Derivation/resource-specific max-jobs

[hacker1024]

Is your feature request related to a problem?

Some derivations use limited hardware resources, such as KVM in NixOS tests.

It would be great if there was a way to limit the max-jobs used for derivations that require these resources, so that conflicts can be avoided.

Proposed solution

An derivation attribute could be added, containing a list of "resource names" that a derivation requires. Then, in nix-conf, the maximum number of concurrent builds for derivations that need each resource can be defined.

Alternative solutions

max-jobs can be set globally while building such derivations, but this prevents their dependencies from building concurrently too, which isn't ideal.

Additional context

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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