puppet-rootless_gitlab_runner/.github/workflows/environment-diff.yml at main · eth-library/puppet-rootless_gitlab_runner · GitHub

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---
# Environment diff workflow: when a PR changes flake.lock, build the pinned
# environment (the Nix shell CI and contributors enter) at the PR head and at
# the merge base, diff the two realized closures, and upsert the package-level
# delta as a single PR comment. No secrets; builds are mostly binary-cache
# downloads.
name: Nix flake environment package diff

on:
  pull_request:
    paths:
      - flake.lock
      - .github/workflows/environment-diff.yml

# Cancel a superseded run when a new push lands on the same PR.
concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

# Dependabot-triggered runs get a read-only GITHUB_TOKEN by default; the
# explicit permissions key is respected and restores comment access.
permissions:
  contents: read
  pull-requests: write

jobs:
  environment-diff:
    name: Environment closure diff
    runs-on: ubuntu-24.04
    timeout-minutes: 20
    steps:
      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
        with:
          persist-credentials: false
          # The pull_request checkout is the PR merge commit; full history so
          # the base side's flake.lock can be read from its first parent.
          fetch-depth: 0

      - name: Install Nix
        # flakes + nix-command are enabled by default in install-nix-action v31,
        # so no extra_nix_config is needed.
        uses: cachix/install-nix-action@13d8dd58da0234aa297dedd986986ccb8e7f3e24  # v31.11.1

      - name: Build both environments and diff their closures
        run: |
          set -euo pipefail
          # The runner is x86_64-linux; hardcode the system rather than
          # evaluating builtins.currentSystem impurely.
          attr='devShells.x86_64-linux.default'
          # HEAD is the PR merge commit, so HEAD^1 is the exact base tip this PR
          # merges onto. Building the checkout against the base lock
          # (--reference-lock-file) pairs head flake.nix with base pins:
          # identical to the true base on the lock-only bumps this workflow
          # targets.
          git show 'HEAD^1:flake.lock' > /tmp/base.lock
          nix build ".#${attr}" --out-link result-head
          nix build ".#${attr}" --reference-lock-file /tmp/base.lock \
            --no-write-lock-file --out-link result-base
          if [ "$(readlink result-base)" = "$(readlink result-head)" ]; then
            # Empty file: the closures are identical (no effective change).
            : > diff.txt
          else
            # Strip ANSI colour codes: diff-closures colours the size delta
            # even when stdout is not a TTY, and the escape codes would
            # render as raw control codes inside the Markdown comment.
            nix store diff-closures ./result-base ./result-head \
              | sed -E 's/\x1b\[[0-9;]*m//g' > diff.txt
          fi

      - name: Upsert the PR comment
        # --edit-last targets the last comment of the workflow token's own
        # identity (github-actions[bot]); this is the only workflow that comments
        # as that bot, so the marker line is informational, not the match key.
        # The unchanged case edits an existing comment but never creates one:
        # no --create-if-none, and the edit's failure when none exists is the
        # intended silence.
        env:
          GH_TOKEN: ${{ github.token }}
          PR_NUMBER: ${{ github.event.pull_request.number }}
        run: |
          set -euo pipefail
          if [ -s diff.txt ]; then
            {
              echo '<!-- environment-diff -->'
              echo '### Environment diff (`flake.lock` update)'
              echo
              echo '```text'
              cat diff.txt
              echo '```'
            } > comment.md
            gh pr comment "$PR_NUMBER" --edit-last --create-if-none \
              --body-file comment.md
          else
            {
              echo '<!-- environment-diff -->'
              echo '### Environment diff (`flake.lock` update)'
              echo
              echo 'No effective change to the environment closure.'
            } > comment.md
            gh pr comment "$PR_NUMBER" --edit-last --body-file comment.md || true
          fi