[Stackless] Stackless Digest, Vol 90, Issue 3

[Christian Tismer]

On 7/8/11 1:53 AM, Richard Tew wrote:
> On Thu, Jul 7, 2011 at 10:10 PM, Hussain Bohra
> <hussainbohra_30 at yahoo.com>  wrote:
>> Hi Christian,
>>
>> Thanks for your feedback.
>>
>> Could you kindly let us know when you are done with the changes, so that we
>> can complete the existing tasks that are dependent on this fix.
> Can't you work around it?  Like check if a frame is a cframe and skip
> it, if it is the case?  I haven't looked at the traceback generation
> in detail, but I imagine that something along the following lines
> would work..
>
> lastTasklet = stackless.current
> thisTasklet = lastTasklet.next
> while True:
>      thisFrame = thisTasklet.frame
>      while thisFrame is not None:
>          if not isinstance(thisFrame, stackless.cframe):
>              # Do traceback related stuff.
>          thisFrame = thisFrame.f_back
>      thisTasklet = thisTasklet.next
>      if lastTasklet is thisTasklet:
>          break

Hi Richard, Hussain,

yes, I had a look now, and you are right. CFrames get into the way,
but are a small problem. Your workaround should be a simple quick
solution in the first place, and I encourage Hussain to try that first.
It is a really simple patch to traceback.py: insert a check into 
extract_stack
before "if limit is None:":

     while f and not isinstance(f, types.FrameType):
         f = f.f_back

That should do.

---------------------------------------------
Now to the omission in stackless:
The special treatment of cframes is quite limited in stackless:

- sys._getframe has a patch to skip over anything that is no normal frame.
- frame objects have a getter function frame_getback() instead of the f_back
    field.

The only way how the treatment of cframes might be circumvented
in extract_stack is when a cframe is given as an argument, and that
was the omission in stackless:
tasklet.frame exposes whatever it has to the user. Only for the running
tasklet, this gets redirected to the threadstate's frame, which in the
from python visible context never is a cframe.

The function slp_get_frame is the culprit (stackless_util.c):

/* CAUTION: This function returns a borrowed reference */
PyFrameObject *
slp_get_frame(PyTaskletObject *task)
{
     PyThreadState *ts = PyThreadState_GET();

     return ts->st.current == task ? ts->frame : task->f.frame;
}

This is correct for builtin API functions, but not for the python
interface. The getter methon tasklet_get_frame (taskletobject.c)
should skip over cframes.

static PyObject *
tasklet_get_frame(PyTaskletObject *task)
{
     PyObject *ret = (PyObject*) slp_get_frame(task);

     if (ret == NULL) ret = Py_None;
     Py_INCREF(ret);
     return ret;
}

I think something like the following should do it:

static PyObject *
tasklet_get_frame(PyTaskletObject *task)
{
     PyFrameObject *f = (PyFrameObject *) slp_get_frame(task);
     PyObject *ret;

     while (f != NULL && !PyFrame_Check(f)) {
             f = f_fback;
     }
     ret = (PyObject *) f;

     if (ret == NULL) ret = Py_None;
     Py_INCREF(ret);
     return ret;
}


Not yet tested, but I guess that should do it.
Do you want to try when removing tasklet_become, or should I?

cheers -- chris

-- 
Christian Tismer             :^)<mailto:tismer at stackless.com>
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