Troubleshooting Munki failing to install Apple updates

Problem

If you see Apple updates (that require a reboot) not installing properly via Munki, it may be because the downloaded update is stale somehow. Not really 100% sure on how this works, since I've seen this fail even on a "stale" update downloaded the same day (not weeks ago).

Symptoms

When you log out to update, you'll see Munki's progress bar over the login screen window, and it will look for a split second as if it's trying to install the pending Apple update but then move on almost immediately to requiring a reboot.

Then, if you check the logs at /Library/Managed Installs/Logs/Install.log, you'll see something like

Apple Software Update install of Security Update 2017-001-10.12.6: FAILED for unknown reason

Workarounds

You could create a script (run from its own Launch Daemon or as part of a Munki run) to clear old updates from the /Library/Updates folder periodically (though, again, I saw this happen even with a recently downloaded update).

I've found that if you run

softwareupdate -d -a
the newly downloaded update will install just fine via Managed Software Center.

This is tricky, because it's not technically a Munki issue (Munki just uses Apple's built-in softwareupdate to install Apple software updates), but clearly there's some flaw in invoking the software update mechanism.

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