rust-cpython: leverage RefCell::borrow() to guarantee there's no mutable ref

Since the underlying value can't be mutably borrowed by PyLeaked, we don't
have to manage yet another mutably-borrowed state. We can just rely on the
RefCell implementation.

Maybe we can add try_leak_immutable(), but this patch doesn't in order to
keep the patch series not too long.

rust/hg-cpython/src/ref_sharing.rs

@@ -1,705 +1,717 b''
 // ref_sharing.rs
 //
 // Copyright 2019 Raphaël Gomès <[e-mail]>
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to
 // deal in the Software without restriction, including without limitation the
 // rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 // sell copies of the Software, and to permit persons to whom the Software is
 // furnished to do so, subject to the following conditions:
 //
 // The above copyright notice and this permission notice shall be included in
 // all copies or substantial portions of the Software.
 //
 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 // IN THE SOFTWARE.
 
 //! Macros for use in the `hg-cpython` bridge library.
 
 use crate::exceptions::AlreadyBorrowed;
 use cpython::{exc, PyClone, PyErr, PyObject, PyResult, Python};
 use std::cell::{Cell, Ref, RefCell, RefMut};
 use std::ops::{Deref, DerefMut};
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 /// Manages the shared state between Python and Rust
 ///
 /// `PySharedState` is owned by `PySharedRefCell`, and is shared across its
 /// derived references. The consistency of these references are guaranteed
 /// as follows:
 ///
 /// - The immutability of `py_class!` object fields. Any mutation of
 ///   `PySharedRefCell` is allowed only through its `borrow_mut()`.
 /// - The `py: Python<'_>` token, which makes sure that any data access is
 ///   synchronized by the GIL.
+/// - The underlying `RefCell`, which prevents `PySharedRefCell` data from
+///   being directly borrowed or leaked while it is mutably borrowed.
 /// - The `borrow_count`, which is the number of references borrowed from
 ///   `PyLeaked`. Just like `RefCell`, mutation is prohibited while `PyLeaked`
 ///   is borrowed.
 /// - The `generation` counter, which increments on `borrow_mut()`. `PyLeaked`
 ///   reference is valid only if the `current_generation()` equals to the
 ///   `generation` at the time of `leak_immutable()`.
 #[derive(Debug, Default)]
 struct PySharedState {
     mutably_borrowed: Cell<bool>,
     // The counter variable could be Cell<usize> since any operation on
     // PySharedState is synchronized by the GIL, but being "atomic" makes
     // PySharedState inherently Sync. The ordering requirement doesn't
     // matter thanks to the GIL.
     borrow_count: AtomicUsize,
     generation: AtomicUsize,
 }
 
 // &PySharedState can be Send because any access to inner cells is
 // synchronized by the GIL.
 unsafe impl Sync for PySharedState {}
 
 impl PySharedState {
     fn borrow_mut<'a, T>(
         &'a self,
         py: Python<'a>,
         pyrefmut: RefMut<'a, T>,
     ) -> PyResult<PyRefMut<'a, T>> {
         if self.mutably_borrowed.get() {
             return Err(AlreadyBorrowed::new(
                 py,
                 "Cannot borrow mutably while there exists another \
                  mutable reference in a Python object",
             ));
         }
         match self.current_borrow_count(py) {
             0 => {
                 self.mutably_borrowed.replace(true);
                 // Note that this wraps around to the same value if mutably
                 // borrowed more than usize::MAX times, which wouldn't happen
                 // in practice.
                 self.generation.fetch_add(1, Ordering::Relaxed);
                 Ok(PyRefMut::new(py, pyrefmut, self))
             }
             _ => Err(AlreadyBorrowed::new(
                 py,
                 "Cannot borrow mutably while immutably borrowed",
             )),
         }
     }
 
