[WIP] bpo-35059: Add assertion to _PyObject_CAST()

[vstinner]

_PyObject_CAST() and _PyVarObject_CAST() macros now require that the
type of the argument has a size. For example, _PyObject_CAST(frame)
now raises a compilation error if frame type is "struct _frame*" but
"struct _frame" is not defined.

_PyObject_CAST() and _PyVarObject_CAST() macros now raise a
compilation error if the argument is a "PyObject" or a "PyObject**".

Partially revert b37672d to show
that _PyObject_CAST() accepts non-trivial expressions.

https://bugs.python.org/issue35059

[vstinner]

This change introduces two backward incompatible changes:

• _PyObject_CAST(keyfile ? keyfile_bytes : certfile_bytes) raises a compilation error, whereas it's legit code and works if Py_TYPE() is a regular function.
• _PyObject_CAST(frame) raises a compilation error if frame type is undefined. Example: struct _frame * without "frameobject.h".

See my commit b37672d for other examples of regressions.

So I don't think that this change should be merged.

[serge-sans-paille]

if op is an expression, then your dereferencing is likely to fail, better use *(op).

[serge-sans-paille]

And you don't have the issue of op being evaluated three times because sizeof does not evaluate the expression \o/

[vstinner]

Oh right, fixed.

[vstinner]

_PyObject_CAST() and _PyVarObject_CAST() macros now require that the
type of the argument has a size. For example, _PyObject_CAST(frame)
now raises a compilation error if frame type is "struct _frame*" but
"struct _frame" is not defined.

IMHO it's a blocker issue, it's a backward incompatible change.

At least, _PyObject_CAST() doesn't require 'op' to have a size of Py_LIMITED_API is defined.

[vstinner]

cc @nascheme: you might like this experiment :-)

[vstinner]

@serhiy-storchaka, @pablogsal: I experimented a C macro to detect "Py_TYPE(&obj)" bug (with "PyObject *obj"), but it doesn't seem possible to write a magic macro because the Python C API is a mess :-)

For example, _PyObject_CAST() should also accept PyTupleObject* which contains a "PyObject ob_base". But there are other type monsters like:

• PyASCIIObject: PyObject ob_base;
• PyCompactUnicodeObject: PyASCIIObject _base;
• PyUnicodeObject: PyCompactUnicodeObject _base;

[serhiy-storchaka]

I did not know about _PyObject_CAST().

[vstinner]

I did not know about _PyObject_CAST()

I added it yesterday :-) commit 2ff8fb7

I wrote it for C macros to avoid "(PyObject*)ob" mistake: missing parenthesis around "ob". By the way, I should fix Include/odictobject.h in Python 3.6 and 3.7.

I have to confess that I also wrote the macro to be able to make such experiment. More generally, it's a hint on cast between object pointer types which helps me on my work on the C API.

[vstinner]

Ok, this PR was fun to write, but since it's breaking the backward compatibility, it's really a no-go and so I close it.