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
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
//! Helper module for some internals, most users don't need to interact with it.

use std::{
    cell::UnsafeCell,
    error::Error,
    fmt::{Display, Error as FormatError, Formatter},
    ops::{Deref, DerefMut},
    sync::atomic::{AtomicUsize, Ordering},
    usize,
};

macro_rules! borrow_panic {
    ($s:expr) => {{
        panic!(
            "Tried to fetch data of type {:?}, but it was already borrowed{}.",
            ::std::any::type_name::<T>(),
            $s,
        )
    }};
}

/// Marker struct for an invalid borrow error
#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub struct InvalidBorrow;

impl Display for InvalidBorrow {
    fn fmt(&self, f: &mut Formatter) -> Result<(), FormatError> {
        write!(f, "Tried to borrow when it was illegal")
    }
}

impl Error for InvalidBorrow {
    fn description(&self) -> &str {
        "This error is returned when you try to borrow immutably when it's already \
         borrowed mutably or you try to borrow mutably when it's already borrowed"
    }
}

/// An immutable reference to data in a `TrustCell`.
///
/// Access the value via `std::ops::Deref` (e.g. `*val`)
#[derive(Debug)]
pub struct Ref<'a, T: ?Sized + 'a> {
    flag: &'a AtomicUsize,
    value: &'a T,
}

impl<'a, T: ?Sized> Ref<'a, T> {
    /// Makes a new `Ref` for a component of the borrowed data which preserves
    /// the existing borrow.
    ///
    /// The `TrustCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as `Ref::map(...)`.
    /// A method would interfere with methods of the same name on the contents
    /// of a `Ref` used through `Deref`. Further this preserves the borrow of
    /// the value and hence does the proper cleanup when it's dropped.
    ///
    /// # Examples
    ///
    /// This can be used to avoid pointer indirection when a boxed item is
    /// stored in the `TrustCell`.
    ///
    /// ```
    /// use shred::cell::{Ref, TrustCell};
    ///
    /// let cb = TrustCell::new(Box::new(5));
    ///
    /// // Borrowing the cell causes the `Ref` to store a reference to the `Box`, which is a
    /// // pointer to the value on the heap, not the actual value.
    /// let boxed_ref: Ref<'_, Box<usize>> = cb.borrow();
    /// assert_eq!(**boxed_ref, 5); // Notice the double deref to get the actual value.
    ///
    /// // By using `map` we can let `Ref` store a reference directly to the value on the heap.
    /// let pure_ref: Ref<'_, usize> = Ref::map(boxed_ref, Box::as_ref);
    ///
    /// assert_eq!(*pure_ref, 5);
    /// ```
    ///
    /// We can also use `map` to get a reference to a sub-part of the borrowed
    /// value.
    ///
    /// ```rust
    /// # use shred::cell::{TrustCell, Ref};
    ///
    /// let c = TrustCell::new((5, 'b'));
    /// let b1: Ref<'_, (u32, char)> = c.borrow();
    /// let b2: Ref<'_, u32> = Ref::map(b1, |t| &t.0);
    /// assert_eq!(*b2, 5);
    /// ```
    pub fn map<U, F>(self, f: F) -> Ref<'a, U>
    where
        F: FnOnce(&T) -> &U,
        U: ?Sized,
    {
        // Extract the values from the `Ref` through a pointer so that we do not run
        // `Drop`. Because the returned `Ref` has the same lifetime `'a` as the
        // given `Ref`, the lifetime we created through turning the pointer into
        // a ref is valid.
        let flag = unsafe { &*(self.flag as *const _) };
        let value = unsafe { &*(self.value as *const _) };

        // We have to forget self so that we do not run `Drop`. Further it's safe
        // because we are creating a new `Ref`, with the same flag, which will
        // run the cleanup when it's dropped.
        std::mem::forget(self);

        Ref {
            flag,
            value: f(value),
        }
    }
}

impl<'a, T: ?Sized> Deref for Ref<'a, T> {
    type Target = T;

    fn deref(&self) -> &T {
        self.value
    }
}

impl<'a, T: ?Sized> Drop for Ref<'a, T> {
    fn drop(&mut self) {
        self.flag.fetch_sub(1, Ordering::Release);
    }
}

impl<'a, T: ?Sized> Clone for Ref<'a, T> {
    fn clone(&self) -> Self {
        self.flag.fetch_add(1, Ordering::Release);
        Ref {
            flag: self.flag,
            value: self.value,
        }
    }
}