     /// Return a reference to the wrapped data and its state with an
     /// artificial static lifetime.
     /// We need to be protected by the GIL for thread-safety.
     ///
     /// # Safety
     ///
     /// This is highly unsafe since the lifetime of the given data can be
     /// extended. Do not call this function directly.
     unsafe fn leak_immutable<T>(
         &self,
         py: Python,
-        data: &PySharedRefCell<T>,
+        data: Ref<T>,
     ) -> PyResult<(&'static T, &'static PySharedState)> {
         if self.mutably_borrowed.get() {
             return Err(AlreadyBorrowed::new(
                 py,
                 "Cannot borrow immutably while there is a \
                  mutable reference in Python objects",
             ));
         }
-        // TODO: it's weird that self is data.py_shared_state. Maybe we
-        // can move stuff to PySharedRefCell?
-        let ptr = data.as_ptr();
-        let state_ptr: *const PySharedState = &data.py_shared_state;
+        let ptr: *const T = &*data;
+        let state_ptr: *const PySharedState = self;
         Ok((&*ptr, &*state_ptr))
     }
 
     fn current_borrow_count(&self, _py: Python) -> usize {
         self.borrow_count.load(Ordering::Relaxed)
     }
 
     fn increase_borrow_count(&self, _py: Python) {
         // Note that this wraps around if there are more than usize::MAX
         // borrowed references, which shouldn't happen due to memory limit.
         self.borrow_count.fetch_add(1, Ordering::Relaxed);
     }
 
     fn decrease_borrow_count(&self, _py: Python) {
         let prev_count = self.borrow_count.fetch_sub(1, Ordering::Relaxed);
         assert!(prev_count > 0);
     }
 
     /// # Safety
     ///
     /// It's up to you to make sure the reference is about to be deleted
     /// when updating the leak count.
     fn decrease_leak_count(&self, py: Python, mutable: bool) {
         if mutable {
             assert_eq!(self.current_borrow_count(py), 0);
             assert!(self.mutably_borrowed.get());
             self.mutably_borrowed.replace(false);
         } else {
             unimplemented!();
         }
     }
 
     fn current_generation(&self, _py: Python) -> usize {
         self.generation.load(Ordering::Relaxed)
     }
 }
 
 /// Helper to keep the borrow count updated while the shared object is
 /// immutably borrowed without using the `RefCell` interface.
 struct BorrowPyShared<'a> {
     py: Python<'a>,
     py_shared_state: &'a PySharedState,
 }
 
 impl<'a> BorrowPyShared<'a> {
     fn new(
         py: Python<'a>,
         py_shared_state: &'a PySharedState,
     ) -> BorrowPyShared<'a> {
         py_shared_state.increase_borrow_count(py);
         BorrowPyShared {
             py,
             py_shared_state,
         }
     }
 }
 
 impl Drop for BorrowPyShared<'_> {
     fn drop(&mut self) {
         self.py_shared_state.decrease_borrow_count(self.py);
     }
 }
 
 /// `RefCell` wrapper to be safely used in conjunction with `PySharedState`.
 ///
 /// This object can be stored in a `py_class!` object as a data field. Any
 /// operation is allowed through the `PySharedRef` interface.
 #[derive(Debug)]
 pub struct PySharedRefCell<T> {
     inner: RefCell<T>,
     py_shared_state: PySharedState,
 }
 
 impl<T> PySharedRefCell<T> {
     pub fn new(value: T) -> PySharedRefCell<T> {
         Self {
             inner: RefCell::new(value),
             py_shared_state: PySharedState::default(),
         }
     }
 
     fn borrow<'a>(&'a self, _py: Python<'a>) -> Ref<'a, T> {
         // py_shared_state isn't involved since
         // - inner.borrow() would fail if self is mutably borrowed,
         // - and inner.borrow_mut() would fail while self is borrowed.
         self.inner.borrow()
     }
 
-    fn as_ptr(&self) -> *mut T {
-        self.inner.as_ptr()
-    }
-
     // TODO: maybe this should be named as try_borrow_mut(), and use
     // inner.try_borrow_mut(). The current implementation panics if
     // self.inner has been borrowed, but returns error if py_shared_state
     // refuses to borrow.
     fn borrow_mut<'a>(&'a self, py: Python<'a>) -> PyResult<PyRefMut<'a, T>> {
         self.py_shared_state.borrow_mut(py, self.inner.borrow_mut())
     }
 }
 
