bpo-25782: avoid hang in PyErr_SetObject when current exception has a cycle in its context chain (GH-27626)

Co-authored-by: Dennis Sweeney 36520290+sweeneyde@users.noreply.github.com
(cherry picked from commit d5c217475c)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Lib/test/test_exceptions.py

@@ -953,6 +953,148 @@ class ExceptionTests(unittest.TestCase):
             pass
         self.assertEqual(e, (None, None, None))
 
+    def test_raise_does_not_create_context_chain_cycle(self):
+        class A(Exception):
+            pass
+        class B(Exception):
+            pass
+        class C(Exception):
+            pass
+
+        # Create a context chain:
+        # C -> B -> A
+        # Then raise A in context of C.
+        try:
+            try:
+                raise A
+            except A as a_:
+                a = a_
+                try:
+                    raise B
+                except B as b_:
+                    b = b_
+                    try:
+                        raise C
+                    except C as c_:
+                        c = c_
+                        self.assertIsInstance(a, A)
+                        self.assertIsInstance(b, B)
+                        self.assertIsInstance(c, C)
+                        self.assertIsNone(a.__context__)
+                        self.assertIs(b.__context__, a)
+                        self.assertIs(c.__context__, b)
+                        raise a
+        except A as e:
+            exc = e
+
+        # Expect A -> C -> B, without cycle
+        self.assertIs(exc, a)
+        self.assertIs(a.__context__, c)
+        self.assertIs(c.__context__, b)
+        self.assertIsNone(b.__context__)
+
+    def test_no_hang_on_context_chain_cycle1(self):
+        # See issue 25782. Cycle in context chain.
+
+        def cycle():
+            try:
+                raise ValueError(1)
+            except ValueError as ex:
+                ex.__context__ = ex
+                raise TypeError(2)
+
+        try:
+            cycle()
+        except Exception as e:
+            exc = e
+
+        self.assertIsInstance(exc, TypeError)
+        self.assertIsInstance(exc.__context__, ValueError)
+        self.assertIs(exc.__context__.__context__, exc.__context__)
+
+    def test_no_hang_on_context_chain_cycle2(self):
+        # See issue 25782. Cycle at head of context chain.
+
+        class A(Exception):
+            pass
+        class B(Exception):
+            pass
+        class C(Exception):
+            pass
+
+        # Context cycle:
+        # +-----------+
+        # V           |
+        # C --> B --> A
+        with self.assertRaises(C) as cm:
+            try:
+                raise A()
+            except A as _a:
+                a = _a
+                try:
+                    raise B()
+                except B as _b:
+                    b = _b
+                    try:
+                        raise C()
+                    except C as _c:
+                        c = _c
+                        a.__context__ = c
+                        raise c
+
+        self.assertIs(cm.exception, c)
+        # Verify the expected context chain cycle
+        self.assertIs(c.__context__, b)
+        self.assertIs(b.__context__, a)
+        self.assertIs(a.__context__, c)
+
+    def test_no_hang_on_context_chain_cycle3(self):
+        # See issue 25782. Longer context chain with cycle.
+
+        class A(Exception):
+            pass
+        class B(Exception):
+            pass
+        class C(Exception):
+            pass
+        class D(Exception):
+            pass
+        class E(Exception):
+            pass
+
+        # Context cycle:
+        #             +-----------+
+        #             V           |
+        # E --> D --> C --> B --> A
+        with self.assertRaises(E) as cm:
+            try:
+                raise A()
+            except A as _a:
+                a = _a
+                try:
+                    raise B()
+                except B as _b:
+                    b = _b
+                    try:
+                        raise C()
+                    except C as _c:
+                        c = _c
+                        a.__context__ = c
+                        try:
+                            raise D()
+                        except D as _d:
+                            d = _d
+                            e = E()
+                            raise e
+
+        self.assertIs(cm.exception, e)
+        # Verify the expected context chain cycle
+        self.assertIs(e.__context__, d)
+        self.assertIs(d.__context__, c)
+        self.assertIs(c.__context__, b)
+        self.assertIs(b.__context__, a)
+        self.assertIs(a.__context__, c)
+
     def test_unicode_change_attributes(self):
         # See issue 7309. This was a crasher.