Issue python#102: Improve stackless.cstack

- Fix stackless.cstack.__str__. It was broken for Python 3.
Now it returns the stack decoded as iso-8859-1 byte string.
- New read-only attribute stackless.cstack.nesting_level.
It can be used to order multiple cstack objects belonging to the same tasklet.
- Added a few test cases

https://bitbucket.org/stackless-dev/stackless/issues/102

Author: Anselm Kruis

Stackless/core/stacklesseval.c

@@ -189,11 +189,12 @@ Note: For inspection, str() can dump it as a string.\
 
 
 static PyMemberDef cstack_members[] = {
-    {"size", T_INT, offsetof(PyCStackObject, ob_base.ob_size), READONLY},
+    {"size", T_PYSSIZET, offsetof(PyCStackObject, ob_base.ob_size), READONLY},
     {"next", T_OBJECT, offsetof(PyCStackObject, next), READONLY},
     {"prev", T_OBJECT, offsetof(PyCStackObject, prev), READONLY},
     {"task", T_OBJECT, offsetof(PyCStackObject, task), READONLY},
     {"startaddr", T_ADDR, offsetof(PyCStackObject, startaddr), READONLY},
+    {"nesting_level", T_INT, offsetof(PyCStackObject, nesting_level), READONLY},
     {0}
 };
 
@@ -203,8 +204,9 @@ static PyObject *
 cstack_str(PyObject *o)
 {
     PyCStackObject *cst = (PyCStackObject*)o;
-    return PyUnicode_FromStringAndSize((char*)&cst->stack,
-        Py_SIZE(cst)*sizeof(cst->stack[0]));
+    return PyUnicode_Decode((char*)&cst->stack,
+        Py_SIZE(cst)*sizeof(cst->stack[0]),
+		"latin_1", "strict");
 }
 
 PyTypeObject PyCStack_Type = {

Stackless/unittests/test_miscell.py

@@ -10,6 +10,7 @@
 import contextlib
 import time
 import os
+import struct
 from stackless import _test_nostacklesscall as apply_not_stackless
 try:
     import _thread as thread
@@ -1224,6 +1225,30 @@ def test_threads(self):
         self.assertEqual(type(stackless.threads), list)
 
 
+class TestCstate(StacklessTestCase):
+    def test_cstate(self):
+        self.assertIsInstance(stackless.main.cstate, stackless.cstack)
+
+    def test_str_size(self):
+        c = stackless.main.cstate
+        s = str(c)
+        self.assertEqual(len(s), c.size * struct.calcsize("P"))
+
+    def test_nesting_level(self):
+        c = stackless.main.cstate
+        l1 = c.nesting_level
+        self.assertIsInstance(l1, int)
+
+    def test_chain(self):
+        start = stackless.main.cstate
+        c = start.next
+        self.assertIsNot(c, start)
+        while(c is not start):
+            self.assertIsInstance(c, stackless.cstack)
+            self.assertIs(c.prev.next, c)
+            c = c.next
+
+
 #///////////////////////////////////////////////////////////////////////////////
 
 if __name__ == '__main__':