1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
//! 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));
    }
}