The archive-mirrors value from repo files are not overridable, displayed nor deduplicated

[kit-ty-kate]

There are two forms of archive-mirrors:

• The archive-mirrors from the global options, displayed in opam option and changeable there (since opamClient: pass dl_cache from initConfig to config #5315). By default empty.
• The archive-mirrors from each repositories currently set in your switch

Currently the latter isn't used very much due to #4830, but as most people who will actually set archive-mirrors manually are most likely using opam >= 2.2, the idea of setting it in opam-repository sounds doable.

However i can see some minor problems with the way opam handles archive-mirrors for repositories.

• If one adds several repositories and the all have their archive-mirrors set, then any cache miss will actually try downloading the archive as many times as there are repositories and these urls are not de-duplicated. Meaning that for opam-repository for example, a cache miss will keep sending request to opam.ocaml.org as many times as there are opam-repository forks in the current switch.
• These values aren't visible anywhere for regular users. The only way for users to know their values is to do grep archive-mirrors: .opam/repo/*/repo or something of the sort.
• These values aren't overridable. If for whatever reason you do not want to use the mirror from a repository, there are currently no ways of doing that without forking it and removing the archive-mirrors field manually, because...
• ... removing archive-mirrors from the repo file cannot currently be done automatically without assuming a certain format (e.g. Avoid duplicated archive-mirrors: field in the repo file ocaml-opam/opam2web#247) or without using opam-ed

To solve these minor issues, i propose to:

• improve the handling of the mirrors by transforming the string list to Set.(OpamUrl.t).t where the values are actually used
• add a Repositories configuration section in opam option that displays the value of archive-mirrors for each repository
• allow to override the repository configuration with a new opam option --repository argument. Setting it could add a note during opam update to remind them that the value from the repository isn't taken into account
• add a new opam admin edit-repo-file command, which would allow to remove or edit fields in the repo file

[mseri]

Adding a comment here so that the status appears in ocaml/opam-repository#28348