pyo3/pycell/
impl_.rs

1#![allow(missing_docs)]
2//! Crate-private implementation of PyClassObject
3
4use std::cell::UnsafeCell;
5use std::marker::PhantomData;
6use std::mem::ManuallyDrop;
7use std::sync::atomic::{AtomicUsize, Ordering};
8
9use crate::impl_::pyclass::{
10    PyClassBaseType, PyClassDict, PyClassImpl, PyClassThreadChecker, PyClassWeakRef,
11};
12use crate::internal::get_slot::TP_FREE;
13use crate::type_object::{PyLayout, PySizedLayout};
14use crate::types::{PyType, PyTypeMethods};
15use crate::{ffi, PyClass, PyTypeInfo, Python};
16
17use super::{PyBorrowError, PyBorrowMutError};
18
19pub trait PyClassMutability {
20    // The storage for this inheritance layer. Only the first mutable class in
21    // an inheritance hierarchy needs to store the borrow flag.
22    type Storage: PyClassBorrowChecker;
23    // The borrow flag needed to implement this class' mutability. Empty until
24    // the first mutable class, at which point it is BorrowChecker and will be
25    // for all subclasses.
26    type Checker: PyClassBorrowChecker;
27    type ImmutableChild: PyClassMutability;
28    type MutableChild: PyClassMutability;
29}
30
31pub struct ImmutableClass(());
32pub struct MutableClass(());
33pub struct ExtendsMutableAncestor<M: PyClassMutability>(PhantomData<M>);
34
35impl PyClassMutability for ImmutableClass {
36    type Storage = EmptySlot;
37    type Checker = EmptySlot;
38    type ImmutableChild = ImmutableClass;
39    type MutableChild = MutableClass;
40}
41
42impl PyClassMutability for MutableClass {
43    type Storage = BorrowChecker;
44    type Checker = BorrowChecker;
45    type ImmutableChild = ExtendsMutableAncestor<ImmutableClass>;
46    type MutableChild = ExtendsMutableAncestor<MutableClass>;
47}
48
49impl<M: PyClassMutability> PyClassMutability for ExtendsMutableAncestor<M> {
50    type Storage = EmptySlot;
51    type Checker = BorrowChecker;
52    type ImmutableChild = ExtendsMutableAncestor<ImmutableClass>;
53    type MutableChild = ExtendsMutableAncestor<MutableClass>;
54}
55
56#[derive(Debug)]
57struct BorrowFlag(AtomicUsize);
58
59impl BorrowFlag {
60    pub(crate) const UNUSED: usize = 0;
61    const HAS_MUTABLE_BORROW: usize = usize::MAX;
62    fn increment(&self) -> Result<(), PyBorrowError> {
63        // relaxed is OK because we will read the value again in the compare_exchange
64        let mut value = self.0.load(Ordering::Relaxed);
65        loop {
66            if value == BorrowFlag::HAS_MUTABLE_BORROW {
67                return Err(PyBorrowError { _private: () });
68            }
69            match self.0.compare_exchange(
70                // only increment if the value hasn't changed since the
71                // last atomic load
72                value,
73                value + 1,
74                // reading the value is happens-after a previous write
75                // writing the new value is happens-after the previous read
76                Ordering::AcqRel,
77                // relaxed is OK here because we're going to try to read again
78                Ordering::Relaxed,
79            ) {
80                Ok(..) => {
81                    break Ok(());
82                }
83                Err(changed_value) => {
84                    // value changed under us, need to try again
85                    value = changed_value;
86                }
87            }
88        }
89    }
90    fn decrement(&self) {
91        // relaxed load is OK but decrements must happen-before the next read
92        self.0.fetch_sub(1, Ordering::Release);
93    }
94}
95
96pub struct EmptySlot(());
97pub struct BorrowChecker(BorrowFlag);
98
99pub trait PyClassBorrowChecker {
100    /// Initial value for self
101    fn new() -> Self;
102
103    /// Increments immutable borrow count, if possible
104    fn try_borrow(&self) -> Result<(), PyBorrowError>;
105
106    /// Decrements immutable borrow count
107    fn release_borrow(&self);
108    /// Increments mutable borrow count, if possible
109    fn try_borrow_mut(&self) -> Result<(), PyBorrowMutError>;
110    /// Decremements mutable borrow count
111    fn release_borrow_mut(&self);
112}
113
