Detect remote `systemFeatures` automatically

[roberth]

Is your feature request related to a problem? Please describe.

• Inadequate forward compatibility in derivations #10876

  • By making requiredFeatures easy to use, we can use it to make forward compatibility fail safe

• Manually duplicating remote information onto clients is not great UX.

Describe the solution you'd like

• Nix can ask the remote builder which features it implements.
• Nix maintains a cache of systemFeatures for each of its builders.
• This information is transmitted not as a mere list features, but a list of combinations of derivation systems and features; not all combinations may be valid!
  • example: macOS host may support aarch64-linux by forwarding to a VM, but does not support nested virtualization, so virtualization features only apply to aarch64-darwin.
• The exposed combinations of features can be derived from the usual config, or set explicitly
  • Do not include (transitive) remote builders by default, at least until Recursion in builders definitions deadlocks builds with waiting for lock on '/nix/store/...' #10740 is solved

Describe alternatives you've considered

Additional context

Related terms: requiredFeatures mandatoryFeatures

Priorities

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