Inside the pw_lock() function in vipw, it is opening /etc/passwd before it
creates /etc/ptmp. This turns out to be a problem because imagine the
following:

1. Another program is making changes to /etc/passwd using the /etc/ptmp
method (i.e. copy passwd to ptmp, alter it, rename it back). It has
completed its changes to /etc/ptmp and is ready to rename it back when its
timeslice expires.

2. vipw comes up and open()s an fd to /etc/passwd, then it's timeslice
expires.

3. The first program comes back up, rename()s /etc/ptmp to /etc/passwd, and
is done.

4. However, vipw stills holds a file descriptor on the _old_ data! Since
the first program's /etc/ptmp no longer exists, vipw copies the old data to
its new /etc/ptmp without ever knowing something is wrong.

I think the proper thing is for vipw to create it's /etc/ptmp first then
open() /etc/passwd.

This condition has apparently existed in vipw for years and still exists in
6.1 and 6.2 beta. If I am misunderstanding something here please let me
know. Thanks for your time.