"a foreign function that will call a COM method" generated by `ctypes.WINFUNCTYPE` works in Python3.7, does not work as same in newer Python

[junkmd]

Bug report

I'm contributing on enthought/comtypes.

In comtypes, there is a test for the behavior of Excel that is currently skipped. If I comment out the unittest.skip marker in that test, it works in Python 3.7 and fails in Python 3.11.

PS ...\comtypes> py -3.7 -m unittest comtypes.test.test_excel -vv
test (comtypes.test.test_excel.Test_EarlyBind) ... ok
test (comtypes.test.test_excel.Test_LateBind) ... ok
----------------------------------------------------------------------
Ran 2 tests in 10.576s
OK
PS ...\comtypes> py -3.7 -m clear_comtypes_cache -y  # <- clear caches, required!
Removed directory "...\comtypes\comtypes\gen"
Removed directory "...\AppData\Roaming\Python\Python37\comtypes_cache"
PS ...\comtypes> py -3.11 -m unittest comtypes.test.test_excel -vv       
test (comtypes.test.test_excel.Test_EarlyBind.test) ... FAIL
test (comtypes.test.test_excel.Test_LateBind.test) ... ok

======================================================================
FAIL: test (comtypes.test.test_excel.Test_EarlyBind.test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "...\comtypes\comtypes\test\test_excel.py", line 62, in test
    self.assertEqual(xl.Range["A1:C3"].Value(),
AssertionError: Tuples differ: ((None, None, 10.0), ('x', 'y', 'z'), (3.0, 2.0, 1.0)) != ((10.0, 20.0, 31.4), ('x', 'y', 'z'), (3.0, 2.0, 1.0))

First differing element 0:
(None, None, 10.0)
(10.0, 20.0, 31.4)

- ((None, None, 10.0), ('x', 'y', 'z'), (3.0, 2.0, 1.0))
?   ------------

+ ((10.0, 20.0, 31.4), ('x', 'y', 'z'), (3.0, 2.0, 1.0))
?       ++++++++++++


----------------------------------------------------------------------
Ran 2 tests in 9.640s

FAILED (failures=1)

xl.Range[... calls a prototype-function generated by ctypes.WinFunctionType and ctypes.WINFUNCTYPE.
This is also reported in enthought/comtypes#212 and the test fails in Python 3.8 as well.
A strange callback behavior also occurs with simple COM libraries.
Therefore, I think that this may not be caused by the Excel specification.

There may be other regressions in ctypes callbacks that have also been reported in #82929.
Also, is #97513 a possible solution to this problem?

Any opinions would be appreciated.

Your environment

• CPython versions tested on: Python 3.7.1 and Python 3.11.0
• Windows 10

Linked PRs

• gh-99952: [ctypes] fix refcount issues in from_param() result. #100169
• [3.11] gh-99952: fix refcount issues in ctypes.Structure from_param() result #101339
• [3.10] [3.11] gh-99952: fix refcount issues in ctypes.Structure from_param() result (GH-101339) #101340

[karpierz]

@michaelDCurran wrote:
For reference python/cpython#82929 has already been fixed in Python main.

Unfortunatelly the bug still exist in the newest (6.12.2022) versions of Python (3.10.9, 3.11.1).

For such code:

xl = CreateObject("Excel.Application")
from comtypes.gen.Excel import xlRangeValueDefault
xl.Workbooks.Add()
xl.Range["A1", "C1"].Value[xlRangeValueDefault] = (10, "20", 31.4)
print(xl.Range["A1", "C1"].Value[xlRangeValueDefault])
xl.Visible = True
input("Press a key")
xl.Quit()

we have results:

D:\Py>py -3.7 z_COM.py
((10.0, 20.0, 31.4),)
Press a key

D:\Py>py -3.10 z_COM.py
10.0
Press a key

D:\Py>py -3.11 z_COM.py
10.0
Press a key

PS: This is an important/real barrier to upgrading Python to versions newer than 3.7.

[michaelDCurran]

Is the bug in the setting or the fetching of that value? In other words, if you make a spreadsheet manually by hand with those values, and then change your test case to just open the file and fetch the values in that cell range, do you get the expected results?

[karpierz]

@michaelDCurran

Is the bug in the setting or the fetching of that value?

The same behaviour.

For such code:

xl = CreateObject("Excel.Application")
from comtypes.gen.Excel import xlRangeValueDefault
xl.Workbooks.Add()
xl.Range["A1", "C1"].Value[xlRangeValueDefault] = (10, "20", 31.4)
xl.Visible = True
print(xl.Range["A1", "C1"].Value[xlRangeValueDefault])
input("Press a key")  # in this place we just edited a row
print(xl.Range["A1", "C1"].Value[xlRangeValueDefault])
input("Press a key")
xl.Quit()

we have results:

D:\Py>py -3.7 z_COM.py
((10.0, 20.0, 31.4),)
Press a key
((11.0, 22.0, 33.0),)
Press a key

D:\Py>py -3.10 z_COM.py
10.0
Press a key
33.0
Press a key

D:\Py>py -3.11 z_COM.py
10.0
Press a key
33.0
Press a key

PS: py>3.7 gets last cell in the row instead of whole row.

