feat: new post on building mix releases

content/posts/build-mix-releases-with-nix-flakes-and-private-packages.md

@@ -0,0 +1,66 @@
+---
+date: '2024-08-09T18:15:23-04:00'
+draft: false
+params:
+  author: "Eric Lee"
+tags: ["elixir", "nix"]
+title: 'Build Mix Releases With Nix Flakes And Private Packages'
+---
+
+We have a license to the fantastic Elixir [Oban Pro](https://getoban.pro/)
+library. To retrieve the Oban Pro and Oban Web dependencies, we must add their
+third-party Hex repository and authenticate with our auth key like so:
+
+```shell
+mix hex.repo add oban https://getoban.pro/repo \
+  --fetch-public-key SHA256:4/OSKi0NRF91QVVXlGAhb/BIMLnK8NHcx/EWs+aIWPc \
+  --auth-key ${OBAN_AUTH_KEY}
+```
+
+However, if we were to build our Elixir application and generate a `mix release`
+using `nix build`, we would have to introduce build-time secrets. Some of the
+approaches can be perused [here in the NixOS wiki](https://nixos.wiki/wiki/Comparison_of_secret_managing_schemes).
+
+In this scenario, we instead opt to retrieve dependencies as a GitHub Actions
+step outside of Nix's responsibility. We populate the `deps/` folder by invoking
+`mix deps.get --only $MIX_ENV` and use the path to allow `nix build` to consider
+files that aren't part of the git repository.
+
+```yaml
+jobs:
+  nix-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+          nix_path: "nixpkgs=channel:nixos-unstable"
+      - run: |
+          nix-shell -p cacert elixir --run '
+            # ----- #
+            set -eo pipefail
+
+            mix local.hex --force
+            mix local.rebar --force
+
+            mix hex.repo add oban https://getoban.pro/repo \
+              --fetch-public-key SHA256:4/OSKi0NRF91QVVXlGAhb/BIMLnK8NHcx/EWs+aIWPc \
+              --auth-key "${{ secrets.OBAN_AUTH_KEY }}"
+
+            mix deps.get --only $MIX_ENV
+            # ----- #
+          '
+        env:
+          ELIXIR_ERL_OPTIONS: "+fnu"
+          MIX_ENV: "prod"
+        shell: bash
+      - run: nix build path://$(pwd)
+```
+
+Additionally, for local testing of the GitHub Actions workflow, we can use [act](https://github.com/nektos/act).
+
+```shell
+sudo $(which act) -W .github/workflows/nix-ci.yml -s GITHUB_TOKEN=$(gh auth token) -s OBAN_AUTH_KEY=$OBAN_AUTH_KEY
+```