Create StringSet DerivationBuilderParams::systemFeatures

[Ericson2314]

Motivation

Do this to avoid checking "system features" from the store config directly, because we rather not have DerivationBuilder depend on Store.

Context

Depends on #13846

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[edolstra]

Do this to avoid checking "system features" from the store config directly.

What exactly is the problem with that?

[Ericson2314]

What exactly is the problem with that?

I am trying to make DerivationBuilder not depend on Store. With this change, and one other smaller one, that is succesfully accomplished except for registerOutputs (and recursive Nix).

I want this for separation of concerns, but also FFI

[edolstra]

Maybe it's easier to just pass systemFeatures in DerivationBuilderParams? That's more generic and doesn't require putting platform-specific parsing in derivation-building-goal.cc.

[Ericson2314]

I would settle for that, but IMO "set of strings with some strings having special meanings" is more an artifact of the A-Term derivation format and nix.confg format than how we actually want things to work. I rather have DerivationBuilder take precisely the booleans (or other typed options) it cares about.

I'm fine ditching the ifdefs though, if you would want that.

[edolstra]

but IMO "set of strings with some strings having special meanings" is more an artifact of the A-Term derivation format

That's not the case. It's a list of strings denoting required platform features, and those are inherently platform-specific. Hard-coding those features into DerivationBuilderParams seems inflexible and a wrong abstraction that simply doesn't match how derivations work. For instance, ExternalDerivationBuilder and remote builds need to be able to forward arbitrary features to the actual builder. (E.g. ExternalDerivationBuilder on macOS still needs to be able to see the KVM feature, if it's doing a Linux build on macOS.)

We want to get rid of platform-specific code in derivation-builder.{cc,hh}, and this is going in the opposite direction. Note that there are other platform features like uid-range that we don't want to handle in the platform-independent files.

[dpulls]

🎉 All dependencies have been resolved !

[Ericson2314]

OK, fair enough with ExternalDerivationBuilder.

[Ericson2314]

OK, updated!