JDK-8021820 adds -D_DARWIN_UNLIMITED_SELECT to the build, but the fd_set struct is still limited to 1024.

Snippet from man select(2):

   "Although the provision of getdtablesize(2) was intended to allow user programs to be written
     independent of the kernel limit on the number of open files, the dimension of a sufficiently
     large bit field for select remains a problem. The default size FD_SETSIZE (currently 1024) is
     somewhat smaller than the current kernel limit to the number of open files. However, in order
     to accommodate programs which might potentially use a larger number of open files with select,
     it is possible to increase this size within a program by providing a larger definition of
     FD_SETSIZE before the inclusion of <sys/types.h>.

Either:
 1) FD_SETSIZE needs to be set to a larger value, but what value, the kernel limit, or other?
     This is wasteful for most typical apps that don't use large numbers of file descriptors. Or,
 2) If fd is greater than 1024, then an appropriate amount of memory could be allocated and cast
     to an fd_set. The FD_SET macro will write past FD_SETSIZE.

Option 2 above seems most reasonable. Will proceed with making the appropriate changes.