ffi: fix PyStatus._type

The field wasn't defined previously. And the enum wasn't defined as
`[repr(C)]`.

This missing field could result in memory corruption if a Rust-allocated
`PyStatus` was passed to a Python API, which could perform an
out-of-bounds write. In my code, the out-of-bounds write corrupted a
variable on the stack, leading to a segfault due to illegal memory
access. However, this crash only occurred on Rust 1.54! So I initially
mis-attribted it as a compiler bug / regression. It appears that a
low-level Rust change in 1.54.0 changed the LLVM IR in such a way to
cause LLVM optimization passes to produce sufficiently different
assembly code, tickling the crash. See
rust-lang/rust#87947 if you want to see
the wild goose chase I went on in Rust / LLVM land to potentially
pin this on a compiler bug.

Lessen learned: Rust crashes are almost certainly due to use of
`unsafe`.

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 - Restrict FFI definitions `PyGILState_Check` and `Py_tracefunc` to the unlimited API. [#1787](https://github.com/PyO3/pyo3/pull/1787)
 - Raise `AttributeError` to avoid panic when calling `del` on a `[setter]` defined class property. [#1775](https://github.com/PyO3/pyo3/issues/1775)
+- Add missing `_type` field to `PyStatus` struct definition.
 
 ## [0.14.2] - 2021-08-09
 

src/ffi/cpython/initconfig.rs

@@ -4,6 +4,7 @@ use crate::ffi::Py_ssize_t;
 use libc::wchar_t;
 use std::os::raw::{c_char, c_int, c_ulong};
 
+#[repr(C)]
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum _PyStatus_TYPE {
     _PyStatus_TYPE_OK = 0,
@@ -14,6 +15,7 @@ pub enum _PyStatus_TYPE {
 #[repr(C)]
 #[derive(Copy, Clone)]
 pub struct PyStatus {
+    pub _type: _PyStatus_TYPE,
     pub func: *const c_char,
     pub err_msg: *const c_char,
     pub exitcode: c_int,