Allow to configure niv directory path

[uvNikita]

New NIV_DIR environment variable configures a path to a directory
where niv will manage sources.{nix,json} files.

[kampka]

@nmattia Is there anything one can contribute to this to get it merged? :)

[nmattia]

Oh wow! Apologies, first time I see this PR, must have missed the notification! Would the -s parameter work for you? See #159 for context

[kampka]

Personally, I'd prefer the environment variable because I can ship that within a projects .envrc or similar, but that problem is easily solvable via a wrapper, so I'm not very attached to that specific solution.

[kampka]

The main difference I guess would be that -s currently only places the sources.json into the source directory, the sources.nix is still placed into the nix folder.
This PR will move sources.nix and sources.json to wherever NIV_PATH points to, which is what I would want out of this.

[shajra]

In #172 I asked for this same feature as a CLI option instead of an environment variable. I think I'd like to do both because the user gets better documentation with help and doesn't need to worry about configuration via another mechanism. But for people doing non-interactive things with .envrc they can have their convenience too.

I may make a PR that implements both the env var and extra command-line switch.

[uvNikita]

@nmattia The use-case this PR solves for me is when I want to use niv to manage multiple sources in one project independently. E.g. I use https://github.com/krebs/krops for deployment of multiple NixOS machines and I would like to have a separate sources.nix file per host.

I guess a workaround would be to use a single sources.nix file, but change the name of dependencies, e.g. nixpkgs-host1, nixpkgs-host2 etc.

I just realized that sources.nix can be configured with sourcesFile parameter:

import sources.nix { sourcesFile = PATH ; };

[steshaw]

I'd really to have NIV_DIR as I use direnv and like to layout my projects with a kind of XDG directory layout, keeping my Nix files in .config/nix. e.g.

$ tree .config/
.config/
└── nix
    ├── shell.nix
    ├── sources.json
    └── sources.nix

1 directory, 3 files

When I use --sources-files, I get the following warning

$ niv --sources-file .config/nix/sources.json update nixpkgs
Update nixpkgs
  WARNING: Could not read nix/sources.nix
     ( nix/sources.nix: openBinaryFile: does not exist (No such file or directory) )
Done: Update nixpkgs

I'm not sure what the downside of the warning is.

I currently workaround with

$ (cd .config && niv update nixpkgs)
Update nixpkgs
Done: Update nixpkgs

[nmattia]

@steshaw that's interesting, can you tell me more about the use case? Do you use niv globally, as opposed to per-project?

[steshaw]

Projects

With projects, I provision the [development] environment using direnv+nix+nix-direnv (though I used Lorri also for caching). In an example project .envrc is like this:

export PROJECT_HOME=$PWD
nix_shell=$PROJECT_HOME/.config/nix/shell.nix
use nix "${nix_shell}"
watch_file "${nix_shell}"
PATH_add scripts

The shell.nix is like this https://gist.github.com/steshaw/5ee72fd409e9cdc87deafb3e59096410. Aside: I'm not sure if I need the niv overlay when importing nixpkgs. IIRC, it'd compile the latest niv when I installed my user packages and I prefer to rely on fetching prebuilt binaries from the cache because I occasionally use my user packages on an underpowered MacBook Air.

Being able to add

export NIV_DIR=$PWD/.config/nix

to my .envrc has the benefit that I could be in any directory of my project and still issue niv commands and have niv find the sources.json along with the sources.nix.

I wrote up an unpublished article about how I use XDG-style layout for projects. I still use this layout as much as possible today, though I've moved PROJECT_CACHE_HOME to PROJECT_HOME/.cache, so it matches more closely with XDG and seems easier to remove with rm -rf .cache without accidentally deleting your .local directory 😄.

Global

Not entirely sure what you mean by global but I have 2 candidates: global NixOS configuration and my user profile. I don't use niv for NixOS configuration (yet). I've just started to use it to pin nixpkgs for my user packages. My user packages configuration is in a project/repo that I share across my desktop (NixOS) and laptops macOS. I don't use a project-based XDG layout for this. I'm not using home-manager or nix-darwin but instead, have adapted lnl-overlay. These configurations can be found the nix subdirectory of my "dotfiles" repo https://github.com/steshaw/shelly/tree/master/nix.

Are you thinking that NIV_DIR could be used globally too? That would make sense. I'd set it up in my .profile to point to my the configuration for my user profile:

export NIV_DIR=~/Code/steshaw/shelly/nix/nix

If I also used niv for my NixOS configuration, then I'd have to choose. Since my user packages change more frequently than my NixOS, I'd probably leave it at that.

[nmattia]

(though I used Lorri also for caching)

Then you may want to give https://github.com/nmattia/sorri a try! It's a work in progress but we already use it at $WORK.

I use XDG-style layout for projects.

Ok, that makes a lot of sense. I want to spend some time cleaning up #279 this week, I might as well add support for NIV_DIR.

[steshaw]

Thanks! I'll be sure to check out https://github.com/nmattia/sorri as well 😄

[nickgarber]

Thanks for sharing your work-around @steshaw!
Initially I'd hoped to set NIV_DIR to .nix, but I've moved to using .config/nix.

Addng DirEnv to the mix, this is loaded by adding use nix .config/nix/shell.nix to the project.envrc.

Looking forward to when this PR is merge so that Niv mgmt will be more "normalized".

Thanks!