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This is a fresh py3k checkout on a fresh Debian Lenny install:
======================================================================
ERROR: test_pool_worker_lifetime (test.test_multiprocessing.WithProcessesTestPoolWorkerLifetime)
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Traceback (most recent call last):
 File "/home/antoine/py3k/py3k/Lib/test/test_multiprocessing.py", line 1076, in test_pool_worker_lifetime
 self.assertNotEqual(sorted(origworkerpids), sorted(finalworkerpids))
TypeError: unorderable types: NoneType() < int()
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Confirmed.
I got a Py3k warning on 2.7 about "unorderable types".

Also witnessed on one of the buildbots:
test test_multiprocessing failed -- Traceback (most recent call last):
 File "/home/pybot/buildarea/3.x.klose-debian-ia64/build/Lib/test/test_multiprocessing.py", line 1076, in test_pool_worker_lifetime
 self.assertNotEqual(sorted(origworkerpids), sorted(finalworkerpids))
TypeError: unorderable types: NoneType() < int()
http://www.python.org/dev/buildbot/3.x.stable/builders/ia64%20Ubuntu%203.x/builds/434/steps/test/logs/stdio 

I can reproduce it almost always under these conditions:
System: Ubuntu Intrepid 64-bit, running on the actual hardware.
CPU: Core 2 Duo.
Load: At least one core maxed out by another process.
On an empty machine I haven't reproduced it yet.

Indeed, I forgot to mention that the Debian Lenny install I reproduce the bug on is a single core (virtual) machine.

I found the problem. On a loaded machine, an attempt is made to
get the pid of the worker before the process has properly started.
There are several places where a sleep could be inserted. Perhaps
Process.start() should wait until the child's pid is not None?

Thanks Stefan for the analysis.
This patch should fix the issue, and make the buildbots happy.

Fixed with r78653 and r78654.

Sometimes it triggers a different issue, with patch applied (ia64 Ubuntu trunk):
test_multiprocessing
test test_multiprocessing failed -- Traceback (most recent call last):
 File "/home/pybot/buildarea/trunk.klose-debian-ia64/build/Lib/test/test_multiprocessing.py", line 1075, in test_pool_worker_lifetime
 while countdown and not all(w.is_alive() for w in p._pool):
 File "/home/pybot/buildarea/trunk.klose-debian-ia64/build/Lib/test/test_multiprocessing.py", line 1075, in <genexpr>
 while countdown and not all(w.is_alive() for w in p._pool):
 File "/home/pybot/buildarea/trunk.klose-debian-ia64/build/Lib/multiprocessing/process.py", line 132, in is_alive
 self._popen.poll()
 File "/home/pybot/buildarea/trunk.klose-debian-ia64/build/Lib/multiprocessing/forking.py", line 106, in poll
 pid, sts = os.waitpid(self.pid, flag)
OSError: [Errno 10] No child processes

It looks like a new incarnation of long standing #1731717.

This last bug is fixed, too.
http://bugs.python.org/issue1731717#msg100643