Proposal to modify behavior of installworld

[John Marino]

The following has happened to me a few times in the last 2 years:

1) I create a modification to world source
2) "make buildworld" completes successfully
3) Halfway through "make installworld" the system coredumps everywhere 
due to bug introduced in step 1).

With the kernel, you can always just launch the previous kernel which 
was backed up prior to the new kernel getting installed.

I propose that /usr/src/Makefile be changed to optionally backup the 
important directories of world prior to overwriting the binaries (and 
set the option default on) so that it's possible to rollback world 
without having to download the latest snapshot image and cpdup it all. 
And "make rollbackworld" option (or similar) should be added to the 
makefile.

I'd even have an option to configuration to keep the last X versions.

What do people think?
This functionality would be invaluable, especially to those systems not 
running hammer (although I don't think hammer would be all that useful 
in the case that dozens or hundreds of binaries are corrupted).

Anyone have an alternative proposal?

John