bpo-40696: Fix a hang that can arise after gen.throw() (GH-20287)

This updates _PyErr_ChainStackItem() to use _PyErr_SetObject()
instead of _PyErr_ChainExceptions(). This prevents a hang in
certain circumstances because _PyErr_SetObject() performs checks
to prevent cycles in the exception context chain while
_PyErr_ChainExceptions() doesn't.

Lib/test/test_asyncio/test_tasks.py

@@ -536,9 +536,42 @@ class BaseTaskTests:
                     self.assertEqual((type(chained), chained.args),
                         (KeyError, (3,)))
 
-        task = self.new_task(loop, run())
-        loop.run_until_complete(task)
-        loop.close()
+        try:
+            task = self.new_task(loop, run())
+            loop.run_until_complete(task)
+        finally:
+            loop.close()
+
+    def test_exception_chaining_after_await_with_context_cycle(self):
+        # Check trying to create an exception context cycle:
+        # https://bugs.python.org/issue40696
+        has_cycle = None
+        loop = asyncio.new_event_loop()
+        self.set_event_loop(loop)
+
+        async def process_exc(exc):
+            raise exc
+
+        async def run():
+            nonlocal has_cycle
+            try:
+                raise KeyError('a')
+            except Exception as exc:
+                task = self.new_task(loop, process_exc(exc))
+                try:
+                    await task
+                except BaseException as exc:
+                    has_cycle = (exc is exc.__context__)
+                    # Prevent a hang if has_cycle is True.
+                    exc.__context__ = None
+
+        try:
+            task = self.new_task(loop, run())
+            loop.run_until_complete(task)
+        finally:
+            loop.close()
+        # This also distinguishes from the initial has_cycle=None.
+        self.assertEqual(has_cycle, False)
 
     def test_cancel(self):