Qnikst blog - Making personal environment and long live dotfiles

Some time ago I installed NixOS, and one thing that I had a problem with was understanding how to set up user environment. You can can configure NixOS declaratively with a description in a single file /etc/nixos/configuration.nix, but all applications set up there get installed system-wide. Instead, you usually want to have your user environment to be configured per your user. Also you may want to move your environment from one host to another (possibly to another distribution or even to another OS). Updating configuration.nix looks like a too heavy-weight solution for such a task.

Other solutions that I’ve seen were building of one’s own environment or using nix-env for installing applications per user. I was not able to adopt the former solution, and the latter does not provide declarative config, and one may collect lots of garbage of applications she runs once. Besides, it would be nice to control dotfiles in the same style.

One day my colleague Nicolas Mattia suggested me to look at the tool he uses, called homies, the one he had excellently described in his blog. I think everyone should read and check that :). Unfortunately, this approach, at least up to my understanding should exist for each user, because it’s not customisable. So I’ve started my project homster that is solving the same problem. Currently it’s primarily based on the homies though in the future it’s going to diverge.

The general structure of the project is as follows. There is a default.nix file that describes all the packages and the modified ones that I use. In each modified package I pass my config, or it’s configured to read system config from the nix package directory so I can configure common options in the homster project and update them on the host by usual means.

So it seems that I have found the solution to all my needs and I can have a declarative configuration for the user environment. I still can use nix-env -i for temporarily needed packages, but nix-env -f homster/default.nix -i --remove-all command updates and cleans my environment. Most of the dotfiles can be kept in the project. Also, I can set up my environment anywhere where I can install nix package manager.

While implementing this the most interesting problem was git. Git searches for its configuration in 3 places according to the man page:

system-wide: (prefix)/gitconfig
user-wide: $(HOME)/.gitconfig or $XDG_CONFIG_DIR/git/gitconfig
project-wide: ~/.git/config

Besides, it takes --config option that overrides config search entirely. We can’t use --config because that overrides project specific options, we can’t update user-wide file with nix, so the only option left is the system-wide config. So we need to understand what is (prefix). Man pages tell that it’s a value of the PREFIX environment option. Let’s check nixpkgs:

    "prefix=\${out}"

https://github.com/NixOS/nixpkgs/blob/54ba2c9afca07b0f14763b3697d00b637b2461e0/pkgs/applications/version-management/git-and-tools/git/default.nix#L86

It seems that nix installs that, but let us check it:

strace git config 2>&1 | grep gitconfig
access("/etc//gitconfig", R_OK)         = -1 ENOENT (No such file or directory)

Seems not to be what we were expecting. The story continues, we need to understand where does git looks for a config. You can find it on GitHub (modulo the version that I’ve used, but it’s irrelevant for the code I’m interested in) https://github.com/git/git/blob/7e8bfb0412581daf8f3c89909f1d37844e8610dd/config.c#L1634-L1640

const char *git_etc_gitconfig(void)
{
	static const char *system_wide;
	if (!system_wide)
		system_wide = system_path(ETC_GITCONFIG);
	return system_wide;
}

Okay, what is ETC_GITCONFIG https://github.com/git/git/blob/1f1cddd558b54bb0ce19c8ace353fd07b758510d/configure.ac#L387-L391

GIT_PARSE_WITH_SET_MAKE_VAR(gitconfig, ETC_GITCONFIG,
			Use VALUE instead of /etc/gitconfig as the
			global git configuration file.
			If VALUE is not fully qualified it will be interpreted
			as a path relative to the computed prefix at runtime.)

Finally, after searching what GIT_PARSE_WITH_SET_MAKE_VAR means, we can find that we need to pass --with-gitconfig=name as a parameter to the configure.

So now we need to patch nix package for git to do that. As usual, we want to make it declaratively and easy to change without redoing the work that NixOS maintainers already did like applying patches and packaging.

Everything described above can be done pretty quickly in nix, in my homster/git/default.nix I can override default package for git as:

git.overrideAttrs (old: {
   configureFlags = [ "--with-gitconfig=$out/etc/gitconfig ];
})

nix interprets the $out variable and substitutes exact hash there. Then we can copy our file to the right place and get a new system-wide configuration that is controlled by us.

strace git config 2>&1 | grep gitconfig
access("/nix/store/66hi8rssnvhlxbwjg3qkc4bcs76fp8np-git-2.16.4/etc/gitconfig", R_OK) = 0
openat(AT_FDCWD, "/nix/store/66hi8rssnvhlxbwjg3qkc4bcs76fp8np-git-2.16.4/etc/gitconfig", O_RDONLY) = 3

exactly what is needed.

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