 /// Sharable data member of type `T` borrowed from the `PyObject`.
 pub struct PySharedRef<'a, T> {
     py: Python<'a>,
     owner: &'a PyObject,
     data: &'a PySharedRefCell<T>,
 }
 
 impl<'a, T> PySharedRef<'a, T> {
     /// # Safety
     ///
     /// The `data` must be owned by the `owner`. Otherwise, the leak count
     /// would get wrong.
     pub unsafe fn new(
         py: Python<'a>,
         owner: &'a PyObject,
         data: &'a PySharedRefCell<T>,
     ) -> Self {
         Self { py, owner, data }
     }
 
     pub fn borrow(&self) -> Ref<'a, T> {
         self.data.borrow(self.py)
     }
 
     pub fn borrow_mut(&self) -> PyResult<PyRefMut<'a, T>> {
         self.data.borrow_mut(self.py)
     }
 
     /// Returns a leaked reference.
+    ///
+    /// # Panics
+    ///
+    /// Panics if this is mutably borrowed.
     pub fn leak_immutable(&self) -> PyResult<PyLeaked<&'static T>> {
         let state = &self.data.py_shared_state;
+        // make sure self.data isn't mutably borrowed; otherwise the
+        // generation number can't be trusted.
+        let data_ref = self.borrow();
         unsafe {
             let (static_ref, static_state_ref) =
-                state.leak_immutable(self.py, self.data)?;
+                state.leak_immutable(self.py, data_ref)?;
             Ok(PyLeaked::new(
                 self.py,
                 self.owner,
                 static_ref,
                 static_state_ref,
             ))
         }
     }
 }
 
 /// Holds a mutable reference to data shared between Python and Rust.
 pub struct PyRefMut<'a, T> {
     py: Python<'a>,
     inner: RefMut<'a, T>,
     py_shared_state: &'a PySharedState,
 }
 
 impl<'a, T> PyRefMut<'a, T> {
     // Must be constructed by PySharedState after checking its leak_count.
     // Otherwise, drop() would incorrectly update the state.
     fn new(
         py: Python<'a>,
         inner: RefMut<'a, T>,
         py_shared_state: &'a PySharedState,
     ) -> Self {
         Self {
             py,
             inner,
             py_shared_state,
         }
     }
 }
 
 impl<'a, T> std::ops::Deref for PyRefMut<'a, T> {
     type Target = RefMut<'a, T>;
 
     fn deref(&self) -> &Self::Target {
         &self.inner
     }
 }
 impl<'a, T> std::ops::DerefMut for PyRefMut<'a, T> {
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.inner
     }
 }
 
 impl<'a, T> Drop for PyRefMut<'a, T> {
     fn drop(&mut self) {
         self.py_shared_state.decrease_leak_count(self.py, true);
     }
 }
 
 /// Allows a `py_class!` generated struct to share references to one of its
 /// data members with Python.
 ///
 /// # Parameters
 ///
 /// * `$name` is the same identifier used in for `py_class!` macro call.
 /// * `$inner_struct` is the identifier of the underlying Rust struct
 /// * `$data_member` is the identifier of the data member of `$inner_struct`
 /// that will be shared.
 /// * `$shared_accessor` is the function name to be generated, which allows
 /// safe access to the data member.
 ///
 /// # Safety
 ///
 /// `$data_member` must persist while the `$name` object is alive. In other
 /// words, it must be an accessor to a data field of the Python object.
 ///
 /// # Example
 ///
 /// ```
 /// struct MyStruct {
 ///     inner: Vec<u32>;
 /// }
 ///
 /// py_class!(pub class MyType |py| {
 ///     data inner: PySharedRefCell<MyStruct>;
 /// });
 ///
 /// py_shared_ref!(MyType, MyStruct, inner, inner_shared);
 /// ```
 macro_rules! py_shared_ref {
     (
         $name: ident,
         $inner_struct: ident,
         $data_member: ident,
         $shared_accessor: ident
     ) => {
         impl $name {
             /// Returns a safe reference to the shared `$data_member`.
             ///
             /// This function guarantees that `PySharedRef` is created with
             /// the valid `self` and `self.$data_member(py)` pair.
             fn $shared_accessor<'a>(
                 &'a self,
                 py: Python<'a>,
             ) -> $crate::ref_sharing::PySharedRef<'a, $inner_struct> {
                 use cpython::PythonObject;
                 use $crate::ref_sharing::PySharedRef;
                 let owner = self.as_object();
                 let data = self.$data_member(py);
                 unsafe { PySharedRef::new(py, owner, data) }
             }
         }
     };
 }
 
