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[Python-Dev] Disabling string interning for null and single-char causes segfaults

Lukas Lueg lukas.lueg at gmail.com
Sat Mar 2 23:49:52 CET 2013 
Debugging a refcount bug? Good. Out of the door, line on the left, one
cross each.


2013/3/2 Stefan Bucur <stefan.bucur at gmail.com>

> On Sat, Mar 2, 2013 at 4:31 PM, Antoine Pitrou <solipsis at pitrou.net>
> wrote:
> > On Fri, 1 Mar 2013 16:24:42 +0100
> > Stefan Bucur <stefan.bucur at gmail.com> wrote:
> >>
> >> However, after applying this modification, when running "make test" I
> get a
> >> segfault in the test___all__ test case.
> >>
> >> Before digging deeper into the issue, I wanted to ask here if there are
> any
> >> implicit assumptions about string identity and interning throughout the
> >> interpreter implementation. For instance, are two single-char strings
> >> having the same content supposed to be identical objects?
> >
> > From a language POV, no, but inside a specific interpreter such as
> > CPython it may be a reasonable expectation.
> >
> >> I'm assuming that it's either this, or some refcount bug in the
> interpreter
> >> that manifests only when certain strings are no longer interned and thus
> >> have a higher chance to get low refcount values.
> >
> > Indeed, if it's a real bug it would be nice to get it fixed :-)
>
> By the way, in that case, what would be the best way to debug such
> type of ref count errors? I recently ran across this document [1],
> which kind of applies to debugging focused on newly introduced code.
> But when some changes potentially impact a good fraction of the
> interpreter, where should I look first?
>
> I'm asking since I re-ran the failing test with gdb, and the segfault
> seems to occur when invoking the kill() syscall, so the error seems to
> manifest at some later point than when the faulty code is executed.
>
> Stefan
>
> [1] http://www.python.org/doc/essays/refcnt/
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