[Python-Dev] Preventing recursion core dumps

[Guido van Rossum]

> I'm looking at preventing core dumps due to recursive calls. With
> simple nested call counters for every function in object.c, limited to
> 500 levels deep recursions, I think this works okay for repr, str and
> print. It solves most of the complaints, like:
> 
> class Crasher:
> 	def __str__(self): print self
> 
> print Crasher()
> 
> With such protection, instead of a core dump, we'll get an exception:
> 
> RuntimeError: Recursion too deep
> 
> 
> So far, so good. 500 nested calls to repr, str or print are likely
> to be programming bugs. Now I wonder whether it's a good idea to do
> the same thing for getattr and setattr, to avoid crashes like:
> 
> class Crasher:
> 	def __getattr__(self, x): return self.x 
> 
> Crasher().bonk
> 
> Solving this the same way is likely to slow things down a bit, but
> would prevent the crash. OTOH, in a complex object hierarchy with
> tons of delegation and/or lookup dispatching, 500 nested calls is
> probably not enough. Or am I wondering too much? Opinions?

In your examples there's recursive Python code involved.  There's
*already* a generic recursion check for that, but the limit is too
high (the latter example segfaults for me too, while a simple def f():
f() gives a RuntimeError).

It seems better to tune the generic check than to special-case str,
repr, and getattr.

--Guido van Rossum (home page: http://www.pythonlabs.com/~guido/)