gh-143145: Fix reference leak in ctypes _build_result()

[hyongtao-code]

In _ctypes._build_result(), a result tuple is allocated when numretvals > 1. If v becomes NULL during result construction, the function returns early without DECREF'ing the tuple, leaking a reference.
Add Py_XDECREF(tup) before returning when numretvals > 1 and v == NULL.

• Issue: Reference leak in ctypes._build_result() when numretvals > 1 and v == NULL #143145

[StanFromIreland]

LGTM, but this should get an issue and blurb.

[hyongtao-code]

Thanks for the review. More testing details are provided below:

Reproducer

import ctypes
import sys
import gc

class BadInt(ctypes.c_int):
    def __ctypes_from_outparam__(self):
        raise RuntimeError("boom from __ctypes_from_outparam__")

msvcrt = ctypes.CDLL("msvcrt.dll")
PROTO = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.POINTER(BadInt), ctypes.POINTER(BadInt), ctypes.c_size_t)

PARAMFLAG_FIN = 1
PARAMFLAG_FOUT = 2
paramflags = (
    (PARAMFLAG_FOUT, "a", BadInt()),
    (PARAMFLAG_FOUT, "b", BadInt()),
    (PARAMFLAG_FIN,  "n"),
)
f = PROTO(("memmove", msvcrt), paramflags)

def totalrefcount():
    return getattr(sys, "gettotalrefcount")()

base = totalrefcount()
print("base totalrefcount:", base)

N = 20000
step = 2000

for i in range(1, N + 1):
    try:
        f(4)
    except RuntimeError:
        pass
    if i % step == 0:
        gc.collect()
        cur = totalrefcount()
        print(f"{i} totalrefcount: {cur}  delta: {cur - base}")

Result without pr

When running the reproducer on a debug build python_d.exe, sys.gettotalrefcount() increases linearly with the number of calls.

d:\MyCode\cpython\PCbuild\amd64>python_d.exe py_leak_outparam_tuple.py
base totalrefcount: 26086
2000 totalrefcount: 27922  delta: 1836
4000 totalrefcount: 29923  delta: 3837
6000 totalrefcount: 31923  delta: 5837
8000 totalrefcount: 33923  delta: 7837
10000 totalrefcount: 35923  delta: 9837
12000 totalrefcount: 37923  delta: 11837
14000 totalrefcount: 39923  delta: 13837
16000 totalrefcount: 41923  delta: 15837
18000 totalrefcount: 43923  delta: 17837
20000 totalrefcount: 45923  delta: 19837

Result with pr

After applying the fix, the total reference count remains stable. No linear growth is observed across repeated calls, confirming that the reference leak is fixed.

d:\MyCode\cpython\PCbuild\amd64>python_d.exe py_leak_outparam_tuple.py
base totalrefcount: 26078
2000 totalrefcount: 25914  delta: -164
4000 totalrefcount: 25915  delta: -163
6000 totalrefcount: 25915  delta: -163
8000 totalrefcount: 25915  delta: -163
10000 totalrefcount: 25915  delta: -163
12000 totalrefcount: 25915  delta: -163
14000 totalrefcount: 25915  delta: -163
16000 totalrefcount: 25915  delta: -163
18000 totalrefcount: 25915  delta: -163
20000 totalrefcount: 25915  delta: -163