Add a nix/build action

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

.forgejo/workflows/nix-build.yaml

@@ -0,0 +1,36 @@
+---
+name: test nix/build
+
+on:
+  push:
+
+jobs:
+  nix/build:
+    runs-on: nixos-flakes
+    steps:
+      - name: checkout
+        uses: actions/checkout@v3
+
+      - name: build the local test package
+        id: build-local
+        uses: ./build
+        with:
+          flake: ./build/test
+          package: "hello"
+          logs: true
+
+      - name: print the output path
+        run: |
+          echo ${{ steps.build-local.outputs.output-path }}
+
+      - name: build a package from nixpkgs
+        id: build-nixpkgs
+        uses: ./build
+        with:
+          flake: nixpkgs
+          package: "hello"
+          logs: true
+
+      - name: print the output path
+        run: |
+          echo ${{ steps.build-nixpkgs.outputs.output-path }}

build/action.yml

@@ -0,0 +1,30 @@
+---
+name: nix-build
+author: Gergely Nagy
+description: "Build packages with Nix"
+inputs:
+  flake:
+    description: "The flake to build packages from"
+    default: "."
+    required: true
+  package:
+    description: "The package to build from a flake"
+    required: false
+  logs:
+    description: "Enable showing the full build logs"
+    default: false
+    required: false
+outputs:
+  output-path:
+    description: "The resulting output paths"
+    value: "${{ steps.nix-build.outputs.output-path }}"
+runs:
+  using: "composite"
+  steps:
+    - id: nix-build
+      shell: bash
+      run: $GITHUB_ACTION_PATH/bin/nix-build
+      env:
+        INPUT_FLAKE: ${{ inputs.flake }}
+        INPUT_PACKAGE: ${{ inputs.package }}
+        INPUT_LOGS: ${{ inputs.logs }}

build/bin/nix-build

@@ -0,0 +1,12 @@
+#! /usr/bin/env bash
+set -eo pipefail
+
+LOGS=""
+case "${INPUT_LOGS}" in
+    [yY][eE][sS]|[tT][rR][uU][eE]|[oO][nN]|1)
+        LOGS=-L
+        ;;
+esac
+
+nix build ${LOGS} "${INPUT_FLAKE}#${INPUT_PACKAGE}"
+echo "output-path=$(readlink result)" >>"${GITHUB_OUTPUT}"

build/test/flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1685566663,
+        "narHash": "sha256-btHN1czJ6rzteeCuE/PNrdssqYD2nIA4w48miQAFloM=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "4ecab3273592f27479a583fb6d975d4aba3486fe",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "23.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

build/test/flake.nix

@@ -0,0 +1,11 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/23.05";
+  };
+
+  outputs = { self, nixpkgs }:
+    let pkgs = nixpkgs.legacyPackages.x86_64-linux;
+    in {
+      packages.x86_64-linux.hello = pkgs.hello;
+    };
+}