check-nix-format.yml

# This file was copied mostly from check-maintainers-sorted.yaml.
# NOTE: Formatting with the RFC-style nixfmt command is not yet stable. See
# https://github.com/NixOS/rfcs/pull/166.
# Because of this, this action is not yet enabled for all files -- only for
# those who have opted in.

name: Check that Nix files are formatted

on:
  pull_request_target:
    # See the comment at the same location in ./nixpkgs-vet.yml
    types: [opened, synchronize, reopened, edited]

permissions:
  contents: read

jobs:
  get-merge-commit:
    uses: ./.github/workflows/get-merge-commit.yml

  nixos:
    name: nixfmt-check
    runs-on: ubuntu-24.04
    needs: get-merge-commit
    if: "needs.get-merge-commit.outputs.mergedSha && !contains(github.event.pull_request.title, '[skip treewide]')"
    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
        with:
          # pull_request_target checks out the base branch by default
          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
          # Fetches the merge commit and its parents
          fetch-depth: 2

      - name: Checking out base branch
        run: |
          base=$(mktemp -d)
          baseRev=$(git rev-parse HEAD^1)
          git worktree add "$base" "$baseRev"
          echo "baseRev=$baseRev" >> "$GITHUB_ENV"
          echo "base=$base" >> "$GITHUB_ENV"

      - name: Get Nixpkgs revision for nixfmt
        run: |
          # pin to a commit from nixpkgs-unstable to avoid e.g. building nixfmt
          # from staging
          # This should not be a URL, because it would allow PRs to run arbitrary code in CI!
          rev=$(jq -r .rev ci/pinned-nixpkgs.json)
          echo "url=https://github.com/NixOS/nixpkgs/archive/$rev.tar.gz" >> "$GITHUB_ENV"

      - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
        with:
          # explicitly enable sandbox
          extra_nix_config: sandbox = true
          nix_path: nixpkgs=${{ env.url }}

      - name: Install nixfmt
        run: "nix-env -f '<nixpkgs>' -iAP nixfmt-rfc-style"

      - name: Check that Nix files are formatted according to the RFC style
        run: |
          unformattedFiles=()

          # TODO: Make this more parallel

          # Loop through all Nix files touched by the PR
          while readarray -d '' -n 2 entry && (( ${#entry[@]} != 0 )); do
            type=${entry[0]}
            file=${entry[1]}
            case $type in
              A*)
                source=""
                dest=$file
                ;;
              M*)
                source=$file
                dest=$file
                ;;
              C*|R*)
                source=$file
                read -r -d '' dest
                ;;
              *)
                echo "Ignoring file $file with type $type"
                continue
            esac

            # Ignore files that weren't already formatted
            if [[ -n "$source" ]] && ! nixfmt --check ${{ env.base }}/"$source" 2>/dev/null; then
              echo "Ignoring file $file because it's not formatted in the base commit"
            elif ! nixfmt --check "$dest"; then
              unformattedFiles+=("$dest")
            fi
          done < <(git diff -z --name-status ${{ env.baseRev }} -- '*.nix')

          if (( "${#unformattedFiles[@]}" > 0 )); then
            echo "Some new/changed Nix files are not properly formatted"
            echo "Please format them using the Nixpkgs-specific \`nixfmt\` by going to the Nixpkgs root directory, running \`nix-shell\`, then:"
            echo "nixfmt ${unformattedFiles[*]@Q}"
            echo "Make sure your branch is up to date with master, rebase if not."
            echo "If you're having trouble, please ping @NixOS/nix-formatting"
            exit 1
          fi