[nix] Add DetSys installer behind feature flag (#2303)

## Summary

* Adds detsys installer.
* Puts it behind disabled flag. Add CICD matrix to test.
* Special casing: Handles missing systemd by just skipping it.

## How was it tested?

* Tested `devbox setup nix` in linux docker container.
* Will test in CICD
* Much more testing needed before removing feature flag

.github/workflows/cli-tests.yaml

@@ -183,6 +183,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-13]
+        use-detsys: [true, false]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
@@ -197,6 +198,7 @@ jobs:
       - name: Install nix and devbox packages
         run: |
           export NIX_INSTALLER_NO_CHANNEL_ADD=1
+          export DEVBOX_FEATURE_DETSYS_INSTALLER=${{ matrix.use-detsys }}
 
           # Setup github authentication to ensure Github's rate limits are not hit.
           # If this works, we can consider refactoring this into a reusable github action helper.

internal/boxcli/featureflag/detsys.go

@@ -0,0 +1,3 @@
+package featureflag
+
+var UseDetSysInstaller = disable("DETSYS_INSTALLER")

internal/boxcli/setup.go

@@ -29,21 +29,27 @@ func setupCmd() *cobra.Command {
 		},
 	}
 
-	installNixCommand.Flags().Bool(nixDaemonFlag, false, "Install Nix in multi-user mode.")
+	installNixCommand.Flags().Bool(
+		nixDaemonFlag,
+		false,
+		"Install Nix in multi-user mode. This flag is not supported if you are using DetSys installer",
+	)
 	setupCommand.AddCommand(installNixCommand)
 	return setupCommand
 }
 
 func runInstallNixCmd(cmd *cobra.Command) error {
 	if nix.BinaryInstalled() {
+		// TODO: If existing installation is not detsys, but new installation is detsys can we detect
+		// that and replace it?
 		ux.Finfof(
 			cmd.ErrOrStderr(),
 			"Nix is already installed. If this is incorrect "+
 				"please remove the nix-shell binary from your path.\n",
 		)
 		return nil
 	}
-	return nix.Install(cmd.ErrOrStderr(), nixDaemonFlagVal(cmd)())
+	return nix.Install(cmd.ErrOrStderr(), nixDaemonFlagVal(cmd))
 }
 
 // ensureNixInstalled verifies that nix is installed and that it is of a supported version

internal/nix/install.go

@@ -16,6 +16,7 @@ import (
 	"github.com/mattn/go-isatty"
 	"github.com/pkg/errors"
 
+	"go.jetpack.io/devbox/internal/boxcli/featureflag"
 	"go.jetpack.io/devbox/internal/boxcli/usererr"
 	"go.jetpack.io/devbox/internal/build"
 	"go.jetpack.io/devbox/internal/cmdutil"
@@ -28,7 +29,7 @@ const rootError = "warning: installing Nix as root is not supported by this scri
 
 // Install runs the install script for Nix. daemon has 3 states
 // nil is unset. false is --no-daemon. true is --daemon.
-func Install(writer io.Writer, daemon *bool) error {
+func Install(writer io.Writer, daemonFn func() *bool) error {
 	if isRoot() && build.OS() == build.OSWSL {
 		return usererr.New("Nix cannot be installed as root on WSL. Please run as a normal user with sudo access.")
 	}
@@ -39,7 +40,18 @@ func Install(writer io.Writer, daemon *bool) error {
 	defer r.Close()
 
 	installScript := "curl -L https://releases.nixos.org/nix/nix-2.24.7/install | sh -s"
-	if daemon != nil {
+	if featureflag.UseDetSysInstaller.Enabled() {
+		// Should we pin version? Or just trust detsys
+		installScript = "curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install"
+		if isLinuxWithoutSystemd() {
+			ux.Fwarningf(
+				writer,
+				"Could not detect systemd on your system. Installing Nix in root only mode (--init none).\n",
+			)
+			installScript += " linux --init none"
+		}
+		installScript += " --no-confirm"
+	} else if daemon := daemonFn(); daemon != nil {
 		if *daemon {
 			installScript += " -- --daemon"
 		} else {
@@ -157,7 +169,7 @@ func EnsureNixInstalled(writer io.Writer, withDaemonFunc func() *bool) (err erro
 		fmt.Scanln() //nolint:errcheck
 	}
 
-	if err = Install(writer, withDaemonFunc()); err != nil {
+	if err = Install(writer, withDaemonFunc); err != nil {
 		return err
 	}
 
@@ -169,3 +181,14 @@ func EnsureNixInstalled(writer io.Writer, withDaemonFunc func() *bool) (err erro
 	fmt.Fprintln(writer, "Nix installed successfully. Devbox is ready to use!")
 	return nil
 }
+
+func isLinuxWithoutSystemd() bool {
+	if build.OS() != build.OSLinux {
+		return false
+	}
+	// My best interpretation of https://github.com/DeterminateSystems/nix-installer/blob/66ad2759a3ecb6da345373e3c413c25303305e25/src/action/common/configure_init_service.rs#L108-L118
+	if _, err := os.Stat("/run/systemd/system"); errors.Is(err, os.ErrNotExist) {
+		return true
+	}
+	return !cmdutil.Exists("systemctl")
+}