Description of problem:
After the "leapp upgrade" phase, all looks good, the system is rebooted on the specific initramfs used for the upgrade, updates are done, and then the system fails to reboot. It ends at the grub prompt.

Version-Release number of selected component (if applicable):
leapp-0.10.0-2.el7_8.noarch
leapp-repository-0.10.0-2.el7_8.noarch

How reproducible:
100%

Steps to Reproduce:
1. Use a custom /etc/default/grub with blank lines between each variable (or with comment after the line containing GRUB_CMDLINE_LINUX=)
2. Proceed to the leapp upgrade and observe after the (second) reboot.

Actual results:
It ends at the grub cli prompt, as if grub.cfg doesn't exist.

Expected results:
Provide the standard grub menu with the RHEL 8 entry (with old entries as well).

Additional info:
After rescuing the el8 system, we can notice /etc/default/grub contains:
~~~
GRUB_CMDLINE_LINUX="elevator=noop
~~~

It lacks the misleading double-quote.

Here is the original file used by the customer, it was valid:
~~~
# WARNING: This file maintained by Puppet.
# Editing is no use unless you avoid running the Puppet agent

GRUB_DEFAULT=0
GRUB_TIMEOUT=5
##GRUB_HIDDEN_TIMEOUT=0
##
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"

GRUB_CMDLINE_LINUX_RECOVERY="true"

GRUB_CMDLINE_LINUX="elevator=noop"

# Uncomment to disable graphical terminal (grub-pc only)

#GRUB_TERMINAL="console"

GRUB_GFXMODE=640x480
GRUB_DISABLE_RECOVERY="true"


# Set a background image
# Image must respect various specifications
GRUB_BACKGROUND="(hd0,0)/grub/splash.xpm.gz"

# Disable OS prober
GRUB_DISABLE_OS_PROBER="true"
~~~

It seems the pattern used in /usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/detectgrubconfigerror/libraries/scanner.py is wrong:
~~~
pattern = r'GRUB_CMDLINE_LINUX="[^"]+"(?!(\s*$)|(\s+GRUB))'
~~~

Looks like it expects the next line begins with GRUB.

Maybe we can use "grub2-mkconfig -o /tmp/test" to check if the file has a good syntax.

For now, the suggested workaround is to remove blank lines and any comments before launching the upgrade:
~~~
# sed -i -e '/^#/d' -e '/^$/d' /etc/default/grub
~~~