Issue python#101: Support debugging Stackless Python with gdb

Enhance Tools/gdb/libpython.py to also support Stackless Python.

https://bitbucket.org/stackless-dev/stackless/issues/101
(grafted from 0dcb4381fdc89601fcb56527805ec387926d3098)

Stackless/changelog.txt

@@ -9,8 +9,13 @@ What's New in Stackless 3.X.X?
 
 *Release date: 20XX-XX-XX*
 
+- https://bitbucket.org/stackless-dev/stackless/issues/101
+  Enhance Tools/gdb/libpython.py to support debugging Stackless Python.
+  You can now debug Stackless Python with gdb. Unfortunately gdb still
+  does not know about inactive tasklets and their frames.
+
 - https://bitbucket.org/stackless-dev/stackless/issues/99
-  On UNIX like systems you can use the command 
+  On UNIX like systems you can use the command
   $ make teststackless
   to run the Stackless unit tests. Previously the command required some non
   POSIX commands.

Tools/gdb/libpython.py

@@ -86,6 +86,10 @@
 
 ENCODING = locale.getpreferredencoding()
 
+# name of the evalframeex function. It CPython and Stackless Python
+# use different names
+EVALFRAMEEX_FUNCTION_NAME = None
+
 class NullPyObjectPtr(RuntimeError):
     pass
 
@@ -1426,7 +1430,17 @@ def is_python_frame(self):
 
     def is_evalframeex(self):
         '''Is this a PyEval_EvalFrameEx frame?'''
-        if self._gdbframe.name() == 'PyEval_EvalFrameEx':
+        global EVALFRAMEEX_FUNCTION_NAME
+        if EVALFRAMEEX_FUNCTION_NAME is None:
+            try:
+                gdb.lookup_type("PyCFrameObject")
+                # it is Stackless Python
+                EVALFRAMEEX_FUNCTION_NAME = 'PyEval_EvalFrame_value'
+            except gdb.error:
+                # regular CPython
+                EVALFRAMEEX_FUNCTION_NAME = 'PyEval_EvalFrameEx'
+
+        if self._gdbframe.name() == EVALFRAMEEX_FUNCTION_NAME:
             '''
             I believe we also need to filter on the inline
             struct frame_id.inline_depth, only regarding frames with