114impl PyClassBorrowChecker for EmptySlot {
115    #[inline]
116    fn new() -> Self {
117        EmptySlot(())
118    }
119
120    #[inline]
121    fn try_borrow(&self) -> Result<(), PyBorrowError> {
122        Ok(())
123    }
124
125    #[inline]
126    fn release_borrow(&self) {}
127
128    #[inline]
129    fn try_borrow_mut(&self) -> Result<(), PyBorrowMutError> {
130        unreachable!()
131    }
132
133    #[inline]
134    fn release_borrow_mut(&self) {
135        unreachable!()
136    }
137}
138
139impl PyClassBorrowChecker for BorrowChecker {
140    #[inline]
141    fn new() -> Self {
142        Self(BorrowFlag(AtomicUsize::new(BorrowFlag::UNUSED)))
143    }
144
145    fn try_borrow(&self) -> Result<(), PyBorrowError> {
146        self.0.increment()
147    }
148
149    fn release_borrow(&self) {
150        self.0.decrement();
151    }
152
153    fn try_borrow_mut(&self) -> Result<(), PyBorrowMutError> {
154        let flag = &self.0;
155        match flag.0.compare_exchange(
156            // only allowed to transition to mutable borrow if the reference is
157            // currently unused
158            BorrowFlag::UNUSED,
159            BorrowFlag::HAS_MUTABLE_BORROW,
160            // On success, reading the flag and updating its state are an atomic
161            // operation
162            Ordering::AcqRel,
163            // It doesn't matter precisely when the failure gets turned
164            // into an error
165            Ordering::Relaxed,
166        ) {
167            Ok(..) => Ok(()),
168            Err(..) => Err(PyBorrowMutError { _private: () }),
169        }
170    }
171
172    fn release_borrow_mut(&self) {
173        self.0 .0.store(BorrowFlag::UNUSED, Ordering::Release)
174    }
175}
176
177pub trait GetBorrowChecker<T: PyClassImpl> {
178    fn borrow_checker(
179        class_object: &PyClassObject<T>,
180    ) -> &<T::PyClassMutability as PyClassMutability>::Checker;
181}
182
183impl<T: PyClassImpl<PyClassMutability = Self>> GetBorrowChecker<T> for MutableClass {
184    fn borrow_checker(class_object: &PyClassObject<T>) -> &BorrowChecker {
185        &class_object.contents.borrow_checker
186    }
187}
188
189impl<T: PyClassImpl<PyClassMutability = Self>> GetBorrowChecker<T> for ImmutableClass {
190    fn borrow_checker(class_object: &PyClassObject<T>) -> &EmptySlot {
191        &class_object.contents.borrow_checker
192    }
193}
194
195impl<T: PyClassImpl<PyClassMutability = Self>, M: PyClassMutability> GetBorrowChecker<T>
196    for ExtendsMutableAncestor<M>
197where
198    T::BaseType: PyClassImpl + PyClassBaseType<LayoutAsBase = PyClassObject<T::BaseType>>,
199    <T::BaseType as PyClassImpl>::PyClassMutability: PyClassMutability<Checker = BorrowChecker>,
200{
201    fn borrow_checker(class_object: &PyClassObject<T>) -> &BorrowChecker {
202        <<T::BaseType as PyClassImpl>::PyClassMutability as GetBorrowChecker<T::BaseType>>::borrow_checker(&class_object.ob_base)
203    }
204}
205
206/// Base layout of PyClassObject.
207#[doc(hidden)]
208#[repr(C)]
209pub struct PyClassObjectBase<T> {
210    ob_base: T,
211}
212
213unsafe impl<T, U> PyLayout<T> for PyClassObjectBase<U> where U: PySizedLayout<T> {}
214
215#[doc(hidden)]
216pub trait PyClassObjectLayout<T>: PyLayout<T> {
217    fn ensure_threadsafe(&self);
218    fn check_threadsafe(&self) -> Result<(), PyBorrowError>;
219    /// Implementation of tp_dealloc.
220    /// # Safety
221    /// - slf must be a valid pointer to an instance of a T or a subclass.
222    /// - slf must not be used after this call (as it will be freed).
223    unsafe fn tp_dealloc(py: Python<'_>, slf: *mut ffi::PyObject);
224}
225
226impl<T, U> PyClassObjectLayout<T> for PyClassObjectBase<U>
227where
228    U: PySizedLayout<T>,
229    T: PyTypeInfo,
230{
231    fn ensure_threadsafe(&self) {}
232    fn check_threadsafe(&self) -> Result<(), PyBorrowError> {
233        Ok(())
234    }
235    unsafe fn tp_dealloc(py: Python<'_>, slf: *mut ffi::PyObject) {
236        unsafe {
237            // FIXME: there is potentially subtle issues here if the base is overwritten
238            // at runtime? To be investigated.
