Stackless issue python#88: Fix assertion violation caused by

stackless.current.switch()

Fix an assertion violation caused by incorrect usage of
ts->st.del_post_switch.
Add a test case for "switch to current".

This commit also adds the test class TestTaskletSwitching, which was
only in 2.7-slp.

https://bitbucket.org/stackless-dev/stackless/issues/88
(grafted from f0db3bdf1061, b85b07e9f10effdbf52adc71d452d1e93bd3c5fd, 782a1b292460
and 7d9b4398011f)

Author: Anselm Kruis

Stackless/module/taskletobject.c

@@ -747,7 +747,7 @@ impl_tasklet_run_remove(PyTaskletObject *task, int remove)
     STACKLESS_GETARG();
     PyThreadState *ts = PyThreadState_GET();
     PyObject *ret;
-    int inserted, fail, removed=0;
+    int inserted, fail, switched, removed=0;
     PyTaskletObject *prev = ts->st.current;
 
     assert(PyTasklet_Check(task));
@@ -814,9 +814,10 @@ impl_tasklet_run_remove(PyTaskletObject *task, int remove)
     if (fail)
         return NULL;
     /* this is redundant in the interthread case, since insert already did the work */
-    fail = slp_schedule_task(&ret, prev, task, stackless, 0);
+    fail = slp_schedule_task(&ret, prev, task, stackless, &switched);
     if (fail) {
         if (removed) {
+            assert(ts->st.del_post_switch == (PyObject *)prev);
             ts->st.del_post_switch = NULL;
             slp_current_unremove(prev);
         }
@@ -825,6 +826,8 @@ impl_tasklet_run_remove(PyTaskletObject *task, int remove)
             slp_current_uninsert(task);
             Py_DECREF(task);
         }
+    } else if (!switched) {
+        Py_CLEAR(ts->st.del_post_switch);
     }
     return ret;
 }

Stackless/unittests/test_miscell.py

@@ -133,6 +133,35 @@ def test_aliveness3(self):
         self.assertEqual(t.recursion_depth, 0)
 
 
+class TestTaskletSwitching(StacklessTestCase):
+    """Test the tasklet's own scheduling methods"""
+
+    def test_raise_exception(self):
+        c = stackless.channel()
+
+        def foo():
+            self.assertRaises(IndexError, c.receive)
+        s = stackless.tasklet(foo)()
+        s.run()  # necessary, since raise_exception won't automatically run it
+        s.raise_exception(IndexError)
+
+    def test_run(self):
+        c = stackless.channel()
+        flag = [False]
+
+        def foo():
+            flag[0] = True
+        s = stackless.tasklet(foo)()
+        s.run()
+        self.assertEqual(flag[0], True)
+
+    def test_switch_to_current(self):
+        # See https://bitbucket.org/stackless-dev/stackless/issues/88
+        current = stackless.current
+        current.switch()
+        current.switch()  # this second switch used to trigger an assertion violation
+
+
 class TestTaskletThrowBase(object):
 
     def test_throw_noargs(self):