 /// Manage immutable references to `PyObject` leaked into Python iterators.
 ///
 /// This reference will be invalidated once the original value is mutably
 /// borrowed.
 pub struct PyLeaked<T> {
     inner: PyObject,
     data: Option<T>,
     py_shared_state: &'static PySharedState,
     /// Generation counter of data `T` captured when PyLeaked is created.
     generation: usize,
 }
 
 // DO NOT implement Deref for PyLeaked<T>! Dereferencing PyLeaked
 // without taking Python GIL wouldn't be safe. Also, the underling reference
 // is invalid if generation != py_shared_state.generation.
 
 impl<T> PyLeaked<T> {
     /// # Safety
     ///
     /// The `py_shared_state` must be owned by the `inner` Python object.
     fn new(
         py: Python,
         inner: &PyObject,
         data: T,
         py_shared_state: &'static PySharedState,
     ) -> Self {
         Self {
             inner: inner.clone_ref(py),
             data: Some(data),
             py_shared_state,
             generation: py_shared_state.current_generation(py),
         }
     }
 
     /// Immutably borrows the wrapped value.
     ///
     /// Borrowing fails if the underlying reference has been invalidated.
     pub fn try_borrow<'a>(
         &'a self,
         py: Python<'a>,
     ) -> PyResult<PyLeakedRef<'a, T>> {
         self.validate_generation(py)?;
         Ok(PyLeakedRef {
             _borrow: BorrowPyShared::new(py, self.py_shared_state),
             data: self.data.as_ref().unwrap(),
         })
     }
 
     /// Mutably borrows the wrapped value.
     ///
     /// Borrowing fails if the underlying reference has been invalidated.
     ///
     /// Typically `T` is an iterator. If `T` is an immutable reference,
     /// `get_mut()` is useless since the inner value can't be mutated.
     pub fn try_borrow_mut<'a>(
         &'a mut self,
         py: Python<'a>,
     ) -> PyResult<PyLeakedRefMut<'a, T>> {
         self.validate_generation(py)?;
         Ok(PyLeakedRefMut {
             _borrow: BorrowPyShared::new(py, self.py_shared_state),
             data: self.data.as_mut().unwrap(),
         })
     }
 
     /// Converts the inner value by the given function.
     ///
     /// Typically `T` is a static reference to a container, and `U` is an
     /// iterator of that container.
     ///
     /// # Panics
     ///
     /// Panics if the underlying reference has been invalidated.
     ///
     /// This is typically called immediately after the `PyLeaked` is obtained.
     /// In which case, the reference must be valid and no panic would occur.
     ///
     /// # Safety
     ///
     /// The lifetime of the object passed in to the function `f` is cheated.
     /// It's typically a static reference, but is valid only while the
     /// corresponding `PyLeaked` is alive. Do not copy it out of the
     /// function call.
     pub unsafe fn map<U>(
         mut self,
         py: Python,
         f: impl FnOnce(T) -> U,
     ) -> PyLeaked<U> {
         // Needs to test the generation value to make sure self.data reference
         // is still intact.
         self.validate_generation(py)
             .expect("map() over invalidated leaked reference");
 
         // f() could make the self.data outlive. That's why map() is unsafe.
         // In order to make this function safe, maybe we'll need a way to
         // temporarily restrict the lifetime of self.data and translate the
         // returned object back to Something<'static>.
         let new_data = f(self.data.take().unwrap());
         PyLeaked {
             inner: self.inner.clone_ref(py),
             data: Some(new_data),
             py_shared_state: self.py_shared_state,
             generation: self.generation,
         }
     }
 
     fn validate_generation(&self, py: Python) -> PyResult<()> {
         if self.py_shared_state.current_generation(py) == self.generation {
             Ok(())
         } else {
             Err(PyErr::new::<exc::RuntimeError, _>(
                 py,
                 "Cannot access to leaked reference after mutation",
             ))
         }
     }
 }
 