239            let type_obj = T::type_object(py);
240            let type_ptr = type_obj.as_type_ptr();
241            let actual_type = PyType::from_borrowed_type_ptr(py, ffi::Py_TYPE(slf));
242
243            // For `#[pyclass]` types which inherit from PyAny, we can just call tp_free
244            if std::ptr::eq(type_ptr, std::ptr::addr_of!(ffi::PyBaseObject_Type)) {
245                let tp_free = actual_type
246                    .get_slot(TP_FREE)
247                    .expect("PyBaseObject_Type should have tp_free");
248                return tp_free(slf.cast());
249            }
250
251            // More complex native types (e.g. `extends=PyDict`) require calling the base's dealloc.
252            #[cfg(not(Py_LIMITED_API))]
253            {
254                // FIXME: should this be using actual_type.tp_dealloc?
255                if let Some(dealloc) = (*type_ptr).tp_dealloc {
256                    // Before CPython 3.11 BaseException_dealloc would use Py_GC_UNTRACK which
257                    // assumes the exception is currently GC tracked, so we have to re-track
258                    // before calling the dealloc so that it can safely call Py_GC_UNTRACK.
259                    #[cfg(not(any(Py_3_11, PyPy)))]
260                    if ffi::PyType_FastSubclass(type_ptr, ffi::Py_TPFLAGS_BASE_EXC_SUBCLASS) == 1 {
261                        ffi::PyObject_GC_Track(slf.cast());
262                    }
263                    dealloc(slf);
264                } else {
265                    (*actual_type.as_type_ptr())
266                        .tp_free
267                        .expect("type missing tp_free")(slf.cast());
268                }
269            }
270
271            #[cfg(Py_LIMITED_API)]
272            unreachable!("subclassing native types is not possible with the `abi3` feature");
273        }
274    }
275}
276
277/// The layout of a PyClass as a Python object
278#[repr(C)]
279pub struct PyClassObject<T: PyClassImpl> {
280    pub(crate) ob_base: <T::BaseType as PyClassBaseType>::LayoutAsBase,
281    pub(crate) contents: PyClassObjectContents<T>,
282}
283
284#[repr(C)]
285pub(crate) struct PyClassObjectContents<T: PyClassImpl> {
286    pub(crate) value: ManuallyDrop<UnsafeCell<T>>,
287    pub(crate) borrow_checker: <T::PyClassMutability as PyClassMutability>::Storage,
288    pub(crate) thread_checker: T::ThreadChecker,
289    pub(crate) dict: T::Dict,
290    pub(crate) weakref: T::WeakRef,
291}
292
293impl<T: PyClassImpl> PyClassObject<T> {
294    pub(crate) fn get_ptr(&self) -> *mut T {
295        self.contents.value.get()
296    }
297
298    /// Gets the offset of the dictionary from the start of the struct in bytes.
299    pub(crate) fn dict_offset() -> ffi::Py_ssize_t {
300        use memoffset::offset_of;
301
302        let offset =
303            offset_of!(PyClassObject<T>, contents) + offset_of!(PyClassObjectContents<T>, dict);
304
305        // Py_ssize_t may not be equal to isize on all platforms
306        #[allow(clippy::useless_conversion)]
307        offset.try_into().expect("offset should fit in Py_ssize_t")
308    }
309
310    /// Gets the offset of the weakref list from the start of the struct in bytes.
