macros: fix panic in __get__ implementation

CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix use of `catch_unwind` in `allow_threads` which can cause fatal crashes. [#1989](https://github.com/PyO3/pyo3/pull/1989)
 - Fix build failure on PyPy when abi3 features are activated. [#1991](https://github.com/PyO3/pyo3/pull/1991)
 - Fix mingw platform detection. [#1993](https://github.com/PyO3/pyo3/pull/1993)
+- Fix panic in `__get__` implementation when accessing descriptor on type object. [#1997](https://github.com/PyO3/pyo3/pull/1997)
 
 ## [0.15.0] - 2021-11-03
 

pyo3-macros-backend/src/pymethod.rs

@@ -483,8 +483,8 @@ const __HASH__: SlotDef = SlotDef::new("Py_tp_hash", "hashfunc")
 const __RICHCMP__: SlotDef = SlotDef::new("Py_tp_richcompare", "richcmpfunc")
     .extract_error_mode(ExtractErrorMode::NotImplemented)
     .arguments(&[Ty::Object, Ty::CompareOp]);
-const __GET__: SlotDef =
-    SlotDef::new("Py_tp_descr_get", "descrgetfunc").arguments(&[Ty::Object, Ty::Object]);
+const __GET__: SlotDef = SlotDef::new("Py_tp_descr_get", "descrgetfunc")
+    .arguments(&[Ty::MaybeNullObject, Ty::MaybeNullObject]);
 const __ITER__: SlotDef = SlotDef::new("Py_tp_iter", "getiterfunc");
 const __NEXT__: SlotDef = SlotDef::new("Py_tp_iternext", "iternextfunc").return_conversion(
     TokenGenerator(|| quote! { ::pyo3::class::iter::IterNextOutput::<_, _> }),
@@ -606,6 +606,7 @@ fn pyproto(method_name: &str) -> Option<&'static SlotDef> {
 #[derive(Clone, Copy)]
 enum Ty {
     Object,
+    MaybeNullObject,
     NonNullObject,
     CompareOp,
     Int,
@@ -617,7 +618,7 @@ enum Ty {
 impl Ty {
     fn ffi_type(self) -> TokenStream {
         match self {
-            Ty::Object => quote! { *mut ::pyo3::ffi::PyObject },
+            Ty::Object | Ty::MaybeNullObject => quote! { *mut ::pyo3::ffi::PyObject },
             Ty::NonNullObject => quote! { ::std::ptr::NonNull<::pyo3::ffi::PyObject> },
             Ty::Int | Ty::CompareOp => quote! { ::std::os::raw::c_int },
             Ty::PyHashT => quote! { ::pyo3::ffi::Py_hash_t },
@@ -645,6 +646,22 @@ impl Ty {
                 );
                 extract_object(cls, arg.ty, ident, extract)
             }
+            Ty::MaybeNullObject => {
+                let extract = handle_error(
+                    extract_error_mode,
+                    py,
+                    quote! {
+                        #py.from_borrowed_ptr::<::pyo3::PyAny>(
+                            if #ident.is_null() {
+                                ::pyo3::ffi::Py_None()
+                            } else {
+                                #ident
+                            }
+                        ).extract()
+                    },
+                );
+                extract_object(cls, arg.ty, ident, extract)
+            }
             Ty::NonNullObject => {
                 let extract = handle_error(
                     extract_error_mode,

tests/test_proto_methods.rs

@@ -548,11 +548,23 @@ fn descr_getset() {
         r#"
 class Class:
     counter = Counter()
+
+# access via type
+counter = Class.counter
+assert counter.count == 1
+
+# access with instance directly
+assert Counter.__get__(counter, Class()).count == 2
+
+# access via instance
 c = Class()
-c.counter # count += 1
-assert c.counter.count == 2
-c.counter = Counter()
 assert c.counter.count == 3
+
+# __set__
+c.counter = Counter()
+assert c.counter.count == 4
+
+# __delete__
 del c.counter
 assert c.counter.count == 1
 "#