Issue python#123: Fix an assertion violation during interpreter shutdown

During interpreter shutdown Stackless kills tasklets on daemon threads, if they
have a non-trivial C-stack. This caused an assertion violation.

https://bitbucket.org/stackless-dev/stackless/issues/123
(grafted from 9fd4f6a9d266250bef5baa692b89516e73b00e62)

Author: Anselm Kruis

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/123
+  Fix a rare assertion violation during interpreter shutdown with active
+  daemon threads.
+
 - https://bitbucket.org/stackless-dev/stackless/issues/121
   Fix a reference leak. If Python clears a thread state and a destructor or a
   weakref-callback runs Python code, Python used to leak a reference to a

Stackless/module/taskletobject.c

@@ -46,8 +46,12 @@ slp_current_remove(void)
     PyThreadState *ts = PyThreadState_GET();
     PyTaskletObject **chain = &ts->st.current, *ret;
 
-    /* make sure tasklet belongs to this thread */
-    assert((*chain)->cstate->tstate == ts);
+    /* Make sure that the tasklet belongs to this thread.
+     * During interpreter shutdown '(*chain)->cstate->tstate' may be already NULL.
+     * See function slp_kill_tasks_with_stacks() in stacklesseval.c
+     */
+    assert((*chain)->cstate->tstate == ts ||
+           (*chain)->cstate->tstate == NULL);
 
     --ts->st.runcount;
     assert(ts->st.runcount >= 0);