Complete implementation of exportReferencesGraph

[domenkozar]

Currently exportReferencesGraph doesn't contain all information needed by nix-store --load-db in order to register paths as valid.

We use following hack, which is IO intense when generating a disk image:
https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-disk-image.nix#L74-L76

What's the scope of this to be finished? @edolstra

[copumpkin]

It really hurts performance to use fixValidity 😦

[copumpkin]

@edolstra not fully understanding what's going on in the relevant code, why is this not as simple as passing true to the makeValidityRegistration call here?

[domenkozar]

Yeah I thought so too, but I fear @edolstra would have done if it was really that simple :)

[copumpkin]

@domenkozar yeah, just figured it might be easier for @edolstra to explain what needs to be done and have another interested party do the actual work 😄

[domenkozar]

@copumpkin the good new is, we need this for perl rework.

[copumpkin]

❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ @edolstra

[copumpkin]

If it's JSON now, does that mean the perl thing we use for exportReferenceGraph needs to be amended to support it?

[copumpkin]

Oh I see now, you write both formats, cool.

[copumpkin]

One possible gotcha: lots of the processors of exportRereferencesGraph blindly feed a globbed closure-* into the perl program, which would probably pick up the JSON form you produce with that commit.

[edolstra]

Good point, maybe they should be renamed to .<foo>.json.

[copumpkin]

Yeah, that'd probably be easier