GitHub Actions Workflow

Public repositories

If your repository is public and only uses public Helm charts, you can use the following GitHub Actions workflow to generate a diff between the main branch and the pull request branch. The diff will then be posted as a comment on the pull request.

name: Argo CD Diff Preview

on:
  pull_request:
    branches:
      - main

jobs:
  build:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pull-requests: write

    steps:
      - uses: actions/checkout@v4
        with:
          path: pull-request

      - uses: actions/checkout@v4
        with:
          ref: main
          path: main

      - name: Generate Diff
        run: |
          docker run \
            --network=host \
            -v /var/run/docker.sock:/var/run/docker.sock \
            -v $(pwd)/main:/base-branch \
            -v $(pwd)/pull-request:/target-branch \
            -v $(pwd)/output:/output \
            -e TARGET_BRANCH=refs/pull/${{ github.event.number }}/merge \
            -e REPO=${{ github.repository }} \
            dagandersen/argocd-diff-preview:v0.1.10

      - name: Post diff as comment
        run: |
          gh pr comment ${{ github.event.number }} --repo ${{ github.repository }} --body-file output/diff.md --edit-last || \
          gh pr comment ${{ github.event.number }} --repo ${{ github.repository }} --body-file output/diff.md
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

Instead of using refs/pull/${{ github.event.number }}/merge, you could also use ${{ github.head_ref }} or simply specify the branch name manually.

More information about this can be found in this blog post

Private repositories and Helm Charts

In the simple code examples above, we do not provide the cluster with any credentials, which only works if the image/Helm Chart registry and the Git repository are public. Since your repository might not be public you need to provide the tool with the necessary read-access credentials for the repository. This can be done by placing the Argo CD repo secrets in folder mounted at /secrets. When the tool starts, it will simply run kubectl apply -f /secrets to apply every resource to the cluster, before starting the rendering process.

...
    - uses: actions/checkout@v4
      with:
        ref: main
        path: main

    - name: Prepare secrets
      run: |
        mkdir secrets
        cat > secrets/secret.yaml << "EOF"
        apiVersion: v1
        kind: Secret
        metadata:
          name: private-repo
          namespace: argocd
          labels:
            argocd.argoproj.io/secret-type: repo-creds
        stringData:
          url: https://github.com/${{ github.repository }}
          password: ${{ secrets.GITHUB_TOKEN }}  ⬅️ Short-lived GitHub Token
          username: not-used
        EOF

    - name: Generate Diff
      run: |
        docker run \
          --network=host \
          -v /var/run/docker.sock:/var/run/docker.sock \
          -v $(pwd)/main:/base-branch \
          -v $(pwd)/pull-request:/target-branch \
          -v $(pwd)/output:/output \
          -v $(pwd)/secrets:/secrets \           ⬅️ Mount the secrets folder
          -e TARGET_BRANCH=refs/pull/${{ github.event.number }}/merge \
          -e REPO=${{ github.repository }} \
          dagandersen/argocd-diff-preview:v0.1.10

If your ArgoCD Applications use SSH to access the private repositories, then you need to configure the secret above using SSH as well.

    - name: Prepare secrets
      run: |
        mkdir secrets
        cat > secrets/secret.yaml << EOF
        apiVersion: v1
        kind: Secret
        metadata:
          name: private-repo
          namespace: argocd
          labels:
            argocd.argoproj.io/secret-type: repo-creds
        stringData:
          type: git
          url: git@github.com/${{ github.repository }}
          sshPrivateKey: |
        $(echo "${{ secrets.REPO_ACCESS_SSH_PRIVATE_KEY }}" | sed 's/^/    /') ⬅️ Private SSH key with proper indentation
        EOF

If you get this type of error:

failed to apply secrets: failed to apply secret secret.yaml: failed to apply manifest: failed to convert new object (namespace/secret-name; /v1, Kind=Secret) to proper version: unable to convert unstructured object to /v1, Kind=Secret: error decoding from json: illegal base64 data at input byte 76 from folder: ./secrets

it is because base64 wraps encoded lines after 76 characters by default:

-w, --wrap=COLS
    Wrap encoded lines after COLS character (default 76). Use 0 to disable line wrapping.

so you need to use the following alternative:

    - name: Prepare secrets
      run: |
        mkdir -p secrets
        SSH_PRIVATE_KEY_B64=$(echo "${{ secrets.REPO_ACCESS_SSH_PRIVATE_KEY }}" | base64 -w 0)
        URL_B64=$(echo "git@github.com/${{ github.repository }}" | base64 -w 0)
        cat > secrets/secret.yaml <<-EOF
        apiVersion: v1
        kind: Secret
        metadata:
          name: github-repo-ssh
          namespace: argocd
          labels:
            argocd.argoproj.io/secret-type: repo-creds
        data:
          url: "${URL_B64}"
          sshPrivateKey: "${SSH_PRIVATE_KEY_B64}"
        EOF

If Helm Charts are stored as OCI images in a Docker registry (such as AWS ECR), additional fields must be added to the stringData section as shown below.

    - name: Prepare secrets
      run: |
        mkdir secrets
        cat > secrets/secret.yaml << "EOF"
        apiVersion: v1
        kind: Secret
        metadata:
          name: private-registry
          namespace: argocd
          labels:
            argocd.argoproj.io/secret-type: repository
        stringData:
          name: privateRegistry
          url: ${{ secrets.REGISTRY_URL }}
          username: ${{ secrets.REGISTRY_USERNAME }}
          password: ${{ secrets.REGISTRY_PASSWORD }}
          type: helm
          enableOCI: "true"
          forceHttpBasicAuth: "true"
        EOF

For more info, see the Argo CD docs