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feat: add persist-nix feature
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‎src/persist-nix/README.md‎

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# Persist Nix store (persist-nix)
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Retain the Nix store (`/nix/store`) across container rebuilds to speed up installation of nix-based dotfiles.
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## What this feature does
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This feature persists the Nix store directory that is used when managing dotfiles with Nix. The persistent Docker volume significantly reduces download and build times during container rebuilds, especially when dotfiles are heavily managed through Nix.
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## How it works
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### Build phase (as root)
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The `install.sh` script creates a Docker volume and mount point:
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- `/.persist-nix-store` – Volume for the Nix store
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The directory is created with permissions `777` so all users can access it.
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### User initialization phase (in user context)
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The `persist-nix-init.sh` script (runs via `postCreateCommand`) performs:
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1. **Nix availability check**: Verifies Nix is installed; skips initialization if not available
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2. **Backup of existing data**: If `/nix/store` exists as a regular directory, it is renamed to `/nix/store.bak` (preserving any existing store)
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3. **Symlink creation**: Creates a symlink from `/nix/store` to the persistent volume `/.persist-nix-store`
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This approach ensures:
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- **No data loss**: Existing Nix store is backed up before being replaced
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- **Automatic recovery**: Symlinks persist across rebuilds; no repeated setup needed
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- **Dependency compatibility**: Only runs if Nix is present; safe to include in devcontainers without Nix
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## Example Usage
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```json
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"features": {
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"ghcr.io/ckagerer/devcontainer-features/persist-nix:1": {
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"keep_going": false
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}
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}
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```
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## Notes
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- **Nix dependency**: This feature requires Nix to be installed in the container. If Nix is not present, the feature initializes safely and exits without error.
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- **Sudo requirement**: The symlink creation uses `sudo` as it modifies the `/nix` directory structure. Ensure your user can execute `sudo` commands, or run the container with elevated privileges.
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- **Volume lifecycle**: The persistent volume is managed by Docker and is not automatically deleted when the container is removed. To clear cached data, manually delete the Docker volume `devcontainer-persist-nix-store`.
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## Clearing the cache
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To clear the Nix store cache and reclaim disk space:
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```bash
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docker volume rm devcontainer-persist-nix-store
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```
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The volume will be automatically recreated on the next container build.

‎src/persist-nix/devcontainer-feature.json‎

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{
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"name": "persist-nix",
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"id": "persist-nix",
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"version": "1.0.0",
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"description": "Persist Nix store across container rebuilds to speed up nix-based dotfiles.",
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"documentationURL": "https://github.com/ckagerer/devcontainer-features/tree/main/src/persist-nix",
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"options": {
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"keep_going": {
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"type": "boolean",
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"default": false,
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"description": "Ignore errors during setup and continue execution."
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}
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},
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"mounts": [
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{
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"source": "devcontainer-persist-nix-store",
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"target": "/.persist-nix-store",
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"type": "volume"
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}
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],
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"postCreateCommand": "/usr/local/share/persist-nix-init.sh"
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}

‎src/persist-nix/install.sh‎

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#!/usr/bin/env sh
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# Initialize Nix store volume directory
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# This script runs as root during container build
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if [ "${KEEP_GOING:-false}" = "true" ]; then
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set +e
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else
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set -e
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fi
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set -x
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# Create Nix store mount point with permissions for all users
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mkdir -p /.persist-nix-store
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chmod 777 /.persist-nix-store
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# Generate the postCreateCommand script that runs in user context
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INIT_SCRIPT_PATH="/usr/local/share/persist-nix-init.sh"
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tee "$INIT_SCRIPT_PATH" >/dev/null <<'EOF'
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#!/usr/bin/env sh
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# Initialize Nix store symlinks
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# This script runs in user context (postCreateCommand)
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set -e
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set -x
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# Check if a path is a mount point
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is_mount_point() {
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if command -v mountpoint >/dev/null 2>&1; then
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mountpoint -q "$1"
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else
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# Fallback for systems without mountpoint command
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mount | grep -q " on $(readlink -f "$1") "
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fi
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return $?
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}
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# Only proceed if Nix is installed
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if ! command -v nix >/dev/null 2>&1; then
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echo "Nix not found, skipping persist-nix initialization"
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exit 0
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fi
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# Handle existing /nix/store directory
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if [ -d /nix/store ] && [ ! -L /nix/store ]; then
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# Check if it's already a mount point (e.g., from another devcontainer feature)
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if is_mount_point /nix/store 2>/dev/null; then
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echo "Note: /nix/store is already mounted by another feature, skipping backup"
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else
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echo "Backing up existing /nix/store to /nix/store.bak"
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sudo mv /nix/store /nix/store.bak
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fi
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fi
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# Ensure /nix directory exists
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sudo mkdir -p /nix
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# Create symlink from persistent volume to /nix/store
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if [ ! -L /nix/store ]; then
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echo "Creating symlink /nix/store -> /.persist-nix-store"
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sudo ln -s /.persist-nix-store /nix/store
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else
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echo "/nix/store symlink already exists"
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fi
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echo "persist-nix initialization complete"
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EOF
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# Make the init script executable
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chmod +x "$INIT_SCRIPT_PATH"

‎test/persist-nix/scenarios.json‎

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{
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"scenarios": [
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{
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"name": "default",
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"description": "Test persist-nix with default settings"
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}
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]
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}

‎test/persist-nix/test.sh‎

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#!/usr/bin/env bash
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# Test script for persist-nix feature
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# This script verifies that the persist-nix feature correctly sets up
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# the Nix store persistence layer
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set -e
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set -x
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# Test 1: Check if the mount point was created
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if [ ! -d /.persist-nix-store ]; then
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echo "ERROR: /.persist-nix-store directory not found"
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exit 1
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fi
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echo "✓ /.persist-nix-store directory exists"
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# Test 2: Check if the init script was created
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if [ ! -f /usr/local/share/persist-nix-init.sh ]; then
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echo "ERROR: /usr/local/share/persist-nix-init.sh not found"
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exit 1
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fi
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echo "✓ /usr/local/share/persist-nix-init.sh exists"
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# Test 3: Check if the init script is executable
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if [ ! -x /usr/local/share/persist-nix-init.sh ]; then
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echo "ERROR: /usr/local/share/persist-nix-init.sh is not executable"
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exit 1
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fi
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echo "✓ /usr/local/share/persist-nix-init.sh is executable"
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# Test 4: If Nix is installed, verify symlink exists
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if command -v nix >/dev/null 2>&1; then
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if [ -L /nix/store ]; then
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echo "✓ /nix/store is a symlink (Nix installed)"
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# Verify it points to the persistent volume
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if [ "$(readlink /nix/store)" = "/.persist-nix-store" ]; then
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echo "✓ /nix/store correctly points to /.persist-nix-store"
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else
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echo "ERROR: /nix/store does not point to /.persist-nix-store"
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exit 1
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fi
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else
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echo "ERROR: /nix/store is not a symlink (Nix is installed but symlink not created)"
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exit 1
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fi
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else
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echo "ℹ Nix not installed, symlink test skipped"
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fi
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echo ""
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echo "All tests passed!"

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