Build daemon without rest of Nix

[Ericson2314]

From NixOS/rfcs#68 (comment)

@Ericson2314 Would you be opposed to closing this RFC and creating a new feature request issue in the Nix repo? Having a minimal daemon is not really a potentially controversial or hard change that requires a shepherd team and all that :-) What do you think?

OK opening this. For me the requirements are this:

• Don't build libexpr libfetchers
• Expose modern nix-daemon and legacy nix-daemon

It seems like then we would have to make libcmd and libmain not depend on libexpr. Interesting!

[edolstra]

IMHO this is fairly low-priority and shouldn't be done if it causes a lot of code churn, since the value of a minimal daemon isn't entirely obvious. (All that libexpr code doesn't actually get used by the daemon, so it shouldn't cause any problems.) We're also moving in the direction of providing Nix as a single binary, and this goes against that.

[Ericson2314]

Well, I did expect this wasn't as uncontroversial as you said it was :).

For me, Nix's frontend has dramatically increased in scope as of late, and this makes me eager to carve out a more stable scope level "thin waist". I know many people interested in using Nix without the nix expression language, and I also hope this would allow us to work with Guix to standardize a common drv layer. (They weren't so thrilled on the idea before, granted.) Such a common effort is, I think, a crucial step to getting Nix more commonplace in data centers of all stripes.

So to me, it will take churn, but it is entirely worth it. I guess I will return to the RFC at some point,

[Ericson2314]

Last thing before I let this lie dormant:

We're also moving in the direction of providing Nix as a single binary, and this goes against that.

So really, I would love to have libfetchers-less/libexpr-less versions of nix hash nix copy nix build and other such "core" commands. Even more churn, but also preservation of the separate binaries idea.

We could have a nix-core binary (name TBD) with just those sorts of commands, without a libfetchers or libexpr, and also the regular nix binary with everything as today. We could just install/build nix-core for regular end users, but offer a separate nix-core package for those of us interested in these sort of applications (big industrial, alternative frontend, and standardization).

I think I could this without duplicated code, but yes with lot's of churn.