Because the previous commit changed how MFBT tests are linked, they now
use mozjemalloc. Mozjemalloc randomizes small allocations, which id does
by using MFBT's RandomNum. The code in RandomNum, on mac, uses a system
API that allocates memory. So mozjemalloc has some code to handle the
recursion gracefully.
When the RandomNum test runs, it essentially only runs the RNG... which
goes on to allocate memory, which then goes into the RNG. Needless to
say, that doesn't go well. In typical cases, this is not the type of
things that would happen, but it does happen for that one test.
We work around the issue by allocating memory first, which is actually
hard, because compilers like to optimize unused allocations away. So we
turn the existing code into one that uses an allocation instead of an
array on the stack.
Differential Revision: https://phabricator.services.mozilla.com/D105242
25c8cf7dd2