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The following test program crashes:
========================================
import threading, sys, subprocess
# subprocess._cleanup = lambda: None
def doit():
 for i in xrange(0, 1000):
 p = subprocess.Popen( "true" )
 p.wait()
t = threading.Thread( target=doit )
t.start()
doit()
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It crashes because when one thread calls subprocess.Popen(), subprocess
calls this _cleanup() function, which might reap the subprocess started
in another thread ! The other thread might be inside
subprocess.Popen.wait(), just about to call waitpid(), and kill itself.
If you uncomment the commented line, then the program runs with no problems.
I imagine the purpose of _cleanup is to protect users from themselves,
i.e., protect a user who calls subprocess.Popen() a lot without ever
calling wait(). I suggest either:
 (1) eliminating this _cleanup() mechanism completely; people who do
not wait() deserve the zombies they get;
 (2) synchronizing _cleanup() with wait() through a lock; or,
 (3) having wait() simply retry if it gets ECHILD. On the retry, it
will discover that returncode is set, and return normally.
-Ken