 /// Immutably borrowed reference to a leaked value.
 pub struct PyLeakedRef<'a, T> {
     _borrow: BorrowPyShared<'a>,
     data: &'a T,
 }
 
 impl<T> Deref for PyLeakedRef<'_, T> {
     type Target = T;
 
     fn deref(&self) -> &T {
         self.data
     }
 }
 
 /// Mutably borrowed reference to a leaked value.
 pub struct PyLeakedRefMut<'a, T> {
     _borrow: BorrowPyShared<'a>,
     data: &'a mut T,
 }
 
 impl<T> Deref for PyLeakedRefMut<'_, T> {
     type Target = T;
 
     fn deref(&self) -> &T {
         self.data
     }
 }
 
 impl<T> DerefMut for PyLeakedRefMut<'_, T> {
     fn deref_mut(&mut self) -> &mut T {
         self.data
     }
 }
 
 /// Defines a `py_class!` that acts as a Python iterator over a Rust iterator.
 ///
 /// TODO: this is a bit awkward to use, and a better (more complicated)
 ///     procedural macro would simplify the interface a lot.
 ///
 /// # Parameters
 ///
 /// * `$name` is the identifier to give to the resulting Rust struct.
 /// * `$leaked` corresponds to `$leaked` in the matching `py_shared_ref!` call.
 /// * `$iterator_type` is the type of the Rust iterator.
 /// * `$success_func` is a function for processing the Rust `(key, value)`
 /// tuple on iteration success, turning it into something Python understands.
 /// * `$success_func` is the return type of `$success_func`
 ///
 /// # Example
 ///
 /// ```
 /// struct MyStruct {
 ///     inner: HashMap<Vec<u8>, Vec<u8>>;
 /// }
 ///
 /// py_class!(pub class MyType |py| {
 ///     data inner: PySharedRefCell<MyStruct>;
 ///
 ///     def __iter__(&self) -> PyResult<MyTypeItemsIterator> {
 ///         let leaked_ref = self.inner_shared(py).leak_immutable()?;
 ///         MyTypeItemsIterator::from_inner(
 ///             py,
 ///             unsafe { leaked_ref.map(py, |o| o.iter()) },
 ///         )
 ///     }
 /// });
 ///
 /// impl MyType {
 ///     fn translate_key_value(
 ///         py: Python,
 ///         res: (&Vec<u8>, &Vec<u8>),
 ///     ) -> PyResult<Option<(PyBytes, PyBytes)>> {
 ///         let (f, entry) = res;
 ///         Ok(Some((
 ///             PyBytes::new(py, f),
 ///             PyBytes::new(py, entry),
 ///         )))
 ///     }
 /// }
 ///
 /// py_shared_ref!(MyType, MyStruct, inner, MyTypeLeakedRef);
 ///
 /// py_shared_iterator!(
 ///     MyTypeItemsIterator,
 ///     PyLeaked<HashMap<'static, Vec<u8>, Vec<u8>>>,
 ///     MyType::translate_key_value,
 ///     Option<(PyBytes, PyBytes)>
 /// );
 /// ```
 macro_rules! py_shared_iterator {
     (
         $name: ident,
         $leaked: ty,
         $success_func: expr,
         $success_type: ty
     ) => {
         py_class!(pub class $name |py| {
             data inner: RefCell<$leaked>;
 
             def __next__(&self) -> PyResult<$success_type> {
                 let mut leaked = self.inner(py).borrow_mut();
                 let mut iter = leaked.try_borrow_mut(py)?;
                 match iter.next() {
                     None => Ok(None),
                     Some(res) => $success_func(py, res),
                 }
             }
 
             def __iter__(&self) -> PyResult<Self> {
                 Ok(self.clone_ref(py))
             }
         });
 
         impl $name {
             pub fn from_inner(
                 py: Python,
                 leaked: $leaked,
             ) -> PyResult<Self> {
                 Self::create_instance(
                     py,
                     RefCell::new(leaked),
                 )
             }
         }
     };
 }
 
 #[cfg(test)]
 #[cfg(any(feature = "python27-bin", feature = "python3-bin"))]
 mod test {
     use super::*;
     use cpython::{GILGuard, Python};
 
     py_class!(class Owner |py| {
         data string: PySharedRefCell<String>;
     });
     py_shared_ref!(Owner, String, string, string_shared);
 
     fn prepare_env() -> (GILGuard, Owner) {
         let gil = Python::acquire_gil();
         let py = gil.python();
         let owner =
             Owner::create_instance(py, PySharedRefCell::new("new".to_owned()))
                 .unwrap();
         (gil, owner)
     }
 
