Fix __dict__ leak on Python 3.11 and 3.12

newsfragments/6234.fixed.md

@@ -0,0 +1 @@
+Fix a memory leak on Python 3.11 and 3.12 where creating an instance of a `#[pyclass(dict)]` class leaked one empty dict per instance.

src/pyclass_init.rs

@@ -155,9 +155,36 @@ impl<T: PyClass> PyClassInitializer<T> {
 
         // SAFETY: `obj` is constructed using `T::Layout` but has not been initialized yet
         let contents = unsafe { <T as PyClassImpl>::Layout::contents_uninit(obj) };
+
+        let new_contents = PyClassObjectContents::new(self.init);
+
+        // CPython 3.11 and 3.12 eagerly create the instance dict for types with a nonzero
+        // `tp_dictoffset` in `_PyObject_InitializeDict`, storing an owned reference in
+        // the `__dict__` slot, which lives inside `contents`. Carry that value over
+        // instead of clobbering it below, otherwise it leaks. Python 3.13 returned to
+        // creating the instance dict lazily
+        //
+        // The condition is `not(Py_3_13)` rather than `all(Py_3_11, not(Py_3_13))`
+        // because an abi3 build with a lower minimum version can still run on 3.11 and
+        // 3.12; on 3.10 and older this is a harmless no-op (the slot is always null
+        // there).
+        #[cfg(not(Py_3_13))]
+        let new_contents = {
+            let mut new_contents = new_contents;
+            // SAFETY: `tp_alloc` zero-initializes the object, so the slot contains either
+            // zeroes (a valid empty slot value) or a valid owned pointer stored by the base
+            // `tp_new` through the type's `tp_dictoffset`.
+            unsafe {
+                let contents_ptr = (*contents).as_mut_ptr();
+                let dict_ptr = core::ptr::addr_of!((*contents_ptr).dict);
+                new_contents.dict = core::ptr::read(dict_ptr);
+            }
+            new_contents
+        };
+
         // SAFETY: `contents` is a non-null pointer to the space allocated for our
         // `PyClassObjectContents` (either statically in Rust or dynamically by Python)
-        unsafe { (*contents).write(PyClassObjectContents::new(self.init)) };
+        unsafe { (*contents).write(new_contents) };
 
         // Safety: obj is a valid pointer to an object of type `target_type`, which` is a known
         // subclass of `T`

tests/test_class_basics.rs

@@ -502,6 +502,57 @@ fn dunder_dict_is_released() {
     });
 }
 
+// The `__dict__` slot must hold exactly what CPython's `tp_new` left there: CPython 3.11
+// and 3.12 eagerly create the instance dict in `object.__new__` for types with a nonzero
+// `tp_dictoffset`, and the pyclass contents initialization must preserve it rather than
+// clobber it. All other versions create the dict lazily on first access.
+#[cfg(all(not(Py_LIMITED_API), not(PyPy), not(GraalPy)))]
+#[test]
+fn instance_dict_slot_is_not_clobbered() {
+    Python::attach(|py| {
+        let inst = Py::new(
+            py,
+            DunderDictSupport {
+                _pad: *b"DEADBEEFDEADBEEFDEADBEEFDEADBEEF",
+            },
+        )
+        .unwrap();
+
+        // Read the `__dict__` slot directly (see `_PyObject_GetDictPtr`), *without*
+        // going through `__dict__`.
+        // SAFETY: `inst` is a valid object whose type has a positive `tp_dictoffset`.
+        let read_slot = || unsafe {
+            let offset = (*pyo3::ffi::Py_TYPE(inst.as_ptr())).tp_dictoffset;
+            assert!(offset > 0);
+            *inst
+                .as_ptr()
+                .cast::<u8>()
+                .offset(offset)
+                .cast::<*mut pyo3::ffi::PyObject>()
+        };
+
+        let slot_dict = read_slot();
+        if cfg!(all(Py_3_11, not(Py_3_13))) {
+            // `object.__new__` created the dict; it must survive pyclass initialization.
+            assert!(
+                !slot_dict.is_null(),
+                "the eagerly created __dict__ was clobbered during pyclass initialization"
+            );
+            // The slot holds the only reference to it.
+            // SAFETY: previous assert guarantees it's a valid PyObject
+            assert_eq!(unsafe { pyo3::ffi::Py_REFCNT(slot_dict) }, 1);
+        } else {
+            // No eager creation on these versions; the slot starts out empty.
+            assert!(slot_dict.is_null());
+        }
+
+        // Whichever way the dict comes into existence, `__dict__` must be the dict
+        // stored in the slot.
+        let dict_attr = inst.bind(py).getattr("__dict__").unwrap();
+        assert_eq!(read_slot(), dict_attr.as_ptr());
+    });
+}
+
 // If the base class has dict support, child class also has dict
 #[pyclass(extends=DunderDictSupport)]
 struct InheritDict {