How to rescue guix system from failed grub install

[ndre]

Hello Guix and Freax!

Two days back I've upgraded my guix system using the common 'guix pull
&& guix reconfigure' routine and after rebooting I couldn't boot any
system generations. Grub was able to unlock and mount my cryptodisk
just fine, but after that when I did try to boot any generation, grub
would error out and show the following messages:

error: can't find command 'search'
error: can't find command 'linux'
error: can't find command 'initrd'
Press any key to continue...

Another issue that I noticed was that grub's resolution was lower than
before. Trying to insmod the missing commands gave out the following
errors:

linux - error: ELF header smaller than expected
initrd - error: file /boot/grub/i386-pc/initrd.mod not found

Moreover, some other commands such as lsmod were missing as well.

With the help of quiliro and nckx, I was able to rescue the system
using the grub from guix installer image and since I didn't find
instructions elsewhere, I'll document here what I did so others can
benefit.

1. Boot guix installer image;
2. Press 'c' to get to grub's command line;
3. type 'insmod luks'; # skip this and the next if disk is not
                       # encrypted
4. type 'cryptomount -a'; # should mount (crypto0)
#  change crypto0 for hd1 if unencrypted
5. type 'configfile (crypto0)/boot/grub/grub.cfg'.

With the above I was able to boot the system and reconfigure it again
without issues this time. Since it worked the second time without a
new pull, it seems I may have a faulty untrustworthy hard disk and
the issue is probably not related to any bug/regressions.

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