MacOS: Disk full; Terminal in Recovery mode won’t delete files; boot panics

I had the same issue with Mojave actually, including the kernel panic at boot. Fortunately I was able to free enough space and find a culprit.

It turned out that not only was the disk very near full after upgrading but a running app process was failing to do something (thanks, new sandbox rules!) and it would fail forever, growing and growing one of its preferences files to gigabyte size.

So in my case at least, Apple’s system “protection” basically created a risk to the system. The OS simultaneously caused a program to trigger a failure condition that the app wouldn’t have otherwise had, and then the OS did not protect the system enough when the app’s failure response was to grow a file on disk without limit! Apple even redesigned the damned filesystem, meaning a lot of tricks to erase files didn’t seem to work. I was desperately blowing away anything I could think of while trying to identify the process working against me. (This is incredibly hard. Processes fail to spawn, etc. and lots of things go to hell. If you don’t already have a couple of shells open somewhere you’re probably screwed.)

Do not let macOS fill up its disk. And I’m really starting to wonder what is up with Apple engineering.

My question, and my extremely lucky answer. Posting here in the hope that this helps others avoid the same situation.

I mean ideally this wouldn't happen in the first place. It seems far too easy to totally screw your system.