
// but the read has to be Acquire because of the requirements mentioned
// above.
// * `Waiter.signaled` is both used as a flag, and to protect a field with
// interior mutability in `Waiter`. `Waiter.thread` is changed in
// `WaiterQueue::Drop` which then sets `signaled` with Release ordering.
// After `wait` loads `signaled` with Acquire and sees it is true, it needs to
// see the changes to drop the `Waiter` struct correctly.
// * There is one place where the two atomics `Once.state_and_queue` and
// `Waiter.signaled` come together, and might be reordered by the compiler or
// processor. Because both use Acquire ordering such a reordering is not
// allowed, so no need for SeqCst.
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;
use crate::cell::Cell;
use crate::fmt;
use crate::marker;
use crate::panic::{RefUnwindSafe, UnwindSafe};
use crate::ptr;
use crate::sync::atomic::{AtomicBool, AtomicPtr, Ordering};
use crate::thread::{self, Thread};
type Masked = ();
/// 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 [`Once::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 {
// `state_and_queue` is actually a pointer to a `Waiter` with extra state
// bits, so we add the `PhantomData` appropriately.
state_and_queue: AtomicPtr<Masked>,
_marker: marker::PhantomData<*const 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 {}
#[stable(feature = "sync_once_unwind_safe", since = "1.59.0")]
impl UnwindSafe for Once {}
#[stable(feature = "sync_once_unwind_safe", since = "1.59.0")]
impl RefUnwindSafe for Once {}
/// State yielded to [`Once::call_once_force()`]’s closure parameter. The state
/// can be used to query the poison status of the [`Once`].
#[stable(feature = "once_poison", since = "1.51.0")]
#[derive(Debug)]
pub struct OnceState {
poisoned: bool,
set_state_on_drop_to: Cell<*mut Masked>,
}
/// Initialization value for static [`Once`] values.
///
/// # Examples
///
/// ```
/// use std::sync::{Once, ONCE_INIT};
///
/// static START: Once = ONCE_INIT;
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(
since = "1.38.0",
note = "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_and_queue` 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, used while in the
// RUNNING state.
// Note: `Waiter` can't hold a mutable pointer to the next thread, because then
// `wait` would both hand out a mutable reference to its `Waiter` node, and keep
// a shared reference to check `signaled`. Instead we hold shared references and
// use interior mutability.
#[repr(align(4))] // Ensure the two lower bits are free to use as state bits.
struct Waiter {
thread: Cell<Option<Thread>>,
signaled: AtomicBool,
next: *const Waiter,
}
// Head of a linked list of waiters.
// Every node is a struct on the stack of a waiting thread.
// Will wake up the waiters when it gets dropped, i.e. also on panic.
struct WaiterQueue<'a> {
state_and_queue: &'a AtomicPtr<Masked>,
set_state_on_drop_to: *mut Masked,
}
impl Once {
/// Creates a new `Once` value.
#[inline]
#[stable(feature = "once_new", since = "1.2.0")]
#[rustc_const_stable(feature = "const_once_new", since = "1.32.0")]
#[must_use]
pub const fn new() -> Once {
Once {
state_and_queue: AtomicPtr::new(ptr::invalid_mut(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 might 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")]
#[track_caller]
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_once_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()`]: Once::call_once
/// [`call_once_force()`]: Once::call_once_force
///
/// # Examples
///
/// ```
/// 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.is_poisoned());
/// });
///
/// // once any success happens, we stop propagating the poison
/// INIT.call_once(|| {});
/// ```
#[stable(feature = "once_poison", since = "1.51.0")]
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()(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
///
/// This function returning `false` does not mean that [`Once`] has not been
/// executed. For example, it may have been executed in the time between
/// when `is_completed` starts executing and when it returns, in which case
/// the `false` return value would be stale (but still permissible).
///
/// [`call_once()`]: Once::call_once
///
/// # Examples
///
/// ```
/// 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);
/// ```
///
/// ```
/// 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);
/// ```
#[stable(feature = "once_is_completed", since = "1.43.0")]
#[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
// `Release` operations on the slow path.
self.state_and_queue.load(Ordering::Acquire).addr() == 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]
#[track_caller]
fn call_inner(&self, ignore_poisoning: bool, init: &mut dyn FnMut(&OnceState)) {
let mut state_and_queue = self.state_and_queue.load(Ordering::Acquire);
loop {
match state_and_queue.addr() {
COMPLETE => break,
POISONED if !ignore_poisoning => {
// Panic to propagate the poison.
panic!("Once instance has previously been poisoned");
}
POISONED | INCOMPLETE => {
// Try to register this thread as the one RUNNING.
