New submission from Josh Rosenberg:
I could be missing something, but it looks like PyStructSequence_NewType is
guaranteed broken. Specifically, it allocates the memory with
PyType_GenericAlloc (use PyType_Type as the base), and PyType declares itself
to have garbage collected instances, so the memory is allocated with
_PyObject_GC_Malloc and added to GC tracking just before PyType_GenericAlloc
returns.
Problem is, PyStructSequence_Init2 copies from a template struct which sets
Py_TPFLAGS_DEFAULT. So even though the new struct sequence is GC allocated and
managed, it doesn't set Py_TPFLAGS_HEAPTYPE, which means when GC tries to
traverse it, type's type_traverse errors out with:
Fatal Python error: type_traverse() called for non-heap type
'NameOfStructSequence'
It's possible I'm missing something here, so I've attached simple test code for
others to confirm (it omits most error checking for simplicity/readability).
Just compile the extension module, then run (with the module in the working
directory):
python -c "import testnewtype; Foo = testnewtype.makeseq('Foo', ['x', 'y'])"
There is a commented out line in the test code that explicitly sets the
HEAPTYPE flag after type construction (no idea if that's supposed to be
supported), and uncommenting it seems to fix the crash (though again, if
retroactively flagging as HEAPTYPE is unsupported, something else may break
here).
I can't find any actual use of PyStructSequence_NewType in the CPython code
base, which probably explains why this hasn't been seen; odds are, most
extensions using struct sequences are using Init, not NewType, and I only ran
into this because I was experimenting with a struct sequence based replacement
for collections.namedtuple (to end the start up time objections to using
namedtuple in the built-in modules, e.g. #28638).
----------
components: Interpreter Core
files: testnewtype.c
messages: 280904
nosy: josh.r
priority: normal
severity: normal
status: open
title: PyStructSequence_NewType is broken
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7
Added file: http://bugs.python.org/file45495/testnewtype.c
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<http://bugs.python.org/issue28709>
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