[Bug 1446552] Re: Unattended upgrades handles new dependencies inconsistently

[Balint Reczey]

** Description changed:

+ [Impact]
+ 
  When an installed package adds a dependency that is not yet installed on
  the system, this sometimes causes the package to not be installed,
  depending on the origin containing the original candidate version.
  
  I believe that the problem is in /usr/bin/unattended-upgrade, line 102.
  Here a check is performed to prevent downgrades. However, as a side
  effect it also prevents adjusting the candidate version for packages
  that have not yet been installed (because pkg.is_upgradable is False for
  packages that have not been installed).
  
  This makes updating private packages using unattended-upgrades
  troublesome, especially when new dependencies have been added. Currently
  it requires repackaging the dependencies with a slightly higher version
  number than what is in the main repository, and then hosting them on the
  private repository, which is time consuming and error-prone. With the
  included patch, it is sufficient to just host the same version on the
  private repository.
+ 
+ [Test Case]
+ 
+ - Create a testing package (doesn't have to really contain anything) that just installs 1 file into /usr/share/testpackage/, and have it depend on some packages.
+ - Put that package on a private repository (which is also configured for APT and unattended-upgrades)
+ - Install the package using `apt-get install testingpackage`
+ - Update the package as follows: 1. Add a dependency which is not yet installed on your machine (and is also not in the security-repository). Up the version number, and add it to the private repository.
+ - Run `unattended-upgrade --debug --apt-debug 2>&1 | tee output.txt`.
+ - Verify the package was not updated (missing dependency).
+ - Host the dependency on your private APT server as well (1-1 copy).
+ - Run `unattended-upgrade --debug --apt-debug 2>&1 | tee output.txt`.
+ - Verify the package was not updated (missing dependency).
+ - Re-build the dependency with a higher version number, and add it to your private APT repository.
+ - Run `unattended-upgrade --debug --apt-debug 2>&1 | tee output.txt`.
+ - Verify the package was now upgraded.
+ 
+ With the proposed patch, the upgrade would already succeed after hosting
+ the exact copy on the private APT repository.
+ 
+ [Regression Potential]
+ 
+ The changed code logic now allows adjusting candidates of packages which
+ are not upgradable and not installed. Since the changed check was there
+ to avoid downgrades the possible regression would be somehow enabling
+ downgrades accidentally. Adjusting _not_ installed packages in itself
+ would not cause downgrading installed packages thus the change seems to
+ be safe.

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https://bugs.launchpad.net/bugs/1446552

Title:
  Unattended upgrades handles new dependencies inconsistently

Status in unattended-upgrades package in Ubuntu:
  Fix Released

Bug description:
  [Impact]

  When an installed package adds a dependency that is not yet installed
  on the system, this sometimes causes the package to not be installed,
  depending on the origin containing the original candidate version.

  I believe that the problem is in /usr/bin/unattended-upgrade, line
  102. Here a check is performed to prevent downgrades. However, as a
  side effect it also prevents adjusting the candidate version for
  packages that have not yet been installed (because pkg.is_upgradable
  is False for packages that have not been installed).

  This makes updating private packages using unattended-upgrades
  troublesome, especially when new dependencies have been added.
  Currently it requires repackaging the dependencies with a slightly
  higher version number than what is in the main repository, and then
  hosting them on the private repository, which is time consuming and
  error-prone. With the included patch, it is sufficient to just host
  the same version on the private repository.

  [Test Case]

  - Create a testing package (doesn't have to really contain anything) that just installs 1 file into /usr/share/testpackage/, and have it depend on some packages.
  - Put that package on a private repository (which is also configured for APT and unattended-upgrades)
  - Install the package using `apt-get install testingpackage`
  - Update the package as follows: 1. Add a dependency which is not yet installed on your machine (and is also not in the security-repository). Up the version number, and add it to the private repository.
  - Run `unattended-upgrade --debug --apt-debug 2>&1 | tee output.txt`.
  - Verify the package was not updated (missing dependency).
  - Host the dependency on your private APT server as well (1-1 copy).
  - Run `unattended-upgrade --debug --apt-debug 2>&1 | tee output.txt`.
  - Verify the package was not updated (missing dependency).
  - Re-build the dependency with a higher version number, and add it to your private APT repository.
  - Run `unattended-upgrade --debug --apt-debug 2>&1 | tee output.txt`.
  - Verify the package was now upgraded.

  With the proposed patch, the upgrade would already succeed after
  hosting the exact copy on the private APT repository.

  [Regression Potential]

  The changed code logic now allows adjusting candidates of packages
  which are not upgradable and not installed. Since the changed check
  was there to avoid downgrades the possible regression would be somehow
  enabling downgrades accidentally. Adjusting _not_ installed packages
  in itself would not cause downgrading installed packages thus the
  change seems to be safe.

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