Comment 0 for bug 1684198

All invokations of `sudo do-release-upgrade` fail due to `An unresolvable problem occurred while calculating the upgrade.` after a previous run has been aborted with Ctrl+C. The output is:

```
Hit http://de.archive.ubuntu.com/ubuntu zesty-backports InRelease
Get:3 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Debian_8.0 Release.gpg [481 B]
Ign http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Debian_8.0 Release.gpg
Fetched 90,7 kB in 0s (0 B/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done

Updating repository information

Third party sources disabled

Some third party entries in your sources.list were disabled. You can
re-enable them after the upgrade with the 'software-properties' tool
or your package manager.

To continue please press [ENTER]

Hit http://de.archive.ubuntu.com/ubuntu zesty InRelease
Get:1 http://security.ubuntu.com/ubuntu zesty-security InRelease [89,2 kB]
Hit http://de.archive.ubuntu.com/ubuntu zesty-updates InRelease
Hit http://de.archive.ubuntu.com/ubuntu zesty-backports InRelease
Fetched 89,2 kB in 0s (0 B/s)

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done

Calculating the changes

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
```

The release in `sources.list` have been altered and the release upgrade should:

  * explain what's going on - why give a list of possible reasons if checking these reasons programmatically is absolutely possible -> apply principles of "Don't make me think" design
  * not say that it restores the original system state without making sure that it actually does that because it does _not_
  * restore the system state reliably, e.g. detect a mismatch between `lsb_release` output and release in `sources.list` (in doubt ask the user for input)
  * warn about potentially unwanted changes of crucial system configuration when pressing Ctrl+C and add require the user to press it a second time/send SIGINT twice
  * after more than 20 version upgrades over more than 10 years just work perfectly better sooner than later

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: ubuntu-release-upgrader-core 1:16.10.10
ProcVersionSignature: Ubuntu 4.8.0-46.49-generic 4.8.17
Uname: Linux 4.8.0-46-generic x86_64
NonfreeKernelModules: zfs zunicode icp zcommon znvpair zavl
ApportVersion: 2.20.3-0ubuntu8.2
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Wed Apr 19 16:51:42 2017
InstallationDate: Installed on 2015-12-12 (494 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to yakkety on 2017-04-19 (0 days ago)