Support repairPath on most stores.

[Ericson2314]

Motivation

More progress on issue #5729

The method trivially generalizes to be store-implementation-agnostic, in fact.

However, we force it to continue to be unimplemented with RemoteStore and LegacySSHStore because the implementation we'd get via the generalization is probably not the one users expect. This keeps our hands untied to do it right going forward.

Context

For more about the tension between the scheduler logic being store-type-agnostic and remote stores doing their own scheduling, see issues #5025 and #5056.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[Ericson2314]

@roberth and I discussed this somewhat. He was concerned overriding a default method to make it fail was a bit tricky.

One alternatively is simply go ahead and implement daemon protocol and legacy ssh protocol. That at least masks the issue.

The better, but much more invasive route, is to remove all these methods from Store and instead put them in a different class; let's call it Scheduler. I don't think this should be a subclass however, because the idea is that we can schedule with any store. (build/entry-points.cc would become a struct WrapStore : Scheduler.) It is just the RPC stores that are also schedulers.

I am worried that taking on the above will make for far more work, and more controversy, so I slightly lean towards merging this (or this + new protocol stuff) now. But if I am mistaken and there is broad agreement about this Scheduler thing, that would be good to know!

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-05-19-nix-team-meeting-minutes-56/28446/1