gh-99952: [ctypes] fix refcount issues in from_param() result. (#100169)

Fixes a reference counting issue with `ctypes.Structure` when a `from_param()` method call is used and the structure size is larger than a C pointer `sizeof(void*)`.

This problem existed for a very long time, but became more apparent in 3.8+ by change likely due to garbage collection cleanup timing changes.

1 files changed, 52 insertions, 0 deletions

diff --git a/Lib/test/test_ctypes/test_parameters.py b/Lib/test/test_ctypes/test_parameters.py
index 22d290d..06cc951 100644
--- a/Lib/test/test_ctypes/test_parameters.py
+++ b/Lib/test/test_ctypes/test_parameters.py
@@ -266,6 +266,58 @@ class SimpleTypesTestCase(unittest.TestCase):
         self.assertRegex(repr(c_wchar_p.from_param('hihi')), r"^<cparam 'Z' \(0x[A-Fa-f0-9]+\)>$")
         self.assertRegex(repr(c_void_p.from_param(0x12)), r"^<cparam 'P' \(0x0*12\)>$")
 
+    @test.support.cpython_only
+    def test_from_param_result_refcount(self):
+        # Issue #99952
+        import _ctypes_test
+        from ctypes import PyDLL, c_int, c_void_p, py_object, Structure
+
+        class X(Structure):
+            """This struct size is <= sizeof(void*)."""
+            _fields_ = [("a", c_void_p)]
+
+            def __del__(self):
+                trace.append(4)
+
+            @classmethod
+            def from_param(cls, value):
+                trace.append(2)
+                return cls()
+
+        PyList_Append = PyDLL(_ctypes_test.__file__)._testfunc_pylist_append
+        PyList_Append.restype = c_int
+        PyList_Append.argtypes = [py_object, py_object, X]
+
+        trace = []
+        trace.append(1)
+        PyList_Append(trace, 3, "dummy")
+        trace.append(5)
+
+        self.assertEqual(trace, [1, 2, 3, 4, 5])
+
+        class Y(Structure):
+            """This struct size is > sizeof(void*)."""
+            _fields_ = [("a", c_void_p), ("b", c_void_p)]
+
+            def __del__(self):
+                trace.append(4)
+
+            @classmethod
+            def from_param(cls, value):
+                trace.append(2)
+                return cls()
+
+        PyList_Append = PyDLL(_ctypes_test.__file__)._testfunc_pylist_append
+        PyList_Append.restype = c_int
+        PyList_Append.argtypes = [py_object, py_object, Y]
+
+        trace = []
+        trace.append(1)
+        PyList_Append(trace, 3, "dummy")
+        trace.append(5)
+
+        self.assertEqual(trace, [1, 2, 3, 4, 5])
+
 ################################################################
 
 if __name__ == '__main__':