Add Py::get_frozen for GIL-independent access to frozen classes.

newsfragments/3158.added.md

@@ -0,0 +1 @@
+Add `Py::get_frozen` for GIL-indepedent access to internally synchronized frozen classes.

src/instance.rs

@@ -1,9 +1,9 @@
-use crate::pyclass::boolean_struct::False;
 // Copyright (c) 2017-present PyO3 Project and Contributors
 use crate::conversion::PyTryFrom;
 use crate::err::{self, PyDowncastError, PyErr, PyResult};
 use crate::gil;
 use crate::pycell::{PyBorrowError, PyBorrowMutError, PyCell};
+use crate::pyclass::boolean_struct::{False, True};
 use crate::types::{PyDict, PyString, PyTuple};
 use crate::{
     ffi, AsPyPointer, FromPyObject, IntoPy, IntoPyPointer, PyAny, PyClass, PyClassInitializer,
@@ -467,6 +467,41 @@ where
     {
         self.as_ref(py).try_borrow_mut()
     }
+
+    /// Provide an immutable borrow of the value `T` without acquiring the GIL.
+    ///
+    /// This is sound if the class is frozen and `Sync`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::atomic::{AtomicUsize, Ordering};
+    /// # use pyo3::prelude::*;
+    ///
+    /// #[pyclass(frozen)]
+    /// struct FrozenCounter {
+    ///     value: AtomicUsize,
+    /// }
+    ///
+    /// let cell  = Python::with_gil(|py| {
+    ///     let counter = FrozenCounter { value: AtomicUsize::new(0) };
+    ///
+    ///     Py::new(py, counter).unwrap()
+    /// });
+    ///
+    /// cell.get_frozen().value.fetch_add(1, Ordering::Relaxed);
+    /// ```
+    pub fn get_frozen(&self) -> &T
+    where
+        T: PyClass<Frozen = True> + Sync,
+    {
+        let any = self.as_ptr() as *const PyAny;
+        // SAFETY: The class itself is frozen and `Sync` and we do not access anything but `cell.contents.value`.
+        unsafe {
+            let cell: &PyCell<T> = PyNativeType::unchecked_downcast(&*any);
+            &*cell.get_ptr()
+        }
+    }
 }
 
 impl<T> Py<T> {

src/pycell.rs

@@ -450,7 +450,7 @@ impl<T: PyClass> PyCell<T> {
         std::mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
     }
 
-    fn get_ptr(&self) -> *mut T {
+    pub(crate) fn get_ptr(&self) -> *mut T {
         self.contents.value.get()
     }