Christopher Chavez - 2015-09-19

This is to follow up on the merge request from a couple of months ago. I think the original merge request can be rejected. I'd like to revise it, but would like to first get feedback on the approach used.

Again, the issue: umount $InstallDir would typically fail on OS X, e.g.

Unmounting install dir
umount(/Volumes/ESP): Resource busy -- try 'diskutil unmount'

As discussed on http://apple.stackexchange.com/a/129244 OS X suggests using diskuitl unmount instead of umount, which suceeds in unmounting the ESP:

Unmounting install dir
Volume EFI on disk0s1 unmounted

I'd rather contribute a quality fix for the issue: in particular, either check if diskutil is available before attempting to use it, or distinguish OS X from any other Darwin (e.g. PureDarwin), e.g. checking for and using sw_vers in addition to uname. (From what an IRC user told me, diskutil and other closed-source components are not to be included--at the same time, I don't know if umount works in their case.)

Would checking for diskutil suffice, or should the installer distinguish OS X from non-OS X Darwin (which might be useful for other installer purposes), or both? I might then be able to provide a revised fix.