/// A mutable reference to data in a `TrustCell`.
///
/// Access the value via `std::ops::DerefMut` (e.g. `*val`)
#[derive(Debug)]
pub struct RefMut<'a, T: ?Sized + 'a> {
    flag: &'a AtomicUsize,
    value: &'a mut T,
}

impl<'a, T: ?Sized> RefMut<'a, T> {
    /// Makes a new `RefMut` for a component of the borrowed data which
    /// preserves the existing borrow.
    ///
    /// The `TrustCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::map(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefMut` used through `DerefMut`.
    /// Further this preserves the borrow of the value and hence does the
    /// proper cleanup when it's dropped.
    ///
    /// # Examples
    ///
    /// This can also be used to avoid pointer indirection when a boxed item is
    /// stored in the `TrustCell`.
    ///
    /// ```
    /// use shred::cell::{RefMut, TrustCell};
    ///
    /// let cb = TrustCell::new(Box::new(5));
    ///
    /// // Borrowing the cell causes the `RefMut` to store a reference to the `Box`, which is a
    /// // pointer to the value on the heap, and not a reference directly to the value.
    /// let boxed_ref: RefMut<'_, Box<usize>> = cb.borrow_mut();
    /// assert_eq!(**boxed_ref, 5); // Notice the double deref to get the actual value.
    ///
    /// // By using `map` we can let `RefMut` store a reference directly to the value on the heap.
    /// let pure_ref: RefMut<'_, usize> = RefMut::map(boxed_ref, Box::as_mut);
    ///
    /// assert_eq!(*pure_ref, 5);
    /// ```
    ///
    /// We can also use `map` to get a reference to a sub-part of the borrowed
    /// value.
    ///
    /// ```rust
    /// # use shred::cell::{TrustCell, RefMut};
    ///
    /// let c = TrustCell::new((5, 'b'));
    ///
    /// let b1: RefMut<'_, (u32, char)> = c.borrow_mut();
    /// let b2: RefMut<'_, u32> = RefMut::map(b1, |t| &mut t.0);
    /// assert_eq!(*b2, 5);
    /// ```
    pub fn map<U, F>(self, f: F) -> RefMut<'a, U>
    where
        F: FnOnce(&mut T) -> &mut U,
        U: ?Sized,
    {
        // Extract the values from the `RefMut` through a pointer so that we do not run
        // `Drop`. Because the returned `RefMut` has the same lifetime `'a` as
        // the given `RefMut`, the lifetime we created through turning the
        // pointer into a ref is valid.
        let flag = unsafe { &*(self.flag as *const _) };
        let value = unsafe { &mut *(self.value as *mut _) };

        // We have to forget self so that we do not run `Drop`. Further it's safe
        // because we are creating a new `RefMut`, with the same flag, which
        // will run the cleanup when it's dropped.
        std::mem::forget(self);

        RefMut {
            flag,
            value: f(value),
        }
    }
}

impl<'a, T: ?Sized> Deref for RefMut<'a, T> {
    type Target = T;

    fn deref(&self) -> &T {
        self.value
    }
}

impl<'a, T: ?Sized> DerefMut for RefMut<'a, T> {
    fn deref_mut(&mut self) -> &mut T {
        self.value
    }
}

impl<'a, T: ?Sized> Drop for RefMut<'a, T> {
    fn drop(&mut self) {
        self.flag.store(0, Ordering::Release)
    }
}

/// A custom cell container that is a `RefCell` with thread-safety.
#[derive(Debug)]
pub struct TrustCell<T> {
    flag: AtomicUsize,
    inner: UnsafeCell<T>,
}

impl<T> TrustCell<T> {
    /// Create a new cell, similar to `RefCell::new`
    pub fn new(val: T) -> Self {
        TrustCell {
            flag: AtomicUsize::new(0),
            inner: UnsafeCell::new(val),
        }
    }

    /// Consumes this cell and returns ownership of `T`.
    pub fn into_inner(self) -> T {
        self.inner.into_inner()
    }

    /// Get an immutable reference to the inner data.
    ///
    /// Absence of write accesses is checked at run-time.
    ///
    /// # Panics
    ///
    /// This function will panic if there is a mutable reference to the data
    /// already in use.
    pub fn borrow(&self) -> Ref<T> {
        self.check_flag_read()
            .unwrap_or_else(|_| borrow_panic!(" mutably"));

        Ref {
            flag: &self.flag,
            value: unsafe { &*self.inner.get() },
        }
    }

    /// Get an immutable reference to the inner data.
    ///
    /// Absence of write accesses is checked at run-time. If access is not
    /// possible, an error is returned.
    pub fn try_borrow(&self) -> Result<Ref<T>, InvalidBorrow> {
        self.check_flag_read()?;

        Ok(Ref {
            flag: &self.flag,
            value: unsafe { &*self.inner.get() },
        })
    }