     #[test]
     fn test_leaked_borrow() {
         let (gil, owner) = prepare_env();
         let py = gil.python();
         let leaked = owner.string_shared(py).leak_immutable().unwrap();
         let leaked_ref = leaked.try_borrow(py).unwrap();
         assert_eq!(*leaked_ref, "new");
     }
 
     #[test]
     fn test_leaked_borrow_mut() {
         let (gil, owner) = prepare_env();
         let py = gil.python();
         let leaked = owner.string_shared(py).leak_immutable().unwrap();
         let mut leaked_iter = unsafe { leaked.map(py, |s| s.chars()) };
         let mut leaked_ref = leaked_iter.try_borrow_mut(py).unwrap();
         assert_eq!(leaked_ref.next(), Some('n'));
         assert_eq!(leaked_ref.next(), Some('e'));
         assert_eq!(leaked_ref.next(), Some('w'));
         assert_eq!(leaked_ref.next(), None);
     }
 
     #[test]
     fn test_leaked_borrow_after_mut() {
         let (gil, owner) = prepare_env();
         let py = gil.python();
         let leaked = owner.string_shared(py).leak_immutable().unwrap();
         owner.string_shared(py).borrow_mut().unwrap().clear();
         assert!(leaked.try_borrow(py).is_err());
     }
 
     #[test]
     fn test_leaked_borrow_mut_after_mut() {
         let (gil, owner) = prepare_env();
         let py = gil.python();
         let leaked = owner.string_shared(py).leak_immutable().unwrap();
         let mut leaked_iter = unsafe { leaked.map(py, |s| s.chars()) };
         owner.string_shared(py).borrow_mut().unwrap().clear();
         assert!(leaked_iter.try_borrow_mut(py).is_err());
     }
 
     #[test]
     #[should_panic(expected = "map() over invalidated leaked reference")]
     fn test_leaked_map_after_mut() {
         let (gil, owner) = prepare_env();
         let py = gil.python();
         let leaked = owner.string_shared(py).leak_immutable().unwrap();
         owner.string_shared(py).borrow_mut().unwrap().clear();
         let _leaked_iter = unsafe { leaked.map(py, |s| s.chars()) };
     }
 
     #[test]
     fn test_borrow_mut_while_leaked_ref() {
         let (gil, owner) = prepare_env();
         let py = gil.python();
         assert!(owner.string_shared(py).borrow_mut().is_ok());
         let leaked = owner.string_shared(py).leak_immutable().unwrap();
         {
             let _leaked_ref = leaked.try_borrow(py).unwrap();
             assert!(owner.string_shared(py).borrow_mut().is_err());
             {
                 let _leaked_ref2 = leaked.try_borrow(py).unwrap();
                 assert!(owner.string_shared(py).borrow_mut().is_err());
             }
             assert!(owner.string_shared(py).borrow_mut().is_err());
         }
         assert!(owner.string_shared(py).borrow_mut().is_ok());
     }
 
     #[test]
     fn test_borrow_mut_while_leaked_ref_mut() {
         let (gil, owner) = prepare_env();
         let py = gil.python();
         assert!(owner.string_shared(py).borrow_mut().is_ok());
         let leaked = owner.string_shared(py).leak_immutable().unwrap();
         let mut leaked_iter = unsafe { leaked.map(py, |s| s.chars()) };
         {
             let _leaked_ref = leaked_iter.try_borrow_mut(py).unwrap();
             assert!(owner.string_shared(py).borrow_mut().is_err());
         }
         assert!(owner.string_shared(py).borrow_mut().is_ok());
     }
+
+    #[test]
+    #[should_panic(expected = "mutably borrowed")]
+    fn test_leak_while_borrow_mut() {
+        let (gil, owner) = prepare_env();
+        let py = gil.python();
+        let _mut_ref = owner.string_shared(py).borrow_mut();
+        let _ = owner.string_shared(py).leak_immutable();
+    }
 }