Use apt preferences to prefer packages from the mirror over the main repos #18793
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Why I did it
Currently, if snapshots are enabled, then packages from the mirror and packages from the main repos have the same priority. This becomes problematic if the main repos have a package that's of a newer version than the snapshots, and something wants to install that newer version, even though it could be installed with that older version. For example, if a -dev package is being installed, and there's a newer version of the -dev and the main package in the main repos (compared to the snapshots), then apt will try to install the newer -dev package, which will have a dependency on the newer main package, but the older main package will still be installed. It could instead try to install the older -dev package from the snapshots, which would be installable.
Work item tracking
How I did it
To fix this, use apt preferences to give the pacakges in the snapshot repo a higher priority than the ones in the main repo. This should be enough to convince apt to use packages from the snapshot repo whenever possible, but it still allows packages from the main repo to be installed, in case some change comes in that requires that for whatever reason.
How to verify it
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Tested branch (Please provide the tested image version)
Description for the changelog
Link to config_db schema for YANG module changes
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