In the case of Unix systems, there is a *far* simpler and more portable solution technique, which is to tell the test postmaster to put its socket in some non-world-accessible directory created by the test scaffolding.
+1 - I'm all for KISS.
Of course that doesn't work for Windows, which is why we looked at the random-password solution. But I wonder whether we shouldn't use the nonstandard-socket-location approach everywhere else, and only use random passwords on Windows. That would greatly reduce the number of cases to worry about for portability of the password-generation code; and perhaps we could also push the crypto issue into reliance on some Windows-supplied functionality (though I'm just speculating about that part).
