gh-126366: Fix crash if __iter__ raises an exception during yield from (#126369)

Author: ZeroIntensity

Lib/test/test_yield_from.py

@@ -1576,6 +1576,19 @@ def outer():
             self.assertIsNone(caught.exception.__context__)
             self.assert_stop_iteration(g)
 
+    def test_throws_in_iter(self):
+        # See GH-126366: NULL pointer dereference if __iter__
+        # threw an exception.
+        class Silly:
+            def __iter__(self):
+                raise RuntimeError("nobody expects the spanish inquisition")
+
+        def my_generator():
+            yield from Silly()
+
+        with self.assertRaisesRegex(RuntimeError, "nobody expects the spanish inquisition"):
+            next(iter(my_generator()))
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2024-11-03-15-15-36.gh-issue-126366.8BBdGU.rst

@@ -0,0 +1,2 @@
+Fix crash when using ``yield from`` on an object that raises an exception in
+its ``__iter__``.

Python/bytecodes.c

@@ -2811,11 +2811,12 @@ dummy_func(
             }
             else {
                 /* `iterable` is not a generator. */
-                iter = PyStackRef_FromPyObjectSteal(PyObject_GetIter(iterable_o));
+                PyObject *iter_o = PyObject_GetIter(iterable_o);
                 DEAD(iterable);
-                if (PyStackRef_IsNull(iter)) {
+                if (iter_o == NULL) {
                     ERROR_NO_POP();
                 }
+                iter = PyStackRef_FromPyObjectSteal(iter_o);
                 DECREF_INPUTS();
             }
         }

Python/executor_cases.c.h

@@ -71,6 +71,7 @@ test.test_socket.RecvmsgSCMRightsStreamTest.testCmsgTruncLen1
 test.test_socket.RecvmsgSCMRightsStreamTest.testCmsgTruncLen2Minus1
 test.test_subprocess.POSIXProcessTestCase.test_exception_bad_args_0
 test.test_subprocess.POSIXProcessTestCase.test_exception_bad_executable
+test.test_subprocess.POSIXProcessTestCase.test_vfork_used_when_expected
 test.test_subprocess.ProcessTestCase.test_cwd_with_relative_arg
 test.test_subprocess.ProcessTestCase.test_cwd_with_relative_executable
 test.test_subprocess.ProcessTestCase.test_empty_env