unattended upgrade returns inconsistent exit code

[192dot]

Issue

When unattended upgrade attempts to update a package that requires conffile prompt, and packages without the conffile prompts, it returns 1.
When unattended upgrade attempts to update a package that requires conffile prompt, it will mark the package to blacklist, then it returns 0.
This is causing flapping in monitoring alerts.

Versions Tested

Tag 2.9.1 (the latest) and earlier: contains this issue.
Master: does not contain this issue.

To Recreate the error

• Downgrade a package which does not prompt for conffile during updates (e.g. nginx)
• Downgrade a package which prompts for conffile during updates (e.g. cloud-init)
• Run unattended-upgrade, then echo $? shows it returned 1
• Run unattended-upgrade again, then echo $? shows it returned 0