llvm dev - Oct 2020 - FileCheck / lit hanging on mac builders

Hi all,

I've got some internal buildbots running lit tests and occasionally when
run on Mac the entire lit test suite deadlocks.  If I dump the process list
on the machine I can see that lit and FileCheck are both running and
deadlocked.

Only happens on Mac, but it doesn't depend on Python version.  I've seen
this with everything from Python 2.7 to Python 3.8.

Does anyone have any theories on what could be going on?  I've audited the
source code for FileCheck and I don't even see how it's *possible* for
FileCheck to deadlock.

Has anyone else ever seen anything like this or have any theories on what
might be happening?
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