Possible cause of segfault in subclassing

[kngwyu]

Related to #220, #407
cc: @konstin @joar @davidhewitt

So as I said now I'm investigating safer layout than our current pyclass, which is constructed (roughly) by

let ptr = alloc(...);  // *mut PyObject
let align = std::mem::align_of::<Self>();  // Self = pyclass
// Suppose 
// struct PyClassLayout { |\
//       base: Self::BASE,      | offset
//       self_: PyClass   |/
// }
let offset = (self::BASE::SIZE + align - 1) / align * align;  
let self_ptr = (ptr as *mut u8).offset(Self::OFFSET) as *mut T;

And what I found problematic is <Py~ as PyTypeInfo>::SIZE is std::mem::size_of::<ffi::PyObject> for all Py~ types, which is not true.
Actually, PyDictObject is defined as (roughly)

typedef struct {
    PyObject ob_base;
    Py_ssize_t ma_used;
    uint64_t ma_version_tag;
    PyDictKeysObject *ma_keys;
    PyObject **ma_values;
} PyDictObject;

So it's size 64bit * 4 = 32bytes larger than PyObject.
Is it the cause of segfault?

[davidhewitt]

It's certainly suspicious that the SIZE is incorrectly defined. However, I can't see where SIZE is actually used for PyDict. The only use I find of ::SIZE is in type_object::initialize_type(), which I don't think ever gets called for native types.

pyo3/src/type_object.rs

Line 327 in 4aa2ff5

type_object.tp_basicsize = <T as PyTypeInfo>::SIZE as ffi::Py_ssize_t;

I would think that the macro in types/mod.rs should perhaps be calculating size differently:

pyo3/src/types/mod.rs

Line 107 in 4aa2ff5

const SIZE: usize = ::std::mem::size_of::<$crate::ffi::PyObject>();

I think SIZE really wants to be (*$typeobject).tp_basicsize? But then it can no longer be a constant.

I see in your upcoming branch #683 you've removed SIZE completely, which is also fine. I'll get round to reviewing that PR asap.

[davidhewitt]

@kngwyu I guess this can now be closed thanks to your new PyClassShell!