Static blog on Nix

NixOS, yay! Nix causes me so frustration, and I’m so happy when it works!

I’ve been idly fixing my NixOS server, and now it’s all working. I can quickly and easily push up blog posts, w00t!

Build locally, deploy results

One thing Nix does well is fetching an already finished result from a cache.

I wanted to do something similar by locally building my static blog on my beefy laptop and then pushing only the result to my wimpy rented server.

I already had a configuration.nix file describing everything on my virtual private server, and I wanted the root of my website to be described in another file.

I ended up with this stanza:

virtualHosts = {
  "scannedinavian.com" = {
    enableACME = true;
    forceSSL = true;
    root = "${pkgs.callPackage ./shaesite.nix {} }/dist";
  };
};

This is nifty because the result of the shaesite.nix is substituted in as the contents of the root directory of the webserver.

The contents of shaesite.nix are:

{ fetchFromGitHub}:
let
  src = fetchFromGitHub {
    owner = "shapr";
    repo = "hakyll-nix-something";
    rev = "e14b0c3bba59e33ca3157d0f460e1193f0886d9b";
    sha256 = "13n6zzcgb51zx21vmlsm4s7jy6lf863zm08wdj3inffpfq3g75mp";
    # date = "2024-04-15T14:59:06-04:00";
 };
  wrappedFlake = import "${src}";
in
wrappedFlake.outputs.packages.x86_64-linux.website

How to deploy?

This is how I deploy my blog: nixos-rebuild switch -I nixpkgs=/home/shae/build/nixpkgs -I nixos-config=./configuration.nix --target-host root@surtr.scannedinavian.com

That says the system configuration is in a file in this directory called ./configuration.nix and the result will be pushed to root@surtr.scannedinavian.com

I’m explicitly pointing nixpkgs to a local checkout because this all worked great when I was running NixOS 22.11 locally and on the server, and it didn’t work at all when I upgraded my laptop from NixOS 22.11 to NixOS 23.05.

I finally figured out that this command looks in the environment for the value of nixpkgs, and I hadn’t upgraded my server!

Because this didn’t match, nixos-rebuild tried to copy up so much stuff that my server ran out of disk space, it was a huge mess that took me days to track down.

This got especially interesting because the symptom I saw was “can’t deploy blog, server doesn’t have enough space”.

My first thought was that I had somehow changed the build to include all of the build tools instead of only the result.

This led to a fun digression into learning about nix-tree for investigating dependencies in a closure, and also led me to getting help from Brian McKenna on how to convert a system configuration into a result that can be inspected.

I ended up checking out a local copy of nixpkgs 22.11 and explicitly pointing the build command to that nixpkgs.

Pinning dependencies is exactly the use case for Nix flakes, and that’s one of my next steps for improving this, switch to flakes!

I got stuck, why no updates?

Last week I changed the value of the git hash in the rev value but I didn’t get a new result for my website, nothing was built. What was happening?

Turns out I needed to change the sha256 value as well, otherwise nix assumes the hash matches the existing result and nothing is built.

I swapped two of the characters in the sha256, started a rebuild, and nix would complain that the hash was wrong, and tell me the correct value. Then I would copy in the correct value.

I kinda consider this a bug, I wish nix would keep track of the git hash and sha256 hash pair.

Update the hash in emacs

I got tired of the extra steps and wanted to use John Wiegley’s nix-update.el to magically update the hash for me. After some confusion and debugging, I had to add nix-prefetch-git to my system packages, and move the point to before the fetchFromGitHub, then it worked!

Next?

I don’t like the extra steps of finding the git revision of my blog repository after making changes, maybe I’ll move the whole thing into the same repo with the system configuration?

Like I said before, I also want to move this to a flake.

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