[Python-Dev] Debugging opportunity :-) (original) (raw)

Phillip J. Eby pje at telecommunity.com
Sat Apr 15 00:26:21 CEST 2006

At 11:51 PM 4/14/2006 +0200, Thomas Wouters wrote:

On 4/14/06, Phillip J. Eby <<mailto:pje at telecommunity.com>pje at telecommunity.com> wrote:

At 03:49 PM 4/14/2006 -0400, Tim Peters wrote: > That eventually >pointed to an off-by-one error in the new PyGenNeedsFinalizing(), >where reading up trash made it very likely that old generators >containing an active loop would falsely claim they need finalization. >Alas, fix that, and Python segfaults when running testgenerators.

Even with the close() gimmicks still in place? If so, it sounds like the safest thing for a2 might be to claim that generators always need finalizing. :( No, it sounds like there's a bug in generator cleanup :-) From Tim's description, it isn't obvious to me that the bug is in the hack to clean up simple generator frames. I had "finding generator-cycle leak in testgenerators" on my TODO list (as I'd noticed it was still leaking, too), I've now replaced that with "finding the generator-cleanup crash Tim encountered" :-) I don't know if I'll get to it this weekend, though, so if anyone else wants to hunt for it, don't let me stop you.

FYI, the smallest code I've found so far that reproduces the crash is:

fun_tests = """

class LazyList: ... def init(self, g): ... self.v = None ... self.g = g ... ... def iter(self): ... yield 1 ... if self.v is None: ... self.v = self.g.next() ... yield self.v

def loop(): ... for i in ll: ... yield i

ll = LazyList(loop()) g=iter(ll) g.next() 1 g.next() 1 """

If you replace the fun_tests in test_generators with this, and remove all the other tests (e.g. pep_tests, tut_tests, etc.) you can still get it to crash. The code above is much shorter, but has the disadvantage that I don't really know what it's supposed to do, other than cause a crash. :)

Interestingly, this code does not crash the interpreter on its own, only when run as a doctest (with or without regrtest). I still haven't figured out what the actual problem is, but at least this cuts out all the merge() and times() crud in the case this was derived from, reducing it to a pure zen essence of non-meaning. :)

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