let exchange_result = self.state_and_queue.compare_exchange(
state_and_queue,
ptr::invalid_mut(RUNNING),
Ordering::Acquire,
Ordering::Acquire,
);
if let Err(old) = exchange_result {
state_and_queue = old;
continue;
}
// `waiter_queue` will manage other waiting threads, and
// wake them up on drop.
let mut waiter_queue = WaiterQueue {
state_and_queue: &self.state_and_queue,
set_state_on_drop_to: ptr::invalid_mut(POISONED),
};
// Run the initialization function, letting it know if we're
// poisoned or not.
let init_state = OnceState {
poisoned: state_and_queue.addr() == POISONED,
set_state_on_drop_to: Cell::new(ptr::invalid_mut(COMPLETE)),
};
init(&init_state);
waiter_queue.set_state_on_drop_to = init_state.set_state_on_drop_to.get();
break;
}
_ => {
// All other values must be RUNNING with possibly a
// pointer to the waiter queue in the more significant bits.
assert!(state_and_queue.addr() & STATE_MASK == RUNNING);
wait(&self.state_and_queue, state_and_queue);
state_and_queue = self.state_and_queue.load(Ordering::Acquire);
}
}
}
}
}
fn wait(state_and_queue: &AtomicPtr<Masked>, mut current_state: *mut Masked) {
// Note: the following code was carefully written to avoid creating a
// mutable reference to `node` that gets aliased.
loop {
// Don't queue this thread if the status is no longer running,
// otherwise we will not be woken up.
if current_state.addr() & STATE_MASK != RUNNING {
return;
}
// Create the node for our current thread.
let node = Waiter {
thread: Cell::new(Some(thread::current())),
signaled: AtomicBool::new(false),
next: current_state.with_addr(current_state.addr() & !STATE_MASK) as *const Waiter,
};
let me = &node as *const Waiter as *const Masked as *mut Masked;
// Try to slide in the node at the head of the linked list, making sure
// that another thread didn't just replace the head of the linked list.
let exchange_result = state_and_queue.compare_exchange(
current_state,
me.with_addr(me.addr() | RUNNING),
Ordering::Release,
Ordering::Relaxed,
);
if let Err(old) = exchange_result {
current_state = old;
continue;
}
// We have enqueued ourselves, now lets wait.
// It is important not to return before being signaled, otherwise we
// would drop our `Waiter` node and leave a hole in the linked list
// (and a dangling reference). Guard against spurious wakeups by
// reparking ourselves until we are signaled.
while !node.signaled.load(Ordering::Acquire) {
// If the managing thread happens to signal and unpark us before we
// can park ourselves, the result could be this thread never gets
// unparked. Luckily `park` comes with the guarantee that if it got
// an `unpark` just before on an unparked thread it does not park.
thread::park();
}
break;
}
}
#[stable(feature = "std_debug", since = "1.16.0")]
impl fmt::Debug for Once {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Once").finish_non_exhaustive()
}
}
impl Drop for WaiterQueue<'_> {
fn drop(&mut self) {
// Swap out our state with however we finished.
let state_and_queue =
self.state_and_queue.swap(self.set_state_on_drop_to, Ordering::AcqRel);
// We should only ever see an old state which was RUNNING.
assert_eq!(state_and_queue.addr() & STATE_MASK, RUNNING);
// Walk the entire linked list of waiters and wake them up (in lifo
// order, last to register is first to wake up).
unsafe {
// Right after setting `node.signaled = true` the other thread may
// free `node` if there happens to be has a spurious wakeup.
// So we have to take out the `thread` field and copy the pointer to
// `next` first.
let mut queue =
state_and_queue.with_addr(state_and_queue.addr() & !STATE_MASK) as *const Waiter;
while !queue.is_null() {
let next = (*queue).next;
let thread = (*queue).thread.take().unwrap();
(*queue).signaled.store(true, Ordering::Release);
// ^- FIXME (maybe): This is another case of issue #55005
// `store()` has a potentially dangling ref to `signaled`.
queue = next;
thread.unpark();
}
}
}
}
impl OnceState {
/// Returns `true` if the associated [`Once`] was poisoned prior to the
/// invocation of the closure passed to [`Once::call_once_force()`].
///
/// # Examples
///
/// A poisoned [`Once`]:
///
/// ```
/// 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.is_poisoned());
/// });
/// ```
///
/// An unpoisoned [`Once`]:
///
/// ```
/// use std::sync::Once;
///
/// static INIT: Once = Once::new();
///
/// INIT.call_once_force(|state| {
/// assert!(!state.is_poisoned());
/// });
#[stable(feature = "once_poison", since = "1.51.0")]
pub fn is_poisoned(&self) -> bool {
self.poisoned
}
/// Poison the associated [`Once`] without explicitly panicking.
// NOTE: This is currently only exposed for the `lazy` module
pub(crate) fn poison(&self) {
self.set_state_on_drop_to.set(ptr::invalid_mut(POISONED));
}
}