Issue python#115: fix an unlikely crash, if a context manager silence…

…s an exception

If Stackless calls the method __exit__() of a context manager with an exception and if the method first soft switches to another tasklet and then returns True, the interpreter stack gets corrupted. Fortunately most of the time nothing bad happens.

https://bitbucket.org/stackless-dev/stackless/issues/115

Python/ceval.c

@@ -1793,22 +1793,10 @@ slp_eval_frame_value(PyFrameObject *f, int throwflag, PyObject *retval)
                     err = 0;
                 Py_DECREF(retval);
 
-                if (err >= 0) {
-                    if (err > 0) {
-                        err = 0;
-                        /* There was an exception and a true return */
-                        v = SECOND();
-                        w = THIRD();
-                        STACKADJ(-2);
-                        Py_INCREF(Py_None);
-                        SET_TOP(Py_None);
-                        Py_DECREF(u);
-                        Py_DECREF(v);
-                        Py_DECREF(w);
-                    } else {
-                        /* The stack was rearranged to remove EXIT
-                           above. Let END_FINALLY do its thing */
-                    }
+                if (err > 0) {
+                    err = 0;
+                    /* There was an exception and a True return */
+                    PUSH(PyLong_FromLong((long) WHY_SILENCED));
                 }
                 /* XXX: The main loop contains a PREDICT(END_FINALLY).
                  * I wonder, if we must use it here?

Stackless/changelog.txt

@@ -9,6 +9,10 @@ What's New in Stackless 3.X.X?
 
 *Release date: 20XX-XX-XX*
 
+- https://bitbucket.org/stackless-dev/stackless/issues/115
+  Fix an unlikely crash caused by an context manager, which silences an
+  exception.
+
 - https://bitbucket.org/stackless-dev/stackless/issues/117
   Fix various reference leaks:
   - Leak of a reference to Py_None in generator.throw()

Stackless/unittests/test_defects.py

@@ -492,6 +492,43 @@ def test_kill_during_cPickle_stack_switch(self):
             raise (self.killed[0], self.killed[1], self.killed[2])
         self.assertIsNone(self.killed)
 
+
+class TestContextManager(StacklessTestCase):
+    def test_crash_on_WHY_SILENCED(self):
+        current_switch = stackless.current.switch
+        steps = []
+
+        class CtxManager:
+            def __enter__(self):
+                steps.append(2)
+
+            def __exit__(self, exc_type, exc_val, exc_tb):
+                steps.append(4)
+                current_switch()  # causes a stack corruption upon resuming __exit__
+                steps.append(5)
+                return True  # silence the exception
+
+        def task():
+            try:
+                steps.append(1)
+                return "OK"
+            finally:
+                with CtxManager():
+                    steps.append(3)
+                    1 // 0
+                steps.append(6)
+                # Stackless issue #115 ()
+                # Leaving this finally block crashes Python,
+                # because the interpreter stack is corrupt.
+
+        t = stackless.tasklet(task)()
+        t.run()
+        self.assertListEqual(steps, [1, 2, 3, 4])
+        t.run()
+        r = t.tempval
+        self.assertListEqual(steps, [1, 2, 3, 4, 5, 6])
+
+
 if __name__ == '__main__':
     if not sys.argv[1:]:
         sys.argv.append('-v')