Bug#1052690: grub2: post-install script overrides manual changes to GRUB_DISABLE_OS_PROBER

[Jordi Bosveld]

Hello all,

It seems that the debconf setting 'grub2/enable_os_prober' gets reset to 
false every time the package is reconfigured, possibly as part of a 
package upgrade or reinstallation. When one invokes 'dpkg-reconfigure 
grub-efi-amd64' (or grub-pc etc.) the previously chosen answer to the 
'enable_os_prober' query is forgotten and the preselected answer is now 
'No'. When dpkg-reconfigure is run with the '-phigh' option this happens 
silently and os-prober is once again disabled. The same occurs when 
reinstalling or upgrading the corresponding grub package.

> Additional information: It turns out that the post-install script
> patches /etc/default/grub here:
Yes, but by the time this postinst script is executed, debconf will have 
already been misconfigured and the 'db_get grub2/enable_os_prober' 
command as seen on line 401 will receive a possibly incorrect 'false'.

The double negative used in the configuration is a bit confusing, 
especially once you deal with a commented line (meaning its contents are 
disabled) declaring that the variable that dictates whether or not to 
disable a certain setting should be set to disabled. The package script 
'config' warns the reader 'Watch for the inverted logic here', and 
ironically this is where the bug is at. I have attached a patch that 
will rectify this issue. For simplicity I decided to compare the config 
variable GRUB_DISABLE_OS_PROBER from /etc/default/grub to 'false', i.e. 
a value of exactly 'false' will set the 'grub2/enable_os_prober' debconf 
setting to 'true' and all other contents will result in 
'grub2/enable_os_prober = false'. One could possibly expand the check to 
include 0, no, False, FALSE and whatnot, but that is ultimately up to 
the maintainers and I wanted to keep things simple. With this patch 
applied the previously chosen answer is remembered and a 
(re)installation or reconfiguration no longer changes the setting. Also, 
manually uncommenting the line with GRUB_DISABLE_OS_PROBER=false (as 
mentioned in the package changelog) will now result in debconf properly 
changing the 'grub2/enable_os_prober' setting to 'true' the next time it 
is invoked, so on upgrades and such that line will no longer be 
rewritten into something that disables os-prober. Changing it to 
anything other than 'false' will result in 'grub2/enable_os_prober' to 
be set to 'false', and consequently the line containing 
GRUB_DISABLE_OS_PROBER=whatever in /etc/default/grub will get rewritten 
to the default commented '#GRUB_DISABLE_OS_PROBER=false', which means 
the setting will now default to 'true', as before.

-- 
Kind regards,

Jordi Bosveld
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