I think I've found a small bug with multiprocessing package on 
Windows. If you try to start a multiprocessing.Process from a Python- 
based Windows service, the child process will fail to run. When 
running the parent process as a regular Python program, everything 
works as expected. 
I've tracked the problem down to how main_path is prepared in 
multiprocessing.forking.get_preparation_data() (lines 370-377): 
def get_preparation_data(name): 
    [...skipped a few lines...] 
    if not WINEXE: 
        main_path = getattr(sys.modules['__main__'], '__file__', None) 
        if not main_path and sys.argv[0] not in ('', '-c'): 
            main_path = sys.argv[0] 
        if main_path is not None: 
            if not os.path.isabs(main_path) and \ 
                                      process.ORIGINAL_DIR is not 
None: 
                main_path = os.path.join(process.ORIGINAL_DIR, 
main_path) 
            d['main_path'] = os.path.normpath(main_path) 
    return d 
When the program is running as a Windows service, but is not packaged 
into a single executable, main_path will become the path to the 
service executable (typically, pythonservice.exe). When this data 
makes it to the child process, the prepare() function will treat 
main_path as a path to a python module, and will try to import it. 
This causes it to fail. 
My quick-and-dirty solution was to check in get_preparation_data() if 
main_path ends with '.exe', and if it does, to not pass it at all. 
This solves the problem in my case, but perhaps there's a better way 
to fix this? Here is my version of get_preparation_data(): 
def get_preparation_data(name): 
    ''' 
    Return info about parent needed by child to unpickle process 
object 
    ''' 
    from .util import _logger, _log_to_stderr 
    d = dict( 
        name=name, 
        sys_path=sys.path, 
        sys_argv=sys.argv, 
        log_to_stderr=_log_to_stderr, 
        orig_dir=process.ORIGINAL_DIR, 
        authkey=process.current_process().authkey, 
        ) 
    if _logger is not None: 
        d['log_level'] = _logger.getEffectiveLevel() 
    if not WINEXE: 
        main_path = getattr(sys.modules['__main__'], '__file__', None) 
        if not main_path and sys.argv[0] not in ('', '-c'): 
            main_path = sys.argv[0] 
        if main_path is not None: 
            if not os.path.isabs(main_path) and \ 
                                      process.ORIGINAL_DIR is not 
None: 
                main_path = os.path.join(process.ORIGINAL_DIR, 
main_path) 
            if not main_path.endswith('.exe'): 
                d['main_path'] = os.path.normpath(main_path) 
    return d

Attached test case demonstrates the issue.

Treating python services like frozen executables solves the issue. The patch is attached.

This looks reasonable to me. If no one beats me to it, I'll check it in tonight.

New changeset 1f41b1ab8924 by brian.curtin in branch '3.1':
Fix #5162. Allow child spawning from Windows services (via pywin32).
http://hg.python.org/cpython/rev/1f41b1ab8924

New changeset 184ae02e3221 by brian.curtin in branch '3.2':
Fix #5162. Allow child spawning from Windows services (via pywin32).
http://hg.python.org/cpython/rev/184ae02e3221

New changeset 3c2bdea18b5c by brian.curtin in branch 'default':
Fix #5162. Allow child spawning from Windows services (via pywin32).
http://hg.python.org/cpython/rev/3c2bdea18b5c

New changeset 6507a5ba5c27 by brian.curtin in branch '2.7':
Fix #5162. Allow child spawning from Windows services (via pywin32).
http://hg.python.org/cpython/rev/6507a5ba5c27

Thanks for the patch!

New changeset a280672d3d8d by brian.curtin in branch '2.7':
Add NEWS item for #5162.
http://hg.python.org/cpython/rev/a280672d3d8d

New changeset c26474c6504a by brian.curtin in branch '3.1':
Add NEWS item for #5162.
http://hg.python.org/cpython/rev/c26474c6504a

New changeset 68ef2bf1aa99 by brian.curtin in branch '3.2':
Add NEWS item for #5162.
http://hg.python.org/cpython/rev/68ef2bf1aa99

New changeset 2c4043070f05 by brian.curtin in branch 'default':
Add NEWS item for #5162.
http://hg.python.org/cpython/rev/2c4043070f05

This issue is not fully fixed, there are some occasions where you can still run into it. One example is if you want to spawn a new multiprocessing.Process as sub process in a multiprocessing.Process:


pythonservice.exe
  - multiprocessing.Process
    - multiprocessing.Process (does not start!)

In this case you get:

  WINSERVICE: False
  WINEXE: False
  _python_exe: C:\Python27\python.exe
  prep data: {'authkey': '...', 'sys_path': [...], 'name': 'test', 'orig_dir': '...', 'sys_argv': ['C:\\Python27\\lib\\site-packages\\win32\\PythonService.exe'], 'main_path': 'C:\\Python27\\lib\\site-packages\\win32\\PythonService.exe', 'log_to_stderr': False}