[ynkdir]

VARIANT("A1") and VARIANT("C1") are created by from_param() for calling com method.
It seems that line 203 _VariantClear() is called after line 214 soon.
Then, memory is freed before calling com method.
Also, SysAllocStringLen() returns same memory address with "A1" for "C1".

comtypes/automation.py:
class tagVARIANT(Structure):
...
199     def __del__(self):
200         if self._b_needsfree_:
201             # XXX This does not work.  _b_needsfree_ is never
202             # set because the buffer is internal to the object.
203             _VariantClear(self)
204
205     def __repr__(self):
206         if self.vt & VT_BYREF:
207             return "VARIANT(vt=0x%x, byref(%r))" % (self.vt, self[0])
208         return "VARIANT(vt=0x%x, %r)" % (self.vt, self.value)
209
210     @classmethod
211     def from_param(cls, value):
212         if isinstance(value, cls):
213             return value
214         return cls(value)

[ynkdir]

Here is minimal reproducible code.

# fixed code for more specific.

import ctypes

# A is not ok

class A(ctypes.Structure):
    _fields_ = [("n1", ctypes.c_void_p), ("n2", ctypes.c_int)]

    def __del__(self):
        print("A.__del__", self.n1)

    @classmethod
    def from_param(cls, value):
        print("A.from_param:", value)
        return cls(n1=value)

ctypes.cdll.msvcrt.wprintf.argtypes = [ctypes.c_wchar_p, A]
print("A: start")
ctypes.cdll.msvcrt.wprintf("A: wprintf: %p\n", 1)
print("A: end")

# A: start
# A.from_param: 1
# A.__del__ 1
# A: wprintf: 00000032F1BEEBB0
# A: end

# B is ok

class B(ctypes.Structure):
    _fields_ = [("n1", ctypes.c_void_p)]

    def __del__(self):
        print("B.__del__", self.n1)

    @classmethod
    def from_param(cls, value):
        print("B.from_param:", value)
        return cls(n1=value)

ctypes.cdll.msvcrt.wprintf.argtypes = [ctypes.c_wchar_p, B]
print("B: start")
ctypes.cdll.msvcrt.wprintf("B: wprintf: %p\n", 2)
print("B: end")

# B: start
# B.from_param: 2
# B: wprintf: 0000000000000002
# B.__del__ 2
# B: end

[karpierz]

@ynkdir

199 def del(self):
200 if self.b_needsfree:
201 # XXX This does not work. b_needsfree is never
202 # set because the buffer is internal to the object.
203 _VariantClear(self)

Yes! I can confim. After commented:
pass #_VariantClear(self)

comtypes works as expected also for Py 3.8, 3.9, 3.10, 3.11

D:\Py>py -3.8 z_COM.py
10.0
Press a key
33.0
Press a key

C:\Python38\Lib\site-packages\comtypes>copy automation.py automation.py.org
        1 file(s) copied.

D:\Py>py -3.7 z_COM.py
((10.0, 20.0, 31.4),)
Press a key
((11.0, 22.0, 33.0),)
Press a key

D:\Py>py -3.8 z_COM.py
((10.0, 20.0, 31.4),)
Press a key
((11.0, 22.0, 33.0),)
Press a key

D:\Py>py -3.9 z_COM.py
((10.0, 20.0, 31.4),)
Press a key
((11.0, 22.0, 33.0),)
Press a key

D:\Py>py -3.10 z_COM.py
((10.0, 20.0, 31.4),)
Press a key
((11.0, 22.0, 33.0),)
Press a key

D:\Py>py -3.11 z_COM.py
((10.0, 20.0, 31.4),)
Press a key
((11.0, 22.0, 33.0),)
Press a key

Thanks,
Adam

[ynkdir]

Perhaps ctypes's StructParam_Type should keep object which copied from.
I am not familiar with python source.

diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
index b9092d3981..55bcf044e8 100644
--- a/Modules/_ctypes/_ctypes.c
+++ b/Modules/_ctypes/_ctypes.c
@@ -412,6 +412,7 @@ _ctypes_alloc_format_string_with_shape(int ndim, const Py_ssize_t *shape,
 typedef struct {
     PyObject_HEAD
     void *ptr;
+    PyObject *keep;
 } StructParamObject;
 
 
@@ -419,6 +420,7 @@ static void
 StructParam_dealloc(PyObject *myself)
 {
     StructParamObject *self = (StructParamObject *)myself;
+    Py_XDECREF(self->keep);
     PyMem_Free(self->ptr);
     Py_TYPE(self)->tp_free(myself);
 }
@@ -466,6 +468,7 @@ StructUnionType_paramfunc(CDataObject *self)
 
         StructParamObject *struct_param = (StructParamObject *)obj;
         struct_param->ptr = ptr;
+        struct_param->keep = Py_NewRef(self);
     } else {
         ptr = self->b_ptr;
         obj = Py_NewRef(self);

[karpierz]

Yes. Seems like a main reason of this bug is a different object lifetime in Py <= 3.7, and Py >= 3.8
so (fortunately) in old Pythons this potential ctypes bug maybe was hidden/masked due to longer lifetime of referenced object?.

Maybe in comtypes will be useful/enough to use (as worarround/fix) pythonapi''s IncRef(obj)/DecRef(obj) ??
This will allow to avoid waiting for the official port and python releases with ev. fix?

from ctypes import py_object, pythonapi

Py_IncRef = pythonapi.Py_IncRef
Py_IncRef.argtypes = [py_object]
Py_IncRef.restype = None

Py_DecRef = pythonapi.Py_DecRef
Py_DecRef.argtypes = [py_object]
Py_DecRef.restype = None

PS: Unfortunately I am also not familiar with Python and ctypes C source code :(