    /// Get a mutable reference to the inner data.
    ///
    /// Exclusive access is checked at run-time.
    ///
    /// # Panics
    ///
    /// This function will panic if there are any references to the data already
    /// in use.
    pub fn borrow_mut(&self) -> RefMut<T> {
        self.check_flag_write()
            .unwrap_or_else(|_| borrow_panic!(""));

        RefMut {
            flag: &self.flag,
            value: unsafe { &mut *self.inner.get() },
        }
    }

    /// Get a mutable reference to the inner data.
    ///
    /// Exclusive access is checked at run-time. If access is not possible, an
    /// error is returned.
    pub fn try_borrow_mut(&self) -> Result<RefMut<T>, InvalidBorrow> {
        self.check_flag_write()?;

        Ok(RefMut {
            flag: &self.flag,
            value: unsafe { &mut *self.inner.get() },
        })
    }

    /// Gets exclusive access to the inner value, bypassing the Cell.
    ///
    /// Exclusive access is checked at compile time.
    pub fn get_mut(&mut self) -> &mut T {
        // safe because we have exclusive access via &mut self
        unsafe { &mut *self.inner.get() }
    }

    /// Make sure we are allowed to aquire a read lock, and increment the read
    /// count by 1
    fn check_flag_read(&self) -> Result<(), InvalidBorrow> {
        // Check that no write reference is out, then try to increment the read count
        // and return once successful.
        loop {
            let val = self.flag.load(Ordering::Acquire);

            if val == usize::MAX {
                return Err(InvalidBorrow);
            }

            if self.flag.compare_and_swap(val, val + 1, Ordering::AcqRel) == val {
                return Ok(());
            }
        }
    }

    /// Make sure we are allowed to aquire a write lock, and then set the write
    /// lock flag.
    fn check_flag_write(&self) -> Result<(), InvalidBorrow> {
        // Check we have 0 references out, and then set the ref count to usize::MAX to
        // indicate a write lock.
        match self.flag.compare_and_swap(0, usize::MAX, Ordering::AcqRel) {
            0 => Ok(()),
            _ => Err(InvalidBorrow),
        }
    }
}

unsafe impl<T> Sync for TrustCell<T> where T: Sync {}

impl<T> Default for TrustCell<T>
where
    T: Default,
{
    fn default() -> Self {
        TrustCell::new(Default::default())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn allow_multiple_reads() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let a = cell.borrow();
        let b = cell.borrow();

        assert_eq!(10, *a + *b);
    }

    #[test]
    fn allow_clone_reads() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let a = cell.borrow();
        let b = a.clone();

        assert_eq!(10, *a + *b);
    }

    #[test]
    fn allow_single_write() {
        let cell: TrustCell<_> = TrustCell::new(5);

        {
            let mut a = cell.borrow_mut();
            *a += 2;
            *a += 3;
        }

        assert_eq!(10, *cell.borrow());
    }

    #[test]
    #[should_panic(expected = "but it was already borrowed mutably")]
    fn panic_write_and_read() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let mut a = cell.borrow_mut();
        *a = 7;

        assert_eq!(7, *cell.borrow());
    }

    #[test]
    #[should_panic(expected = "but it was already borrowed")]
    fn panic_write_and_write() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let mut a = cell.borrow_mut();
        *a = 7;

        assert_eq!(7, *cell.borrow_mut());
    }

    #[test]
    #[should_panic(expected = "but it was already borrowed")]
    fn panic_read_and_write() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let _a = cell.borrow();

        assert_eq!(7, *cell.borrow_mut());
    }

    #[test]
    fn try_write_and_read() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let mut a = cell.try_borrow_mut().unwrap();
        *a = 7;

        assert!(cell.try_borrow().is_err());
    }

    #[test]
    fn try_write_and_write() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let mut a = cell.try_borrow_mut().unwrap();
        *a = 7;

        assert!(cell.try_borrow_mut().is_err());
    }

    #[test]
    fn try_read_and_write() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let _a = cell.try_borrow().unwrap();

        assert!(cell.try_borrow_mut().is_err());
    }

    #[test]
    fn cloned_borrow_does_not_allow_write() {
        let cell: TrustCell<_> = TrustCell::new(5);

        let a = cell.borrow();
        let _b = a.clone();
        drop(a);

        assert!(cell.try_borrow_mut().is_err());
    }

    #[test]
    fn ref_with_non_sized() {
        let r: Ref<'_, [i32]> = Ref {
            flag: &AtomicUsize::new(1),
            value: &[2, 3, 4, 5][..],
        };

        assert_eq!(&*r, &[2, 3, 4, 5][..]);
    }

    #[test]
    fn ref_with_non_sized_clone() {
        let r: Ref<'_, [i32]> = Ref {
            flag: &AtomicUsize::new(1),
            value: &[2, 3, 4, 5][..],
        };
        let rr = r.clone();

        assert_eq!(&*r, &[2, 3, 4, 5][..]);
        assert_eq!(r.flag.load(Ordering::SeqCst), 2);

        assert_eq!(&*rr, &[2, 3, 4, 5][..]);
        assert_eq!(rr.flag.load(Ordering::SeqCst), 2);
    }

