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http://hg.python.org/sandbox/sbt#spawn now contains support for starting processes via a separate server process. This depends on fd passing support.
This also solves the problem of mixing threads and processes, but is much faster than using fork+exec. It seems to be just as fast as using plain fork.
I have tested it successfully on Linux and a MacOSX buildbot. (OpenSolaris does not seem to support fd passing.)
At the begining of your program you write
 multiprocessing.set_start_method('forkserver')
to use the fork server.
Alternatively you can use
 multiprocessing.set_start_method('spawn')
to use _posixsubprocess.fork_exec() with closefds=True on Unix or 
 multiprocessing.set_start_method('fork')
to use the standard fork method.
This branch also stops child processes on Windows from automatically inheriting inheritable handles.
The test suite can be run with the different start methods by doing
 python -m test.test_multiprocessing_fork
 python -m test.test_multiprocessing_spawn
 python -m test.test_multiprocessing_forkserver