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
// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.

use crate::util::UncheckedOptionExt;
use core::{
    fmt, mem,
    sync::atomic::{fence, AtomicU8, Ordering},
};
use parking_lot_core::{self, SpinWait, DEFAULT_PARK_TOKEN, DEFAULT_UNPARK_TOKEN};

const DONE_BIT: u8 = 1;
const POISON_BIT: u8 = 2;
const LOCKED_BIT: u8 = 4;
const PARKED_BIT: u8 = 8;

/// Current state of a `Once`.
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
pub enum OnceState {
    /// A closure has not been executed yet
    New,

    /// A closure was executed but panicked.
    Poisoned,

    /// A thread is currently executing a closure.
    InProgress,

    /// A closure has completed successfully.
    Done,
}

impl OnceState {
    /// Returns whether the associated `Once` has been poisoned.
    ///
    /// Once an initialization routine for a `Once` has panicked it will forever
    /// indicate to future forced initialization routines that it is poisoned.
    #[inline]
    pub fn poisoned(self) -> bool {
        match self {
            OnceState::Poisoned => true,
            _ => false,
        }
    }

    /// Returns whether the associated `Once` has successfully executed a
    /// closure.
    #[inline]
    pub fn done(self) -> bool {
        match self {
            OnceState::Done => true,
            _ => false,
        }
    }
}

/// A synchronization primitive which can be used to run a one-time
/// initialization. Useful for one-time initialization for globals, FFI or
/// related functionality.
///
/// # Differences from the standard library `Once`
///
/// - Only requires 1 byte of space, instead of 1 word.
/// - Not required to be `'static`.
/// - Relaxed memory barriers in the fast path, which can significantly improve
///   performance on some architectures.
/// - Efficient handling of micro-contention using adaptive spinning.
///
/// # Examples
///
/// ```
/// use parking_lot::Once;
///
/// static START: Once = Once::new();
///
/// START.call_once(|| {
///     // run initialization here
/// });
/// ```
pub struct Once(AtomicU8);

impl Once {
    /// Creates a new `Once` value.
    #[inline]
    pub const fn new() -> Once {
        Once(AtomicU8::new(0))
    }

    /// Returns the current state of this `Once`.
    #[inline]
    pub fn state(&self) -> OnceState {
        let state = self.0.load(Ordering::Acquire);
        if state & DONE_BIT != 0 {
            OnceState::Done
        } else if state & LOCKED_BIT != 0 {
            OnceState::InProgress
        } else if state & POISON_BIT != 0 {
            OnceState::Poisoned
        } else {
            OnceState::New
        }
    }

    /// 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).
    ///
    /// # Examples
    ///
    /// ```
    /// use parking_lot::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.
    #[inline]
    pub fn call_once<F>(&self, f: F)
    where
        F: FnOnce(),
    {
        if self.0.load(Ordering::Acquire) == DONE_BIT {
            return;
        }

        let mut f = Some(f);
        self.call_once_slow(false, &mut |_| unsafe { f.take().unchecked_unwrap()() });
    }

    /// Performs the same function as `call_once` except ignores poisoning.
    ///
    /// If this `Once` has been poisoned (some initialization panicked) then
    /// this function will continue to attempt to call initialization functions
    /// until one of them doesn't panic.
    ///
    /// The closure `f` is yielded a structure which can be used to query the
    /// state of this `Once` (whether initialization has previously panicked or
    /// not).
    #[inline]
    pub fn call_once_force<F>(&self, f: F)
    where
        F: FnOnce(OnceState),
    {
        if self.0.load(Ordering::Acquire) == DONE_BIT {
            return;
        }

        let mut f = Some(f);
        self.call_once_slow(true, &mut |state| unsafe {
            f.take().unchecked_unwrap()(state)
        });
    }

