Feature request: add mirroring support

[Synss]

It seems one can only specify a single URL in the lock file. So the tools are always fetched from the internet. I'd really prefer mirroring the files and archives on a CI-internal server and only have the internet as a fallback.
I think an API similar to that of the BCR would be applicable here as well. That is, adding an optional mirror_urls field to the lock file, or a global, top-level mirrors field, or both.

Have you considered something like this and would you be interested in the feature?

[willstranton]

I haven't tried it myself, but I think you can get a similar result using --downloader_config.

Here's a blog post on it with an example config file (when it was an experimental flag): https://blog.aspect.build/configuring-bazels-downloader

And here's an issue discussing how you can configure it so that it tries the mirror first, and then the original url: bazelbuild/bazel#23083 (comment)

[Synss]

Thank you for your feedback! I'll look into those as well.

Now, would you accept a pull request adding the mirroring features directly into rules_multitool? I still think there would be some value in a direct and builtin mechanism for this use-case.

[markelliot]

Given that there's a way to do this at the Bazel level I think I'd prefer to avoid adding extra config and extra logic to the ruleset to cover this functionality.