Can't launch do-release-upgrade as root (No new release found)

[Francesco Pretto]

Trying to update to Ubuntu 15.04 to Ubuntu 15.10 update-manager GUI silently fails or I constantly receive "No new release found" from command-line acting as root user.

In /etc/update-manager/release-upgrades I correctly have "Prompt=normal"

Attempted programs used:

1) update-manger (GUI, non-root)
2) update-manager -d (CLI/GUI, non-root)
3) sudo update-manager -d (CLI)
3) sudo do-release-manager -d (CLI)

GUI launched as non-root prompts for a new release (update-manager1.png), but as soon as prompted to log to root (update-manager2.png) it fails silently. GUI launched as root *doesn't* prompt for new release.

WORKAROUND:
Only working mean to upgrade is by command line with non-root "do-release-manager", which correctly reports the available release.

TROUBLESHOOTING:
Debugging do-release-upgrade[1][2][3] led me to the following discovery: in MetaRelease.py[4] root succeeds in "opening" the non-existing file "/var/lib/update-manager/meta-release-development", leading to timeout exception while trying to download the remote file[6] checking for changes at line 330. Instead non-root fails trying to access meta-release-development and fallabacks to a file in the home directory[7]. Downloading at line 330 succeeds and the main script can continue.

meta-release-development[5] in my system wasn't present. This is probably a LTS system switched to non-LTS at some point. Everything else seems to be working.

STEPS TO REPRODUCE:
- In a 15.04 system, delete system meta-release-development[5], if present;
- Try to upgrade as a root user (GUI or CLI).

PROPOSED SOLUTION:
Better handle non existing meta-release-development[5] file as root in MetaRelease.py[4].

VERSIONS INFO:
Description: Ubuntu 15.04
Release: 15.04

ubuntu-release-upgrader-core:
  Installed: 1:15.04.14.3
  Candidate: 1:15.04.14.3

[1] /usr/bin/do-release-upgrade
[2]debug-do-release-upgrade-root.png
[3] debug-do-release-upgrade-nonroot.png
[4] /usr/lib/python3/dist-packages/UpdateManager/Core/MetaRelease.py
[5] /var/lib/update-manager/meta-release-development
[6] http://changelogs.ubuntu.com/meta-release-development
[7] ~/.cache/update-manager-core/meta-release

[Francesco Pretto (ceztko)]

Correction in the original description: fixed reference to system wide meta-release-development, *not* meta-release.

"meta-release-development" is not present on my system, only "meta-release-lts".

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Francesco Pretto (ceztko)]

Also affecting Ubuntu 15.10 trying to upgrade to 16.04 LTS

ubuntu-release-upgrader-core:
  Installed: 1:15.10.14.4
  Candidate: 1:15.10.14.4