311    pub(crate) fn weaklist_offset() -> ffi::Py_ssize_t {
312        use memoffset::offset_of;
313
314        let offset =
315            offset_of!(PyClassObject<T>, contents) + offset_of!(PyClassObjectContents<T>, weakref);
316
317        // Py_ssize_t may not be equal to isize on all platforms
318        #[allow(clippy::useless_conversion)]
319        offset.try_into().expect("offset should fit in Py_ssize_t")
320    }
321}
322
323impl<T: PyClassImpl> PyClassObject<T> {
324    pub(crate) fn borrow_checker(&self) -> &<T::PyClassMutability as PyClassMutability>::Checker {
325        T::PyClassMutability::borrow_checker(self)
326    }
327}
328
329unsafe impl<T: PyClassImpl> PyLayout<T> for PyClassObject<T> {}
330impl<T: PyClass> PySizedLayout<T> for PyClassObject<T> {}
331
332impl<T: PyClassImpl> PyClassObjectLayout<T> for PyClassObject<T>
333where
334    <T::BaseType as PyClassBaseType>::LayoutAsBase: PyClassObjectLayout<T::BaseType>,
335{
336    fn ensure_threadsafe(&self) {
337        self.contents.thread_checker.ensure();
338        self.ob_base.ensure_threadsafe();
339    }
340    fn check_threadsafe(&self) -> Result<(), PyBorrowError> {
341        if !self.contents.thread_checker.check() {
342            return Err(PyBorrowError { _private: () });
343        }
344        self.ob_base.check_threadsafe()
345    }
346    unsafe fn tp_dealloc(py: Python<'_>, slf: *mut ffi::PyObject) {
347        // Safety: Python only calls tp_dealloc when no references to the object remain.
348        let class_object = unsafe { &mut *(slf.cast::<PyClassObject<T>>()) };
349        if class_object.contents.thread_checker.can_drop(py) {
350            unsafe { ManuallyDrop::drop(&mut class_object.contents.value) };
351        }
352        class_object.contents.dict.clear_dict(py);
353        unsafe {
354            class_object.contents.weakref.clear_weakrefs(slf, py);
355            <T::BaseType as PyClassBaseType>::LayoutAsBase::tp_dealloc(py, slf)
356        }
357    }
358}
359
360#[cfg(test)]
361#[cfg(feature = "macros")]
362mod tests {
363    use super::*;
364
365    use crate::prelude::*;
366    use crate::pyclass::boolean_struct::{False, True};
367
368    #[pyclass(crate = "crate", subclass)]
369    struct MutableBase;
370
371    #[pyclass(crate = "crate", extends = MutableBase, subclass)]
372    struct MutableChildOfMutableBase;
373
374    #[pyclass(crate = "crate", extends = MutableBase, frozen, subclass)]
375    struct ImmutableChildOfMutableBase;
376
377    #[pyclass(crate = "crate", extends = MutableChildOfMutableBase)]
378    struct MutableChildOfMutableChildOfMutableBase;
379
380    #[pyclass(crate = "crate", extends = ImmutableChildOfMutableBase)]
381    struct MutableChildOfImmutableChildOfMutableBase;
382
383    #[pyclass(crate = "crate", extends = MutableChildOfMutableBase, frozen)]
384    struct ImmutableChildOfMutableChildOfMutableBase;
385
386    #[pyclass(crate = "crate", extends = ImmutableChildOfMutableBase, frozen)]
387    struct ImmutableChildOfImmutableChildOfMutableBase;
388
389    #[pyclass(crate = "crate", frozen, subclass)]
390    struct ImmutableBase;
391
392    #[pyclass(crate = "crate", extends = ImmutableBase, subclass)]
393    struct MutableChildOfImmutableBase;
394
395    #[pyclass(crate = "crate", extends = ImmutableBase, frozen, subclass)]
396    struct ImmutableChildOfImmutableBase;
397
398    #[pyclass(crate = "crate", extends = MutableChildOfImmutableBase)]
399    struct MutableChildOfMutableChildOfImmutableBase;
400
401    #[pyclass(crate = "crate", extends = ImmutableChildOfImmutableBase)]
402    struct MutableChildOfImmutableChildOfImmutableBase;
403
404    #[pyclass(crate = "crate", extends = MutableChildOfImmutableBase, frozen)]
405    struct ImmutableChildOfMutableChildOfImmutableBase;
406
407    #[pyclass(crate = "crate", extends = ImmutableChildOfImmutableBase, frozen)]
408    struct ImmutableChildOfImmutableChildOfImmutableBase;
409
410    fn assert_mutable<T: PyClass<Frozen = False, PyClassMutability = MutableClass>>() {}
411    fn assert_immutable<T: PyClass<Frozen = True, PyClassMutability = ImmutableClass>>() {}
412    fn assert_mutable_with_mutable_ancestor<
413        T: PyClass<Frozen = False, PyClassMutability = ExtendsMutableAncestor<MutableClass>>,
414    >() {
415    }
416    fn assert_immutable_with_mutable_ancestor<
417        T: PyClass<Frozen = True, PyClassMutability = ExtendsMutableAncestor<ImmutableClass>>,
418    >() {
419    }
420
421    #[test]
422    fn test_inherited_mutability() {
423        // mutable base
424        assert_mutable::<MutableBase>();
425
