gh-108549: In asyncio, do not cancel the current task in Task.__step when a SystemExit exception was just raised

Lib/asyncio/tasks.py

@@ -268,7 +268,11 @@ def __step(self, exc=None):
             raise exceptions.InvalidStateError(
                 f'__step(): already done: {self!r}, {exc!r}')
         if self._must_cancel:
-            if not isinstance(exc, exceptions.CancelledError):
+            if not isinstance(exc, (exceptions.CancelledError,
+                                   # gh-108549: do not swallow these
+                                   # 2 exceptions
+                                   SystemExit, KeyboardInterrupt)
+                              ):
                 exc = self._make_cancelled_error()
             self._must_cancel = False
         self._fut_waiter = None

Lib/test/test_asyncio/test_tasks.py

@@ -1891,6 +1891,25 @@ async def notmuch():
             self.loop.run_until_complete(task),
             'ko')
 
+    def test_step_dont_swallow_systemexit(self):
+        # gh-108549: do not swallow
+        # KeybordInterrupt too.
+        async def sub_task():
+            raise SystemExit
+
+        async def gather():
+            try:
+                await asyncio.gather(sub_task(),)
+            except SystemExit:
+                pass
+
+        t = self.new_task(self.loop, gather())
+        with self.assertRaises(SystemExit):
+            self.loop.run_until_complete(t)
+        t.cancel()
+        self.loop.run_until_complete(t)
+        self.assertTrue(t.done())
+
     def test_step_result_future(self):
         # If coroutine returns future, task waits on this future.
 

Misc/NEWS.d/next/Library/2026-08-24-14-28-47.gh-issue-108549.XZ34WD.rst

@@ -0,0 +1 @@
+In ``asyncio.Task.__step``, do not cancel the current task when a ``SystemExit`` (or a ``KeyboardInterrupt``) exception was just raised.

Modules/_asynciomodule.c

@@ -3051,8 +3051,13 @@ task_step_impl(asyncio_state *state, TaskObj *task, PyObject *exc)
     if (task->task_must_cancel) {
         assert(exc != Py_None);
 
-        if (!exc || !PyErr_GivenExceptionMatches(exc, state->asyncio_CancelledError)) {
-            /* exc was not a CancelledError */
+        if (!exc ||
+            (!PyErr_GivenExceptionMatches(exc, state->asyncio_CancelledError) &&
+            !PyErr_GivenExceptionMatches(exc, PyExc_KeyboardInterrupt) &&
+            !PyErr_GivenExceptionMatches(exc, PyExc_SystemExit))
+            ) {
+            /* exc was not a CancelledError,
+            neither SystemExit or KeyboardInterrupt (see gh-108549 )*/
             exc = create_cancelled_error(state, (FutureObj*)task);
 
             if (!exc) {