    // 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_once_slow(&self, ignore_poison: bool, f: &mut dyn FnMut(OnceState)) {
        let mut spinwait = SpinWait::new();
        let mut state = self.0.load(Ordering::Relaxed);
        loop {
            // If another thread called the closure, we're done
            if state & DONE_BIT != 0 {
                // An acquire fence is needed here since we didn't load the
                // state with Ordering::Acquire.
                fence(Ordering::Acquire);
                return;
            }

            // If the state has been poisoned and we aren't forcing, then panic
            if state & POISON_BIT != 0 && !ignore_poison {
                // Need the fence here as well for the same reason
                fence(Ordering::Acquire);
                panic!("Once instance has previously been poisoned");
            }

            // Grab the lock if it isn't locked, even if there is a queue on it.
            // We also clear the poison bit since we are going to try running
            // the closure again.
            if state & LOCKED_BIT == 0 {
                match self.0.compare_exchange_weak(
                    state,
                    (state | LOCKED_BIT) & !POISON_BIT,
                    Ordering::Acquire,
                    Ordering::Relaxed,
                ) {
                    Ok(_) => break,
                    Err(x) => state = x,
                }
                continue;
            }

            // If there is no queue, try spinning a few times
            if state & PARKED_BIT == 0 && spinwait.spin() {
                state = self.0.load(Ordering::Relaxed);
                continue;
            }

            // Set the parked bit
            if state & PARKED_BIT == 0 {
                if let Err(x) = self.0.compare_exchange_weak(
                    state,
                    state | PARKED_BIT,
                    Ordering::Relaxed,
                    Ordering::Relaxed,
                ) {
                    state = x;
                    continue;
                }
            }

            // Park our thread until we are woken up by the thread that owns the
            // lock.
            let addr = self as *const _ as usize;
            let validate = || self.0.load(Ordering::Relaxed) == LOCKED_BIT | PARKED_BIT;
            let before_sleep = || {};
            let timed_out = |_, _| unreachable!();
            unsafe {
                parking_lot_core::park(
                    addr,
                    validate,
                    before_sleep,
                    timed_out,
                    DEFAULT_PARK_TOKEN,
                    None,
                );
            }

            // Loop back and check if the done bit was set
            spinwait.reset();
            state = self.0.load(Ordering::Relaxed);
        }

        struct PanicGuard<'a>(&'a Once);
        impl<'a> Drop for PanicGuard<'a> {
            fn drop(&mut self) {
                // Mark the state as poisoned, unlock it and unpark all threads.
                let once = self.0;
                let state = once.0.swap(POISON_BIT, Ordering::Release);
                if state & PARKED_BIT != 0 {
                    let addr = once as *const _ as usize;
                    unsafe {
                        parking_lot_core::unpark_all(addr, DEFAULT_UNPARK_TOKEN);
                    }
                }
            }
        }

        // At this point we have the lock, so run the closure. Make sure we
        // properly clean up if the closure panicks.
        let guard = PanicGuard(self);
        let once_state = if state & POISON_BIT != 0 {
            OnceState::Poisoned
        } else {
            OnceState::New
        };
        f(once_state);
        mem::forget(guard);

        // Now unlock the state, set the done bit and unpark all threads
        let state = self.0.swap(DONE_BIT, Ordering::Release);
        if state & PARKED_BIT != 0 {
            let addr = self as *const _ as usize;
            unsafe {
                parking_lot_core::unpark_all(addr, DEFAULT_UNPARK_TOKEN);
            }
        }
    }
}

impl Default for Once {
    #[inline]
    fn default() -> Once {
        Once::new()
    }
}

impl fmt::Debug for Once {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Once")
            .field("state", &self.state())
            .finish()
    }
}

#[cfg(test)]
mod tests {
    use crate::Once;
    use std::panic;
    use std::sync::mpsc::channel;
    use std::thread;

    #[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());
    }

    #[test]
    fn test_once_debug() {
        static O: Once = Once::new();

        assert_eq!(format!("{:?}", O), "Once { state: New }");
    }
}