426        // children of mutable base have a mutable ancestor
427        assert_mutable_with_mutable_ancestor::<MutableChildOfMutableBase>();
428        assert_immutable_with_mutable_ancestor::<ImmutableChildOfMutableBase>();
429
430        // grandchildren of mutable base have a mutable ancestor
431        assert_mutable_with_mutable_ancestor::<MutableChildOfMutableChildOfMutableBase>();
432        assert_mutable_with_mutable_ancestor::<MutableChildOfImmutableChildOfMutableBase>();
433        assert_immutable_with_mutable_ancestor::<ImmutableChildOfMutableChildOfMutableBase>();
434        assert_immutable_with_mutable_ancestor::<ImmutableChildOfImmutableChildOfMutableBase>();
435
436        // immutable base and children
437        assert_immutable::<ImmutableBase>();
438        assert_immutable::<ImmutableChildOfImmutableBase>();
439        assert_immutable::<ImmutableChildOfImmutableChildOfImmutableBase>();
440
441        // mutable children of immutable at any level are simply mutable
442        assert_mutable::<MutableChildOfImmutableBase>();
443        assert_mutable::<MutableChildOfImmutableChildOfImmutableBase>();
444
445        // children of the mutable child display this property
446        assert_mutable_with_mutable_ancestor::<MutableChildOfMutableChildOfImmutableBase>();
447        assert_immutable_with_mutable_ancestor::<ImmutableChildOfMutableChildOfImmutableBase>();
448    }
449
450    #[test]
451    fn test_mutable_borrow_prevents_further_borrows() {
452        Python::with_gil(|py| {
453            let mmm = Py::new(
454                py,
455                PyClassInitializer::from(MutableBase)
456                    .add_subclass(MutableChildOfMutableBase)
457                    .add_subclass(MutableChildOfMutableChildOfMutableBase),
458            )
459            .unwrap();
460
461            let mmm_bound: &Bound<'_, MutableChildOfMutableChildOfMutableBase> = mmm.bind(py);
462
463            let mmm_refmut = mmm_bound.borrow_mut();
464
465            // Cannot take any other mutable or immutable borrows whilst the object is borrowed mutably
466            assert!(mmm_bound
467                .extract::<PyRef<'_, MutableChildOfMutableChildOfMutableBase>>()
468                .is_err());
469            assert!(mmm_bound
470                .extract::<PyRef<'_, MutableChildOfMutableBase>>()
471                .is_err());
472            assert!(mmm_bound.extract::<PyRef<'_, MutableBase>>().is_err());
473            assert!(mmm_bound
474                .extract::<PyRefMut<'_, MutableChildOfMutableChildOfMutableBase>>()
475                .is_err());
476            assert!(mmm_bound
477                .extract::<PyRefMut<'_, MutableChildOfMutableBase>>()
478                .is_err());
479            assert!(mmm_bound.extract::<PyRefMut<'_, MutableBase>>().is_err());
480
481            // With the borrow dropped, all other borrow attempts will succeed
482            drop(mmm_refmut);
483
484            assert!(mmm_bound
485                .extract::<PyRef<'_, MutableChildOfMutableChildOfMutableBase>>()
486                .is_ok());
487            assert!(mmm_bound
488                .extract::<PyRef<'_, MutableChildOfMutableBase>>()
489                .is_ok());
490            assert!(mmm_bound.extract::<PyRef<'_, MutableBase>>().is_ok());
491            assert!(mmm_bound
492                .extract::<PyRefMut<'_, MutableChildOfMutableChildOfMutableBase>>()
493                .is_ok());
494            assert!(mmm_bound
495                .extract::<PyRefMut<'_, MutableChildOfMutableBase>>()
496                .is_ok());
497            assert!(mmm_bound.extract::<PyRefMut<'_, MutableBase>>().is_ok());
498        })
499    }
500
501    #[test]
502    fn test_immutable_borrows_prevent_mutable_borrows() {
503        Python::with_gil(|py| {
504            let mmm = Py::new(
505                py,
506                PyClassInitializer::from(MutableBase)
507                    .add_subclass(MutableChildOfMutableBase)
508                    .add_subclass(MutableChildOfMutableChildOfMutableBase),
509            )
510            .unwrap();
511
512            let mmm_bound: &Bound<'_, MutableChildOfMutableChildOfMutableBase> = mmm.bind(py);
513
514            let mmm_refmut = mmm_bound.borrow();
515
516            // Further immutable borrows are ok
517            assert!(mmm_bound
518                .extract::<PyRef<'_, MutableChildOfMutableChildOfMutableBase>>()
519                .is_ok());
520            assert!(mmm_bound
521                .extract::<PyRef<'_, MutableChildOfMutableBase>>()
522                .is_ok());
