[Freewx-maint] Bug#242737: libwxgtk2.4-python: Segfaults on invocation of SetTimeT

[Robin Dunn]

On 10/13/11 4:50 AM, Olly Betts wrote:
> # I've reproduced this with some recent versions:
> found 2.8.10.1-3
> found 2.8.10.1+dfsg-4
> found 2.8.12.1-1
> thanks
>
> On Fri, Jul 30, 2004 at 07:25:12PM +0930, Ron wrote:
>> I can reproduce this one in 2.4.2.4, in 2.4 branch head, and
>> in 2.5.1.  Should he just not be doing this (DateTime() doesn't
>> return a valid object reference?) or is this a real bug?
>
> I suspect it's a reference counting bug in the SWIG generated code.  I've
> not looked at the code here though.
>
>>> When creating a new wxDateTime object with a specified number of seconds
>>> since 01-01-1970, SetTimeT returns random dates and eventually
>>> segfaults.  See the following interaction between me and the python
>>> interpreter:
>>>
>>>>>> import wx
>>>>>> d=wx.DateTime().SetTimeT(0)
>>>>>> d
>>> <wxDateTime: "wo 28 mei 633986 15:01:41 CET" at _828ea68_wxDateTime_p>
>
> Interestingly, if I run under valgrind, I consistently get the answer I'd
> expect.  Python is a bit noisy under valgrind, so it's hard to see if it
> is reporting an issue, but I suspect what is happening is that d gets
> released and normally is partly overwritten by the time we try to print it
> out, but valgrind's malloc replacement doesn't overwrite that part of the
> object.


SWIG has a problem with methods that return a reference to self, and it 
treats the intermediate results as temporaries and creates new proxies 
for them.  When chaining calls together like that it ends up thinking 
that the original object is no longer referenced and it lets Python 
delete it.  Since the reference saved in d is actually a proxy to the 
same C++ object then it ends up with an invalid pointer.

I'll add the code to stop SWIG's confusion in this case.  In the 
meantime you can work around it by not chaining the constructor and 
method calls together so you can hold on to the original reference.

  >>> import wx
  >>> d = wx.DateTime()
  >>> d.SetTimeT(0)
  <wx.DateTime: "Wednesday, December 31, 1969 04:00:00 PM" at 
_e06b3200_p_wxDateTime>
  >>> print d
  Wednesday, December 31, 1969 04:00:00 PM


-- 
Robin Dunn
Software Craftsman
http://wxPython.org