Functions for managing repository cache management based on a max
desirable size for the entire dnf-json cache directory.
While none of the functions are currently used, the workflow should
be as follows:
- Update the timestamp of a repository whenever it's used in a
transaction by calling `touchRepo()` with the repository ID and the
current time.
- Update the internal cache information when desired by calling
`updateInfo()`. This should be called for example after multiple
depsolve transactions are run for a single build request.
- Shrink the cache to below the configured maxSize by calling
`shrink()`.
The most important work happens in `updateInfo()`. It collects all the
information it needs from the on-disk cache directories and organises it
in a way that makes it convenient for the `shrink()` function to run
efficiently. It stores three important pieces of information:
1. repoElements: a map that links a repository ID with all the
information about a repository's cache:
- the top-level elements (files and directories) for the cache
- size of the repository cache (total of all elements)
- most recent mtime from all the elements which, if the
`touchRepo()` call is consistently used, should reflect the most
recent time the repository was used
2. repoRecency: a list of repository IDs sorted by mtime (oldest first)
3. size: the total size of the cache (total of all repository caches)
This way, when `shrink()` is called, the paths associated with the
least-recently-used repositories can be easily deleted by iterating on
repoRecency, obtaining the repository info from the map, deleting every
path in the repoElements array, and subtracting the repository's size
from the total. The `shrink()` function stops when the new size is
below the maxSize (or when all repositories have been deleted).
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