gh-101430: Update tracemalloc to handle presize properly.

[corona10]

• Issue: _tracemalloc__get_object_traceback doesn't handle objects with pre-headers correctly #101430

[corona10]

AS-IS

# ./python.exe -Wd -X tracemalloc=2 gh-101430.py
/Users/user/oss/cpython/gh-101430.py:5: ResourceWarning: Uh oh
  warnings.warn("Uh oh", ResourceWarning, source=self)

TO-BE

# ./python.exe -Wd -X tracemalloc=2 gh-101430.py
/Users/user/oss/cpython/gh-101430.py:5: ResourceWarning: Uh oh
  warnings.warn("Uh oh", ResourceWarning, source=self)
Object allocated at (most recent call last):
  File "/Users/user/oss/cpython/gh-101430.py", lineno 10
    func()
  File "/Users/user/oss/cpython/gh-101430.py", lineno 8
    m = MyClass()

[corona10]

I now read all the spec of the preheader from #29879.
Looks like preheader should be considered when the PyType_IS_GC is true.

[corona10]

@colesbury PTAL

[colesbury]

This looks correct, but I think it would be more straightforward if you skip the PyType_IS_GC check (and always subtract the pre-header size.) _PyType_PreHeaderSize returns zero for non-GC types.

For context, here's an example in _PyType_AllocNoTrack that operates on GC and non-GC types and unconditionally adds the pre-header size:

cpython/Objects/typeobject.c

Lines 1300 to 1301 in ecfd2d3

	const size_t presize = _PyType_PreHeaderSize(type);
	char *alloc = PyObject_Malloc(size + presize);

[colesbury]

@corona10 There's also one more tracemalloc related use of sizeof(PyGC_Head) that I missed in the bug report (_PyMem_DumpTraceback calls into tracemalloc):

cpython/Objects/object.c

Lines 2390 to 2398 in ecfd2d3

	void *ptr;
	PyTypeObject *type = Py_TYPE(obj);
	if (_PyType_IS_GC(type)) {
	ptr = (void *)((char *)obj - sizeof(PyGC_Head));
	}
	else {
	ptr = (void *)obj;
	}
	_PyMem_DumpTraceback(fileno(stderr), ptr);

[corona10]

@corona10 There's also one more tracemalloc related use of sizeof(PyGC_Head) that I missed in the bug report (_PyMem_DumpTraceback calls into tracemalloc):

Thank you for catching all of them!

[gpshead]

I believe this is good, but I've looped in @vstinner for an extra pair of expert eyes on this.

[corona10]

@gpshead

AFAIK, Victor is enjoying his detoxing for OSS.
I will merge the PR since there looks like no serious issue.

cc @vstinner
If there needs something to fix, please let me know :)

[vstinner]

Thanks for this nice fix @corona10! _PyType_PreHeaderSize() is convenient and better than previously copied-pasted code ;-)