523            assert!(mmm_bound.extract::<PyRef<'_, MutableBase>>().is_ok());
524
525            // Further mutable borrows are not ok
526            assert!(mmm_bound
527                .extract::<PyRefMut<'_, MutableChildOfMutableChildOfMutableBase>>()
528                .is_err());
529            assert!(mmm_bound
530                .extract::<PyRefMut<'_, MutableChildOfMutableBase>>()
531                .is_err());
532            assert!(mmm_bound.extract::<PyRefMut<'_, MutableBase>>().is_err());
533
534            // With the borrow dropped, all mutable borrow attempts will succeed
535            drop(mmm_refmut);
536
537            assert!(mmm_bound
538                .extract::<PyRefMut<'_, MutableChildOfMutableChildOfMutableBase>>()
539                .is_ok());
540            assert!(mmm_bound
541                .extract::<PyRefMut<'_, MutableChildOfMutableBase>>()
542                .is_ok());
543            assert!(mmm_bound.extract::<PyRefMut<'_, MutableBase>>().is_ok());
544        })
545    }
546
547    #[test]
548    #[cfg(not(target_arch = "wasm32"))]
549    fn test_thread_safety() {
550        #[crate::pyclass(crate = "crate")]
551        struct MyClass {
552            x: u64,
553        }
554
555        Python::with_gil(|py| {
556            let inst = Py::new(py, MyClass { x: 0 }).unwrap();
557
558            let total_modifications = py.allow_threads(|| {
559                std::thread::scope(|s| {
560                    // Spawn a bunch of threads all racing to write to
561                    // the same instance of `MyClass`.
562                    let threads = (0..10)
563                        .map(|_| {
564                            s.spawn(|| {
565                                Python::with_gil(|py| {
566                                    // Each thread records its own view of how many writes it made
567                                    let mut local_modifications = 0;
568                                    for _ in 0..100 {
569                                        if let Ok(mut i) = inst.try_borrow_mut(py) {
570                                            i.x += 1;
571                                            local_modifications += 1;
572                                        }
573                                    }
574                                    local_modifications
575                                })
576                            })
577                        })
578                        .collect::<Vec<_>>();
579
580                    // Sum up the total number of writes made by all threads
581                    threads.into_iter().map(|t| t.join().unwrap()).sum::<u64>()
582                })
583            });
584
585            // If the implementation is free of data races, the total number of writes
586            // should match the final value of `x`.
587            assert_eq!(total_modifications, inst.borrow(py).x);
588        });
589    }
590
591    #[test]
592    #[cfg(not(target_arch = "wasm32"))]
593    fn test_thread_safety_2() {
594        struct SyncUnsafeCell<T>(UnsafeCell<T>);
595        unsafe impl<T> Sync for SyncUnsafeCell<T> {}
596
597        impl<T> SyncUnsafeCell<T> {
598            fn get(&self) -> *mut T {
599                self.0.get()
600            }
601        }
602
603        let data = SyncUnsafeCell(UnsafeCell::new(0));
604        let data2 = SyncUnsafeCell(UnsafeCell::new(0));
605        let borrow_checker = BorrowChecker(BorrowFlag(AtomicUsize::new(BorrowFlag::UNUSED)));
606
607        std::thread::scope(|s| {
608            s.spawn(|| {
609                for _ in 0..1_000_000 {
610                    if borrow_checker.try_borrow_mut().is_ok() {
611                        // thread 1 writes to both values during the mutable borrow
612                        unsafe { *data.get() += 1 };
613                        unsafe { *data2.get() += 1 };
614                        borrow_checker.release_borrow_mut();
615                    }
616                }
617            });
618
619            s.spawn(|| {
620                for _ in 0..1_000_000 {
621                    if borrow_checker.try_borrow().is_ok() {
622                        // if the borrow checker is working correctly, it should be impossible
623                        // for thread 2 to observe a difference in the two values
624                        assert_eq!(unsafe { *data.get() }, unsafe { *data2.get() });
625                        borrow_checker.release_borrow();
626                    }
627                }
628            });
629        });
630    }
631}
pyo3/pycell/impl_.rs

pyo3/pycell/
impl_.rs
