[Python-Dev] stack check on Unix: any suggestions? (original) (raw)

Guido van Rossum guido@beopen.com
Thu, 31 Aug 2000 11:07:16 -0500

I'm confused now: how is this counting-stack-limit different from the maximum recursion depth we already have?

The whole point of PyOSStackCheck is to do an actual check of whether there's space left for the stack so we can hopefully have an orderly cleanup before we hit the hard limit. If computing it is too difficult because getrlimit isn't available or doesn't do what we want we should probe it, as the windows code does or my example code posted yesterday does. Note that the testing only has to be done every first time the stack goes past a certain boundary: the probing can remember the deepest currently known valid stack location, and everything that is shallower is okay from that point on (making PyOSStackCheck a subroutine call and a compare in the normal case).

The point is that there's no portable way to do PyOS_CheckStack(). Not even for Unix. So we use a double strategy:

(1) Use a user-settable recursion limit with a conservative default. This can be done portably. It is set low by default so that under reasonable assumptions it will stop runaway recursion long before the stack is actually exhausted. Note that Emacs Lisp has this feature and uses a default of 500. I would set it to 1000 in Python. The occasional user who is fond of deep recursion can set it higher and tweak his ulimit -s to provide the actual stack space if necessary.

(2) Where implementable, use actual stack probing with PyOS_CheckStack(). This provides an additional safeguard for e.g. (1) extensions allocating lots of C stack space during recursion; (2) users who set the recursion limit too high; (3) long-running server processes.

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