[Python-Dev] stack check on Unix: any suggestions?

[Jeremy Hylton]

Does anyone have suggestions for how to detect unbounded recursion in
the Python core on Unix platforms?

Guido assigned me bug 112943 yesterday and gave it priority 9.
http://sourceforge.net/bugs/?func=detailbug&bug_id=112943&group_id=5470

The bug in question causes a core dump on Unix because of a broken
__radd__.  There's another bug (110615) that does that same thing with
recursive invocations of __repr__.

And, of course, there's:
def foo(x): 
    return foo(x)
foo(None)

I believe that these bugs have been fixed on Windows.  Fredrik
confirmed this for one of them, but I don't remember which one.  Would
someone mind confirming and updating the records in the bug tracker?

I don't see an obvious solution.  Is there any way to implement
PyOS_CheckStack on Unix?  I imagine that each platform would have its
own variant and that there is no hope of getting them debugged before
2.0b1. 

We could add some counters in eval_code2 and raise an exception after
some arbitrary limit is reached.  Arbitrary limits seem bad -- and any
limit would have to be fairly restrictive because each variation on
the bug involves a different number of C function calls between
eval_code2 invocations.

We could special case each of the __special__ methods in C to raise an
exception upon recursive calls with the same arguments, but this is
complicated and expensive.  It does not catch the simplest version, 
the foo function above.

Does stackless raise exceptions cleanly on each of these bugs?  That
would be an argument worth mentioning in the PEP, eh?

Any other suggestions are welcome.

Jeremy