APT and dpkg status database interoperability during aborted distribution upgrades

Recovery Behavior for Aborted Upgrades

APT does not automatically identify and purge all obsolete packages during a recovery attempt. While apt-get install -f (fix-broken) resolves unmet dependencies to stabilize the system, it does not treat the presence of legacy packages from a previous release as a trigger for removal unless those packages directly conflict with a required dependency of a newer package.

Obsolete Package Handling

Obsolete packages—those no longer present in the current configured repositories—typically remain in the dpkg status database as installed. They are not flagged for removal by default during recovery because APT prioritizes system stability over cleanup. To remove these, you must either use apt autoremove --purge (for packages marked as automatically installed that are no longer needed) or manually identify and purge them using tools like aptitude search '~o'.

Dependency Loops and Resolution

When a recovery attempt encounters a dependency loop between a partially upgraded package and a legacy package, the expected behavior is as follows:

• Resolver Stalling: APT may report that it cannot resolve the dependencies and will stop without making changes to avoid removing critical system components.
• Priority: The resolver prioritizes dependency consistency. If a legacy package is still technically required by a partially upgraded package, APT will attempt to keep it.
• Blocking: Packages in a non-configured state (e.g., half-configured or unpacked) can block the removal of other packages until their own configuration is completed or they are forced out.

Recommended Recovery Sequence

To stabilize the system without risking the removal of essential recovery tools, follow this scoped sequence:

1. Configure pending packages: Complete the dpkg state transitions first.

   sudo dpkg --configure -a

2. Fix broken dependencies: Resolve missing or conflicting dependencies.

   sudo apt-get install -f

3. Simulate cleanup: Before purging obsolete packages, simulate the action to ensure no essential packages are flagged for removal.

   sudo apt-get -s autoremove --purge

Assumptions and Verification

This behavior assumes a standard Debian-based environment (e.g., Debian or Ubuntu). Because resolver logic varies by version, you should verify your current state before executing removals:

• Check for broken packages: dpkg --audit
• Verify OS version: cat /etc/os-release
• Review the interrupted transaction: tail -n 100 /var/log/dpkg.log

Note: If you are encountering a specific dependency loop that apt-get install -f cannot resolve, please provide the output of apt-get check, as this will determine if manual dpkg --purge intervention is required for a specific blocking package.