The symptom is an exact duplicate of the symptom reported in the following (closed) issue:

http://bugs.python.org/issue9775

The issue is also related to the following other issues:

http://bugs.python.org/issue4106
http://bugs.python.org/issue9205
http://bugs.python.org/issue9207

To reproduce the symptom driving the patches that will be attached to this issue:

  git clone git://github.com/pypa/pip.git
  cd pip
  /any/python setup.py dev
  /any/python setup.py test

You can either wait for the entire test suite to finish or you can press ctrl-C at any time (the tests take a long time).  In either case, a traceback like the following will be printed to the console.

  Error in atexit._run_exitfuncs:
  Traceback (most recent call last):
    File "/home/chrism/opt/Python-2.7.3/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
    func(*targs, **kargs)
    File "/home/chrism/opt/Python-2.7.3/lib/python2.7/multiprocessing/util.py", line 284, in _exit_function
      info('process shutting down')
  TypeError: 'NoneType' object is not callable
  Error in sys.exitfunc:
  Traceback (most recent call last):
    File "/home/chrism/opt/Python-2.7.3/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
      func(*targs, **kargs)
    File "/home/chrism/opt/Python-2.7.3/lib/python2.7/multiprocessing/util.py", line 284, in _exit_function
      info('process shutting down')
  TypeError: 'NoneType' object is not callable

From what I understand in other issues, multiprocessing.util._exit_function shouldn't actually be called *after*  the containing module's globals are destroyed (it should be called before), but this does indeed happen.

Patches will be attached that anticipate the symptom and prevent a shutdown error.  One will be attached for Python 2.7 branch, the other for the Python tip.   Each causes functions that are called at shutdown time to keep a reference around to other functions and globals used within the function, and each does some checks for the insane state and prevents an error from happening in this insane state.