[Nix-dev] Unstable Nixpkgs on stable NixOS (was: Automatically locking the screen with xautolock)

[Michael Alyn Miller]

The 02/08/2015 12:21, Jeffrey David Johnson wrote:
> Nice! I'm currently binding "gksu 'i3lock -c000000 &
> pm-suspend'" to a hotkey using i3, but this is better.  Is
> there a standard way to get it in my fork of nixpkgs, which is
> tracking the release-14.12 branch?  (I could just copy and
> paste but if there's a better way now would be a good time to
> learn.)

I certainly don't have an authoritative answer to this question,
but I'll tell you what I am doing and then maybe someone with
much more NixOS/Nixpkgs experience will chime in with
tweaks/guidance.

In general I have the following goals for my NixOS system:

1. I want everything to be reproducible, which of course means that my
   system is configured in /etc/nixos/configuration.nix, but also that
   any packages that I install as an individual user are reproducible as
   well.
2. Related to the above, I only want to put "system" packages into
   configuration.nix.  My theory here is that most of my package
   management is going to be done as a regular user, by creating
   project-specific nix-shell environments, etc.  Also, `nix-env -q` is
   a nice way to see everything that is installed and it doesn't appear
   that similar tools exist at the NixOS level.

Goal #1 means that configuration.nix is pretty basic -- the bare minimum
number of packages to get my system working and launch an X environment
(the window manager, for example, but no apps).

Goal #2 was more complex.  I found the following two references on this
topic:

- <https://nixos.org/wiki/FAQ#How_can_I_manage_software_with_nix-env_like_with_configuration.nix.3F>
- <https://nixos.org/wiki/Howto_keep_multiple_packages_up_to_date_at_once>

I went with the first option because I wanted to be able to use `nix-env
-q` to see what I had installed, regardless of how I did the install
(manually or through the "myPackageSelections.nix" file).  I can provide
more details here if you like.

With that choice out of the way, the next problem that I ran into was
getting access to newer packages.  I have contributed a couple of
packages to Nixpkgs, fixed some others, etc. and so far I had just been
manually installing them out of my nixpkgs checkout.  This was not
entirely compatible with my first goal, because now I had installed
packages that were not listed in my configuration.

Enter the nixpkgs-unstable channel!  That channel seems to be updated
pretty frequently and means that changes are available in binary form
within a few days.  Thankfully my second goal meant that I already had a
perfect split between stable and unstable: my NixOS configuration could
stay on 14.12 and my user-level Nixpkgs was free to go unstable.

Getting this working was relatively straightforward, although I had to
modify my environment in a couple of unexpected ways (more on this
later).

The simple part is the `nix-channel --add https://nixos.org/channels/nixpkgs-unstable`
and `nix-channel --update` as a normal user.  Some things worked after
that, but not everything.  `nix-env -i whatever` will happily use the
nixpkgs-unstable channel because it finds that channel in ~/.nix-defexpr.
Adding in the -f flag to specify your own expression means that nix-env
no longer knows about the channel though.  Instead, nix-env falls back
to NIX_PATH, which by default points at the stable NixOS channel and
therefore has no idea that you want to use nixpkgs-unstable.

I found that very confusing, although that's probably because of the
subtlety that is the myPackageSelections.nix-based way of managing my
package list.  In any event, I had to dig through the docs to understand
the situation.

I fixed this problem by setting NIX_PATH to `nixpkgs=~/.nix-defexpr/channels/nixpkgs`
and removing the channels_root symlink from ~/nix-defexpr.  That last
step probably isn't necessary, but every now and then nix-env would
complain about duplicate packages and besides that I don't want to get
(user) packages out of the stable NixOS channel anyway.

This all seems to work pretty well and lets me have fast access to
new/changed packages without compiling everything myself.  I am a bit
worried that changing NIX_PATH and removing channels_root is going to
cause me trouble one day, but so far everything seems fine.  As I said,
I would welcome input here from the NixOS/Nixpkgs experts.

I am curious to see what folks think about my installation approach as
well.  I feel like this split between stable OS and unstable packages is
a good one and that declarative user-level package management is also
valuable.  That said, NixOS doesn't support either one "out of the box"
so I am wondering if there is a better approach that I should be using.

Hope that helps!

Thanks,
Michael Alyn Miller