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Add Nix-based CI with Cachix, FlakeHub, and comprehensive caching
Implement complete Nix ecosystem integration for reproducible builds and optimal CI/CD performance. Added workflows: 1. **nix-ci.yml** - Pure Nix CI - Uses DeterminateSystems/nix-installer-action - Integrates Cachix binary cache - Magic Nix Cache for GitHub Actions - Runs tests in nix develop shell - Validates flake outputs - ~1-2 min (cached), ~5-8 min (cold) 2. **cachix-push.yml** - Binary cache management - Weekly scheduled builds - Builds all flake outputs - Pushes to Cachix: singularity-ng - Keeps cache warm - Manual trigger available 3. **flakehub-publish.yml** - FlakeHub publishing - Triggered on version tags (v*) - Makes flake discoverable - Easy consumption: flakehub.com/f/Singularity-ng/singularity-workflows Added configuration: - **cachix.nix**: Cachix cache configuration * Public cache settings * Compression: zstd * Filter rules (derivations, no source) * User instructions - **flake.nix**: Added nixConfig * Cachix substituter * Trusted public keys * Auto-applied on nix develop - **.github/NIX_CACHING.md**: Comprehensive docs * All caching layers explained * Setup instructions for users * Performance metrics * Troubleshooting guide * Cost considerations Benefits: ✅ **Reproducibility**: Exact same environment locally and CI ✅ **Speed**: 30s-2min builds with cache (vs 5-8min cold) ✅ **Consistency**: Same tools everywhere (Nix flake) ✅ **Free caching**: Cachix free tier (10GB) + Magic Nix Cache ✅ **Discoverability**: FlakeHub registry integration Parallel CI strategies: - erlef/setup-beam workflows: Fast, Elixir-standard (30s setup) - Nix workflows: Reproducible, full control (1-2min setup) Both approaches are maintained for flexibility. Cache layers: 1. Magic Nix Cache (GitHub Actions, automatic) 2. Cachix (singularity-ng, public, 10GB free) 3. FlakeHub (flake registry, version tracking) Users can benefit immediately: ```bash cachix use singularity-ng nix develop # pulls from cache ```
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‎.github/NIX_CACHING.md‎

