[Bug 1786484] Update Released

[Chris Halse Rogers]

The verification of the Stable Release Update for ubuntu-release-
upgrader has completed successfully and the package has now been
released to -updates.  Subsequently, the Ubuntu Stable Release Updates
Team is being unsubscribed and will not receive messages about this bug
report.  In the event that you encounter a regression using the package
from -updates please report a new bug using ubuntu-bug and tag the bug
report regression-update so we can easily find any regressions.

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https://bugs.launchpad.net/bugs/1786484

Title:
  free space need for /usr could be negative which is wrong

Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Released
Status in ubuntu-release-upgrader source package in Cosmic:
  Fix Released

Bug description:
  [Impact]
  The checkFreeSpace function doesn't ensure that the space needed is a positive number thereby causing a situation where a partition, like /usr, doesn't have enough free space which then results in a failure during upgrade.

  [Test Case]
  It's not clear to me how to recreate this scenario, however I believe this falls into the "obviously safe" patch category. The specific change to checkFreeSpace, which is described as "this checks if we have enough free space", from DistUpgradeCache.py follows:

  1200             # we are ensuring we have more than enough free space not less
  1201             if size < 0:
  1202                 continue

  This stops the needed space from being a negative number.

  [Regression Potential]
  Other than a typo in the change there won't be any.

  [Original Description]
  In my case I ran out of diskspace in /usr which required a temporary 4 GB of more data during upgrade. Things just start to fail, and I can tell you - then you are screwed. I find this unacceptable for a wide-spread official LTS upgrader.

  ProblemType: BugDistroRelease: Ubuntu 18.04
  Package: ubuntu-release-upgrader-core 1:18.04.22
  ProcVersionSignature: Ubuntu 4.15.0-30.32-generic 4.15.18
  Uname: Linux 4.15.0-30-generic x86_64
  NonfreeKernelModules: nvidia_modeset nvidia
  ApportVersion: 2.20.9-0ubuntu7.2
  Architecture: amd64
  CrashDB: ubuntu
  CurrentDesktop: KDE
  Date: Fri Aug 10 15:14:33 2018
  InstallationDate: Installed on 2016-01-11 (941 days ago)
  InstallationMedia: Kubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
  PackageArchitecture: allSourcePackage: ubuntu-release-upgrader
  Symptom: release-upgrade
  UpgradeStatus: Upgraded to bionic on 2018-08-09 (0 days ago)
  mtime.conffile..etc.update-manager.release-upgrades: 2018-08-06T15:08:30.181962

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