Since it looks like multiprocessing_fork is not going to be fixable for macOS, the main issue remaining is how to help users avoid this trap (literally).  Should we add a check and issues a warning or error at run time?  Or is a doc change sufficient?

In the meantime, I've merged changes to disable running test_multiprocessing_fork which will sometimes (but not always) segfault on 10.14 Mojave.  I should apologize to Barry and others who have run into this.  I did notice the occasional segfault when testing with Mojave just prior to its release but it wasn't always reproducible and I didn't follow up on it.  Now that the change in 10.14 behavior makes this existing problem with fork no exec more obvious, it's clear that the test segfaults are another manifestation of this.