do-release-upgrade breaks if /etc/apt/preferences.d has a rule against a release

[timeless]

The system was Ubuntu Xenial (16.04.3) LTS.

I occasionally try to install a newer version of OpenSSH by using three files (note that in previous incarnations they had "zenity" instead of "artful", the idea is "a slightly newer release of the platform I'm using for a specific set of packages -- but don't upgrade the farm"):

/etc/apt/preferences.d/01-artful:
Package: *
Pin: release n=artful
Pin-Priority: -10

/etc/apt/preferences.d/02-artful-ssh:
Package: openssh-client
Pin: release n=artful
Pin-Priority: 600

Package: libgssapi-krb5-2
Pin: release n=artful
Pin-Priority: 600

Package: libkrb5-3
Pin: release n=artful
Pin-Priority: 600

Package: libkrb5support0
Pin: release n=artful
Pin-Priority: 600

Package: libk5crypto3
Pin: release n=artful
Pin-Priority: 600

Package: openssh-server
Pin: release n=artful
Pin-Priority: 600

Package: openssh-sftp-server
Pin: release n=artful
Pin-Priority: 600

/etc/apt/sources.list.d/artful.list:
deb http://ca.archive.ubuntu.com/ubuntu/ artful main restricted
deb-src http://ca.archive.ubuntu.com/ubuntu/ artful main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://ca.archive.ubuntu.com/ubuntu/ artful-updates main restricted
deb-src http://ca.archive.ubuntu.com/ubuntu/ artful-updates main restricted
root@mail-ca:~# cat /etc/apt/sources.list.d/artful.list.distUpgrade
deb http://ca.archive.ubuntu.com/ubuntu/ artful main restricted
deb-src http://ca.archive.ubuntu.com/ubuntu/ artful main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://ca.archive.ubuntu.com/ubuntu/ artful-updates main restricted
deb-src http://ca.archive.ubuntu.com/ubuntu/ artful-updates main restricted

----
This didn't work for OpenSSH as the artful version requires a newer version of libc6. So, I decided to do a release upgrade (but left the files in place...)
----
I set `Prompt=normal` in `/etc/update-manager/release-upgrades` and ran
do-release-upgrade

It eventually gave me:
```
The software on this computer is up to date.

There are no upgrades available for your system. The upgrade will now
be canceled.

Do you want to start the upgrade?

 Continue [yN] Details [d]

```

Which was very confusing... Along this path, it even encouraged me to reboot my computer (which I did).

Nothing was harmed, but nothing particularly useful was achieved either.
----

Expected results:
When do-release-upgrade performs an update, it has enough knowledge to ignore the files in /etc/apt/sources.list.d/ because it understands they're likely to conflict with the release upgrade process. It should either do the same form /etc/apt/preferences.d/ or warn to the user in case it runs into a circumstance like mine.

[Brian Murray (brian-murray)]

Could you add your log files about the upgrade? They'll be found in /var/log/dist-upgrade/.

[timeless (timeless)]

Sorry, the logs I have right now are from after I removed the preferences and the system was able to upgrade.

I suppose I could at some point set this up in a VM, but I don't have time now.

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]