[Nix-dev] Troubleshooting boot failures & rollbacks

[Stephen Whitmore]

Hello nix-dev,

I've been enjoying nixos for everyday use for the last few months: it's
been a pleasure, and my system has been delightfully stable.

At long last though the inevitable occurred: I rebooted, and my system
only partially boots. This wasn't quite as obvious to solve as booting
to the previous configuration: I had made a dozen system rebuilds since
I last rebooted, so I had to spend a bit of time figuring out which
snapshot introduced the failure.

Now I'm in the position of trying to deduce what configuration change
corresponds to that failure. I pinged some folks on #nixos, and an
obvious solution wasn't made apparent. One user noted that they keep
their .nix configuration files in a git repo and put the commit hash in
their system name to correlate snapshots. Smart! But alas, it did not
help me in the moment.

Are there any tips or strategies for dealing with this situation?
Failing that, how do y'all deal with debugging your system when this
happens? Any relevant nix config repos you could point me at would be
appreciated.

Cheers,

  Stephen