From 26d42496f4986ba027e630d24447c48f06bfd79a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Sun, 9 Nov 2025 13:41:08 +0100
Subject: [PATCH] gh-140530: fix a reference leak in an error path for `raise
 exc from cause` (GH-140908)
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Fix a reference leak in `raise E from T` when `T` is an exception
subtype for which `T.__new__` does not return an exception instance.
(cherry picked from commit 0c77e7c23b5c270a3142105542c56c59b59c52a0)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
---
 Lib/test/test_raise.py                         | 18 +++++++-----------
 ...5-11-02-12-47-38.gh-issue-140530.S934bp.rst |  2 ++
 Python/ceval.c                                 |  1 +
 3 files changed, 10 insertions(+), 11 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-11-02-12-47-38.gh-issue-140530.S934bp.rst

diff --git a/Lib/test/test_raise.py b/Lib/test/test_raise.py
index dcf0753bc828f3..645ef291a58499 100644
--- a/Lib/test/test_raise.py
+++ b/Lib/test/test_raise.py
@@ -186,18 +186,14 @@ def test_class_cause(self):
             self.fail("No exception raised")
 
     def test_class_cause_nonexception_result(self):
-        class ConstructsNone(BaseException):
-            @classmethod
+        # See https://github.com/python/cpython/issues/140530.
+        class ConstructMortal(BaseException):
             def __new__(*args, **kwargs):
-                return None
-        try:
-            raise IndexError from ConstructsNone
-        except TypeError as e:
-            self.assertIn("should have returned an instance of BaseException", str(e))
-        except IndexError:
-            self.fail("Wrong kind of exception raised")
-        else:
-            self.fail("No exception raised")
+                return ["mortal value"]
+
+        msg = ".*should have returned an instance of BaseException.*"
+        with self.assertRaisesRegex(TypeError, msg):
+            raise IndexError from ConstructMortal
 
     def test_instance_cause(self):
         cause = KeyError()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-11-02-12-47-38.gh-issue-140530.S934bp.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-02-12-47-38.gh-issue-140530.S934bp.rst
new file mode 100644
index 00000000000000..e3af493893afcb
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-02-12-47-38.gh-issue-140530.S934bp.rst
@@ -0,0 +1,2 @@
+Fix a reference leak when ``raise exc from cause`` fails. Patch by Bénédikt
+Tran.
diff --git a/Python/ceval.c b/Python/ceval.c
index a181d1861d0131..6ce20af62e7249 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -2108,6 +2108,7 @@ do_raise(PyThreadState *tstate, PyObject *exc, PyObject *cause)
                               "calling %R should have returned an instance of "
                               "BaseException, not %R",
                               cause, Py_TYPE(fixed_cause));
+                Py_DECREF(fixed_cause);
                 goto raise_error;
             }
             Py_DECREF(cause);