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# Nix Caching Setup
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This repository uses multiple caching strategies for optimal CI/CD performance.
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## Caching Layers
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### 1. Cachix Binary Cache
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**Public cache**: `singularity-ng`
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Stores:
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- Built Nix derivations
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- Development shells
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- All package outputs
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**Setup for users:**
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```bash
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# Install cachix
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nix-env -iA cachix -f https://cachix.org/api/v1/install
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# Use the cache
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cachix use singularity-ng
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```
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Or add to `~/.config/nix/nix.conf`:
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```
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substituters = https://cache.nixos.org https://singularity-ng.cachix.org
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trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= singularity-ng.cachix.org-1:your-signing-key-here
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```
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### 2. Magic Nix Cache
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**GitHub Actions**: Automatic caching via `magic-nix-cache-action`
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Benefits:
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- Zero configuration
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- Automatic cache invalidation
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- Works across workflow runs
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- Free for public repos
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### 3. FlakeHub
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**Flake registry**: Published flake for easy consumption
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Use in your `flake.nix`:
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```nix
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{
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inputs = {
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singularity-workflows.url = "https://flakehub.com/f/Singularity-ng/singularity-workflows/*.tar.gz";
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};
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}
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```
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## CI Workflows
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### nix-ci.yml
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Full Nix-based CI:
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- Uses DeterminateSystems/nix-installer-action
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- Integrates Cachix + Magic Nix Cache
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- Runs tests in nix develop shell
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- ~2-3 minutes (cached)
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### cachix-push.yml
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Weekly cache refresh:
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- Rebuilds all derivations
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- Pushes to Cachix
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- Runs on schedule + manual trigger
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- Keeps cache warm
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### flakehub-publish.yml
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FlakeHub publishing:
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- Triggered on version tags
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- Makes flake available via FlakeHub
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- Easy consumption for users
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## Cache Performance
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**First build (cold cache):**
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- ~5-8 minutes (downloads everything)
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**Subsequent builds (warm cache):**
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- ~1-2 minutes (everything cached)
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**With Cachix:**
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- Dev shell: ~30s (pull from cache)
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- Full build: ~1-2 minutes
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## Secrets Required
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Setup in GitHub repository secrets:
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1. `CACHIX_AUTH_TOKEN`
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- Get from: https://app.cachix.org
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- Needed for: cachix-push.yml
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2. FlakeHub (optional)
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- Automatic via GitHub App
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- Or set `FLAKEHUB_TOKEN`
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## Local Development
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The flake includes Cachix configuration:
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```nix
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nixConfig = {
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extra-substituters = [ "https://singularity-ng.cachix.org" ];
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extra-trusted-public-keys = [ "singularity-ng.cachix.org-1:key" ];
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};
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```
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This is automatically applied when you run:
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```bash
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nix develop
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```
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## Monitoring
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- Cachix dashboard: https://app.cachix.org/cache/singularity-ng
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- FlakeHub page: https://flakehub.com/flake/Singularity-ng/singularity-workflows
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- GitHub Actions: Check workflow runs for cache hit rates
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## Best Practices
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1. **Weekly cache updates**: Scheduled via cachix-push.yml
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2. **Lock file updates**: Keep flake.lock current for security
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3. **Cache warming**: Run cachix-push after major dependency updates
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4. **Monitor size**: Large caches cost more on Cachix (free tier: 10GB)
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## Troubleshooting
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### Cache misses
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```bash
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# Clear local Nix store cache
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nix-collect-garbage -d
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# Rebuild with fresh cache
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nix develop --refresh
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```
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### Cachix authentication
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```bash
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# Re-authenticate
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cachix authtoken <YOUR_TOKEN>
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# Test push
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nix build .#devShells.x86_64-linux.default
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cachix push singularity-ng result
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```
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### FlakeHub updates not visible
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- Check workflow runs in Actions tab
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- Verify tag format: `v*` (e.g., v0.1.6)
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- Check FlakeHub dashboard for errors
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## Cost Considerations
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- **Cachix free tier**: 10GB storage, unlimited downloads
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- **Magic Nix Cache**: Free (GitHub Actions feature)
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- **FlakeHub**: Free for public flakes
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If cache exceeds 10GB:
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- Consider paid Cachix plan
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- Or switch to self-hosted cache (nixserve, S3)

‎.github/workflows/cachix-push.yml‎

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name: Build and Push to Cachix
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on:
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push:
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branches: [main]
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schedule:
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# Rebuild cache weekly to keep it fresh
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- cron: '0 0 * * 0'
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workflow_dispatch:
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permissions:
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contents: read
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id-token: write
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jobs:
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build-and-cache:
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name: Build and Cache Nix Derivations
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runs-on: ubuntu-latest
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strategy:
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matrix:
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system: [x86_64-linux]
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steps:
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- name: Checkout code
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uses: actions/checkout@v4
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- name: Install Nix
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uses: DeterminateSystems/nix-installer-action@v13
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- name: Setup Cachix
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uses: cachix/cachix-action@v15
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with:
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name: singularity-ng
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authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
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pushFilter: '(-source$|\.drv$)'
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- name: Setup Magic Nix Cache
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uses: DeterminateSystems/magic-nix-cache-action@v7
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- name: Build dev shell
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run: |
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echo "Building development shell..."
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nix build .#devShells.${{ matrix.system }}.default \
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--print-build-logs \
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--fallback
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- name: Build all outputs
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run: |
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echo "Building all flake outputs..."
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nix flake show --json | \
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jq -r '.packages."${{ matrix.system }}" | keys[]' | \
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while read package; do
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echo "Building package: $package"
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nix build .#$package --print-build-logs || echo "Failed to build $package"
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done
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- name: Push to Cachix
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run: |
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echo "✅ All builds pushed to Cachix cache: singularity-ng"
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echo "📦 Users can use this cache with:"
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echo " cachix use singularity-ng"
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echo ""
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echo "Or add to /etc/nix/nix.conf:"
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echo " substituters = https://cache.nixos.org https://singularity-ng.cachix.org"
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echo " trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= singularity-ng.cachix.org-1:your-key-here"

