name: nix

on:
  push:
    branches:
      - main
  pull_request:
    paths:
      - "go.mod"
      - "go.sum"
      - "nix/**"
      - "flake.nix"
      - "flake.lock"
  workflow_call:
    inputs:
      ref:
        description: "Ref to check out. Defaults to the triggering ref."
        type: string
        required: false

permissions:
  contents: read

concurrency:
  group: nix-${{ github.event_name }}-${{ github.event.pull_request.number || github.ref }}
  # Keyed on the PR author, which is stable across the fix-up push, so the run
  # started by that push cannot cancel the run that made it.
  cancel-in-progress: ${{ github.event_name == 'pull_request' && github.event.pull_request.user.login != 'dependabot[bot]' }}

env:
  SELF_HOSTED_PR: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository }}
  IS_DEPENDABOT: ${{ github.actor == 'dependabot[bot]' }}

jobs:
  build:
    name: build
    runs-on: ubuntu-latest
    permissions:
      # The only write is git push, which authenticates with the PAT that
      # checkout persists, not with GITHUB_TOKEN.
      contents: read
      pull-requests: write
    steps:
      - uses: actions/checkout@v7
        with:
          # head.sha, not the merge ref: the hash is computed for the tree that
          # gets pushed, and the run that verifies it must build that same tree.
          ref: ${{ inputs.ref || github.event.pull_request.head.sha || github.ref }}
          token: ${{ (env.IS_DEPENDABOT == 'true' && secrets.DEPENDABOT) || secrets.GITHUB_TOKEN }}

      - uses: cachix/install-nix-action@v31
        with:
          github_access_token: ${{ secrets.GITHUB_TOKEN }}

      - name: nix build
        id: build
        continue-on-error: ${{ env.SELF_HOSTED_PR == 'true' }}
        run: nix build .#default --no-link --print-build-logs

      - name: Recompute vendorHash
        id: hash
        if: steps.build.outcome == 'failure' && env.SELF_HOSTED_PR == 'true'
        run: |
          nix run nixpkgs#nix-update -- --flake --version=skip --no-src default

          # An unchanged hash means the build broke for some other reason. Say
          # nothing and let the job fail on the real error.
          if git diff --quiet -- nix/package.nix; then
            echo "stale=false" >> "$GITHUB_OUTPUT"
            exit 0
          fi

          echo "stale=true" >> "$GITHUB_OUTPUT"
          sed -n 's/^[[:space:]]*vendorHash = "\([^"]*\)".*/hash=\1/p' nix/package.nix >> "$GITHUB_OUTPUT"

      - name: Push vendorHash fix
        if: steps.hash.outputs.stale == 'true' && env.IS_DEPENDABOT == 'true'
        env:
          HEAD_REF: ${{ github.head_ref }}
          PUSH_TOKEN: ${{ secrets.DEPENDABOT }}
        run: |
          if [ -z "$PUSH_TOKEN" ]; then
            echo "::error::the DEPENDABOT push token is missing from the Dependabot secret store"
            exit 1
          fi
          git config user.name "github-actions[bot]"
          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
          git add nix/package.nix
          git commit -m "fix(nix): update vendorHash [dependabot skip]"
          git push origin "HEAD:refs/heads/$HEAD_REF"
          echo "::notice::vendorHash updated and pushed; the run on the new commit decides this PR"

      - name: Comment stale vendorHash
        if: steps.hash.outputs.stale == 'true' && env.IS_DEPENDABOT != 'true'
        env:
          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          PR: ${{ github.event.pull_request.number }}
          HASH: ${{ steps.hash.outputs.hash }}
        run: |
          marker='<!-- vendorhash-bot -->'
          body=$(cat <<EOF
          $marker
          ### \`nix/package.nix\` vendorHash is stale

          This branch changes the Go dependencies, so the vendor derivation no longer matches the pinned hash and \`nix build\` fails.

          \`\`\`nix
          vendorHash = "$HASH";
          \`\`\`

          Run \`task nix-hash\` locally and commit the result, or paste the hash above.
          EOF
          )

          # Keep one sticky comment instead of a new one on every push. Without
          # the explicit check a failed lookup reads as "no comment yet" and
          # posts a duplicate; piping into head would hide the exit status.
          if ! matches=$(gh api --paginate "repos/$GITHUB_REPOSITORY/issues/$PR/comments?per_page=100" \
                           --jq ".[] | select(.user.login == \"github-actions[bot]\" and ((.body // \"\") | contains(\"$marker\"))) | .id"); then
            echo "::error::could not list the pull request comments"
            exit 1
          fi
          id=$(printf '%s\n' "$matches" | head -n 1)
          if [ -n "$id" ]; then
            gh api -X PATCH "repos/$GITHUB_REPOSITORY/issues/comments/$id" -f body="$body" >/dev/null
          else
            gh pr comment "$PR" --body "$body"
          fi

      # Once the hash is fixed the build passes and the step above stops running,
      # so without this the comment keeps claiming the branch is broken under a
      # green check.
      - name: Resolve stale vendorHash comment
        # Gated on the hash rather than the build: a later push can fix the hash
        # while the build still fails for an unrelated reason, and the comment
        # must stop blaming the vendor derivation either way. Skipped when the
        # recompute itself errored, since then we do not know.
        if: ${{ !cancelled() && env.SELF_HOSTED_PR == 'true' && steps.hash.outputs.stale != 'true' && steps.hash.outcome != 'failure' }}
        continue-on-error: true
        env:
          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          PR: ${{ github.event.pull_request.number }}
          BUILD: ${{ steps.build.outcome }}
        run: |
          marker='<!-- vendorhash-bot -->'
          if ! matches=$(gh api --paginate "repos/$GITHUB_REPOSITORY/issues/$PR/comments?per_page=100" \
                           --jq ".[] | select(.user.login == \"github-actions[bot]\" and ((.body // \"\") | contains(\"$marker\"))) | .id"); then
            echo "::error::could not list the pull request comments"
            exit 1
          fi
          id=$(printf '%s\n' "$matches" | head -n 1)
          [ -n "$id" ] || exit 0

          if [ "$BUILD" = "success" ]; then
            detail='`nix build` passes on this branch.'
          else
            detail='The build is failing for another reason; see the workflow log.'
          fi
          body=$(cat <<EOF
          $marker
          ### \`nix/package.nix\` vendorHash is up to date

          $detail
          EOF
          )
          gh api -X PATCH "repos/$GITHUB_REPOSITORY/issues/comments/$id" -f body="$body" >/dev/null

      - name: Report build failure
        if: ${{ !cancelled() && steps.build.outcome == 'failure' && !(env.IS_DEPENDABOT == 'true' && steps.hash.outputs.stale == 'true') }}
        run: |
          echo "::error file=nix/package.nix::nix build failed; see the log above"
          exit 1