Bug#1032937: breaks upgrades if systemd-resolved can't run

[Michael Biebl]

Am 14.03.23 um 17:31 schrieb Luca Boccassi:
> On Tue, 14 Mar 2023 18:03:53 +0200 =?utf-8?Q?Samuli_Suonp=C3=A4=C3=A4?=
> <suonpaa at diurnalis.fi> wrote:
>>
>>> On 14. Mar 2023, at 17.34, Luca Boccassi <bluca at debian.org> wrote:
>>>
>>> Exactly, so the admin shouldn't install a package that in the
>>> description says:
>>>
>>> "Installing this package automatically overwrites /etc/resolv.conf
> and
>>> switches it to be managed by systemd-resolved."
>>>
>>> if that's not what they want to achieve. I agree that the
>>> /etc/resolv.conf interface is garbage, but there's nothing we can
> do
>>> about it, that's just how it works.
>>> This is the only way read-only images can be supported sanely.
>>
>> Why exactly is it necessary to overwrite /etc/resolv.conf at
> postinst, even if
>> the service will not and can not be started?
>>
>> Wouldn’t it make more sense to make the symlink at a later stage? For
>> Instance when the service is actually started?
> 
> No, because at that point /etc might be part of a read-only filesystem
> and thus it's too late.
> 
>> There’s no point in having resolv.conf symlink to
>> /run/systemd/resolve/stub-resolv.conf when systemd-resolved is not
> running.
> 
> Don't install it if you don't want it to run.
> 
> I might accept a patch that switches to using tmpfiles.d, as long as
> the default is still to enable on install, and that taking a copy of
> the existing resolv.conf is also done, and that it is proven to work on
> a read-only image. That way it's easier to override it with a drop-in.
> I am not going to write and test such patch though, someone else needs
> to do it and send a MR on Salsa.
> 

A simple solution to this issue could be, to copy the existing 
/etc/resolv.conf to /run/systemd/resolve/stub-resolv.conf in postinst.

If systemd-resolved is started, it will be overwritten properly, if not, 
the original resolv.conf should continue to work.

This is how resolvconf does it and how the systemd-resolved package in 
Ubuntu does it:
https://salsa.debian.org/debian/resolvconf/-/blob/unstable/debian/postinst#L101
https://git.launchpad.net/ubuntu/+source/systemd/tree/debian/systemd-resolved.postinst#n31

Would be a simple, 2-line patch

mkdir -p "${DPKG_ROOT}/run/systemd/resolve"
cp "${DPKG_ROOT}/etc/resolv.conf" 
"${DPKG_ROOT}/run/systemd/resolve/stub-resolv.conf"

Regards,
Michael
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