    #[test]
    fn ref_with_trait_obj() {
        let ra: Ref<'_, dyn std::any::Any> = Ref {
            flag: &AtomicUsize::new(1),
            value: &2i32,
        };

        assert_eq!(ra.downcast_ref::<i32>().unwrap(), &2i32);
    }

    #[test]
    fn ref_mut_with_non_sized() {
        let mut r: RefMut<'_, [i32]> = RefMut {
            flag: &AtomicUsize::new(1),
            value: &mut [2, 3, 4, 5][..],
        };

        assert_eq!(&mut *r, &mut [2, 3, 4, 5][..]);
    }

    #[test]
    fn ref_mut_with_trait_obj() {
        let mut ra: RefMut<'_, dyn std::any::Any> = RefMut {
            flag: &AtomicUsize::new(1),
            value: &mut 2i32,
        };

        assert_eq!(ra.downcast_mut::<i32>().unwrap(), &mut 2i32);
    }

    #[test]
    fn ref_map_box() {
        let cell = TrustCell::new(Box::new(10));

        let r: Ref<'_, Box<usize>> = cell.borrow();
        assert_eq!(&**r, &10);

        let rr: Ref<'_, usize> = cell.borrow().map(Box::as_ref);
        assert_eq!(&*rr, &10);
    }

    #[test]
    fn ref_map_preserves_flag() {
        let cell = TrustCell::new(Box::new(10));

        let r: Ref<'_, Box<usize>> = cell.borrow();
        assert_eq!(cell.flag.load(Ordering::SeqCst), 1);
        let _nr: Ref<'_, usize> = r.map(Box::as_ref);
        assert_eq!(cell.flag.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn ref_map_retains_borrow() {
        let cell = TrustCell::new(Box::new(10));

        let _r: Ref<'_, usize> = cell.borrow().map(Box::as_ref);
        assert_eq!(cell.flag.load(Ordering::SeqCst), 1);

        let _rr: Ref<'_, usize> = cell.borrow().map(Box::as_ref);
        assert_eq!(cell.flag.load(Ordering::SeqCst), 2);
    }

    #[test]
    fn ref_map_drops_borrow() {
        let cell = TrustCell::new(Box::new(10));

        let r: Ref<'_, usize> = cell.borrow().map(Box::as_ref);

        assert_eq!(cell.flag.load(Ordering::SeqCst), 1);
        drop(r);
        assert_eq!(cell.flag.load(Ordering::SeqCst), 0);
    }

    #[test]
    fn ref_mut_map_box() {
        let cell = TrustCell::new(Box::new(10));

        {
            let mut r: RefMut<'_, Box<usize>> = cell.borrow_mut();
            assert_eq!(&mut **r, &mut 10);
        }
        {
            let mut rr: RefMut<'_, usize> = cell.borrow_mut().map(Box::as_mut);
            assert_eq!(&mut *rr, &mut 10);
        }
    }

    #[test]
    fn ref_mut_map_preserves_flag() {
        let cell = TrustCell::new(Box::new(10));

        let r: RefMut<'_, Box<usize>> = cell.borrow_mut();
        assert_eq!(cell.flag.load(Ordering::SeqCst), std::usize::MAX);
        let _nr: RefMut<'_, usize> = r.map(Box::as_mut);
        assert_eq!(cell.flag.load(Ordering::SeqCst), std::usize::MAX);
    }

    #[test]
    #[should_panic(expected = "but it was already borrowed")]
    fn ref_mut_map_retains_mut_borrow() {
        let cell = TrustCell::new(Box::new(10));

        let _rr: RefMut<'_, usize> = cell.borrow_mut().map(Box::as_mut);

        let _ = cell.borrow_mut();
    }

    #[test]
    fn ref_mut_map_drops_borrow() {
        let cell = TrustCell::new(Box::new(10));

        let r: RefMut<'_, usize> = cell.borrow_mut().map(Box::as_mut);

        assert_eq!(cell.flag.load(Ordering::SeqCst), std::usize::MAX);
        drop(r);
        assert_eq!(cell.flag.load(Ordering::SeqCst), 0);
    }
}