‎.github/workflows/flakehub-publish.yml‎

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name: Publish to FlakeHub
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on:
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push:
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tags:
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- 'v*'
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workflow_dispatch:
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permissions:
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id-token: write
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contents: read
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jobs:
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flakehub-publish:
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name: Publish to FlakeHub
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runs-on: ubuntu-latest
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steps:
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- name: Checkout code
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uses: actions/checkout@v4
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- name: Install Nix
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uses: DeterminateSystems/nix-installer-action@v13
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- name: Publish to FlakeHub
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uses: DeterminateSystems/flakehub-push@v4
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with:
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visibility: public
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name: Singularity-ng/singularity-workflows
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rolling: false
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tag: ${{ github.ref_name }}
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- name: Summary
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run: |
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echo "✅ Published to FlakeHub!"
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echo "📦 Available at: https://flakehub.com/flake/Singularity-ng/singularity-workflows"
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echo "🔧 Use in flake.nix:"
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echo " inputs.singularity-workflows.url = \"https://flakehub.com/f/Singularity-ng/singularity-workflows/*.tar.gz\";"

‎.github/workflows/nix-ci.yml‎

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name: Nix CI
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on:
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push:
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branches: [main]
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pull_request:
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branches: [main]
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workflow_dispatch:
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concurrency:
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group: ${{ github.workflow }}-${{ github.ref }}
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cancel-in-progress: true
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jobs:
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nix-build:
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name: Nix Build & Test
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runs-on: ubuntu-latest
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permissions:
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contents: read
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checks: write
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id-token: write
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steps:
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- name: Checkout code
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uses: actions/checkout@v4
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- name: Install Nix
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uses: DeterminateSystems/nix-installer-action@v13
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with:
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extra-conf: |
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experimental-features = nix-command flakes
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substituters = https://cache.nixos.org https://singularity-ng.cachix.org
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trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= singularity-ng.cachix.org-1:your-key-here
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- name: Setup Cachix
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uses: cachix/cachix-action@v15
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with:
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name: singularity-ng
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authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
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- name: Setup Magic Nix Cache
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uses: DeterminateSystems/magic-nix-cache-action@v7
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- name: Check flake
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run: nix flake check --all-systems --show-trace
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- name: Build dev shell
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run: nix build .#devShells.x86_64-linux.default
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- name: Enter dev shell and run tests
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run: |
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nix develop --command bash -c '
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echo "=== Environment Info ==="
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elixir --version
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mix --version
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psql --version
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echo ""
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echo "=== Installing dependencies ==="
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mix deps.get
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echo "=== Running tests ==="
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mix test --trace
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echo "=== Running quality checks ==="
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mix format --check-formatted
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mix credo --strict
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'
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- name: Build release
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run: nix build .#singularity_workflow || echo "No release build defined"
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nix-flake-metadata:
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name: Flake Metadata
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runs-on: ubuntu-latest
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steps:
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- name: Checkout code
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uses: actions/checkout@v4
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- name: Install Nix
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uses: DeterminateSystems/nix-installer-action@v13
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- name: Setup Magic Nix Cache
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uses: DeterminateSystems/magic-nix-cache-action@v7
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- name: Show flake metadata
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run: nix flake metadata
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- name: Show flake outputs
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run: nix flake show

‎cachix.nix‎

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# Cachix configuration for singularity-workflows
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# This file is used by cachix-action in CI
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{
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name = "singularity-ng";
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# Public cache - anyone can read
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public = true;
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# What to push to the cache
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# We want to cache all our build outputs
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filter = {
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# Push all derivations
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derivations = true;
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# Don't push source derivations (saves space)
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source = false;
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};
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# Compression settings
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compression = "zstd";
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# Cache configuration for users
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instructions = ''
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# To use this cache, add to your nix.conf or use cachix:
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# Option 1: Using cachix CLI
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cachix use singularity-ng
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# Option 2: Manual nix.conf configuration
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# Add to /etc/nix/nix.conf or ~/.config/nix/nix.conf:
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substituters = https://cache.nixos.org https://singularity-ng.cachix.org
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trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= singularity-ng.cachix.org-1:your-signing-key-here
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# Option 3: In your flake.nix
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nixConfig = {
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extra-substituters = [ "https://singularity-ng.cachix.org" ];
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extra-trusted-public-keys = [ "singularity-ng.cachix.org-1:your-signing-key-here" ];
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};
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'';
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}

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