[issue2138] Add a factorial function

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

It looks like there's a refcounting bug in the code: if the call to 
PyNumber_Multiply fails then iobj gets DECREF'd twice.  This means that a 
keyboard interrupt of factorial() can exit the interpreter:

Python 2.6a3+ (trunk:64341M, Jun 17 2008, 13:19:01) 
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from math import factorial
[36374 refs]
>>> factorial(10**9)
^CFatal Python error: 
/Users/dickinsm/python_source/trunk/Modules/mathmodule.c:562 object at 
0x81f63c has negative ref count -1
Abort trap

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<http://bugs.python.org/issue2138>
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