Specifying "search --label" instead of "search --file" in grub

[Christopher Allan Webber <cwebber@dustycloud.org>]

Heya all,

Today while installing GuixSD I hit a really painful and hard to track
down bug.  It can happen when you've got multiple Guix installs which
both have the same kernel.

  # Set 'root' to the partition that contains the kernel.
  search --file --set ~a/~a~%

Since this searches across multiple devices for something that matches
that file (probably not the most efficient route if the label is known
anyway?) it can break things if multiple filesystems contain the same
kernel file.

In my case, I had the usb key with the Guix install plugged in, and I
had just installed Guix to disk.  I had seemingly messed up my install,
so my system wasn't booting, but then I discovered I couldn't even boot
to the USB key either.

Well, the USB key had the same kernel as my borked install.... so... I
couldn't boot my system!

After nearly an hour of frustration I hit the revelation that this was
probably the case, hit "e" in the grub menu, switched switch from --file
to --label, and I got things working.

I wonder if this might bite anyone else?  I paniced because I seemed
unable to boot into either and thought I had totally borked my fancy
~new machine.  I'd hate for anyone else to go through the same thing!

 - Chris