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Looking over the code a bit more, I see that my last message
wasn't entirely accurate.  There does seem to be only one
lock for both gethostbyname and gethostbyaddr
(gethostbyname_lock is used for both).

This is a pretty simple test that illustrates the problem
I'm seeing.  My previous work was on my OS X machine, but
this is Python 2.2 (#3, Mar  6 2002, 18:30:37) [C] on irix6.

#!/usr/bin/env python
#
# Copyright (c) 2002  Dustin Sallings <dustin@spy.net>
# $Id$

import threading
import socket
import time

class ResolveMe(threading.Thread):

    hosts=['propaganda.spy.net', 'bleu.west.spy.net',
'mail.west.spy.net']

    def __init__(self):
        threading.Thread.__init__(self)
        self.setDaemon(1)

    def run(self):
        # Run 100 times
        for i in range(100):
            for h in self.hosts:
                nrv=socket.gethostbyname_ex(h)
                arv=socket.gethostbyaddr(nrv[2][0])

                try:
                    # Verify the hostname is correct
                    assert(h==nrv[0])
                    # Verify the two hostnames match
                    assert(nrv[0]==arv[0])
                    # Verify the two addresses match
                    assert(nrv[2]==arv[2])
                except AssertionError:
                    print "Failed!  Checking " + `h` + "
got, " \
                        + `nrv` + " and " + `arv`

if __name__=='__main__':
    for i in range(1,10):
        print "Starting " + `i` + " threads."
        threads=[]
        for n in range(i):
            rm=ResolveMe()
            rm.start()
            threads.append(rm)
        for t in threads:
            t.join()
        print `i` + " threads complete."
    time.sleep(60)

The output looks like this:

verde:/tmp 190> ./pytest.py
Starting 1 threads.
1 threads complete.
Starting 2 threads.
Failed!  Checking 'propaganda.spy.net' got,
('mail.west.spy.net', [], ['66.149.231.226']) and
('mail.west.spy.net', [], ['66.149.231.226'])
Failed!  Checking 'bleu.west.spy.net' got,
('mail.west.spy.net', [], ['66.149.231.226']) and
('mail.west.spy.net', [], ['66.149.231.226'])

[...]