fix(greeter): update NixOS declarative configuration

- Report NixOS greeter state from `var/lib/dms-greeter`
- Allows `dms greeter status` use
- Prevent imperative greeter changes on module-managed NixOS systems
- Preserve immutable-distro sync policy
- Closes #2728

core/cmd/dms/commands_greeter_test.go

@@ -3,9 +3,11 @@ package main
 import (
 	"errors"
 	"reflect"
+	"strings"
 	"testing"
 
 	sharedpam "github.com/AvengeMedia/DankMaterialShell/core/internal/pam"
+	"github.com/spf13/cobra"
 )
 
 func TestSyncGreeterConfigsAndAuthDelegatesSharedAuth(t *testing.T) {
@@ -85,3 +87,57 @@ func TestSyncGreeterConfigsAndAuthStopsOnConfigError(t *testing.T) {
 		t.Fatal("expected auth sync not to run after config sync failure")
 	}
 }
+
+func TestGreeterStatusStateDirUsesNixOSDefault(t *testing.T) {
+	if got := greeterStatusStateDir("", true); got != nixOSGreeterStateDir {
+		t.Fatalf("greeterStatusStateDir() = %q, want %q", got, nixOSGreeterStateDir)
+	}
+}
+
+func TestGreeterStatusStateDirHonorsExplicitOverrideOnNixOS(t *testing.T) {
+	command := "dms-greeter --cache-dir /srv/dms-greeter --command niri"
+	if got := greeterStatusStateDir(command, true); got != "/srv/dms-greeter" {
+		t.Fatalf("greeterStatusStateDir() = %q, want %q", got, "/srv/dms-greeter")
+	}
+}
+
+func TestRejectNixOSGreeterMutationBlocksImperativeCommands(t *testing.T) {
+	origGreeterIsNixOSFn := greeterIsNixOSFn
+	greeterIsNixOSFn = func() bool { return true }
+	t.Cleanup(func() {
+		greeterIsNixOSFn = origGreeterIsNixOSFn
+	})
+
+	for _, commandName := range []string{"install", "enable", "sync", "uninstall"} {
+		t.Run(commandName, func(t *testing.T) {
+			root := &cobra.Command{Use: "dms"}
+			greeterCommand := &cobra.Command{Use: "greeter"}
+			mutationCommand := &cobra.Command{Use: commandName}
+			root.AddCommand(greeterCommand)
+			greeterCommand.AddCommand(mutationCommand)
+
+			err := rejectNixOSGreeterMutation(mutationCommand)
+			if err == nil {
+				t.Fatalf("expected NixOS greeter %s to be rejected", commandName)
+			}
+			if !strings.Contains(err.Error(), "dms greeter "+commandName+" is disabled on NixOS") {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if strings.Contains(err.Error(), "/var/cache/dms-greeter") {
+				t.Fatalf("NixOS remediation should not recommend the non-NixOS cache path: %v", err)
+			}
+		})
+	}
+}
+
+func TestRejectNixOSGreeterMutationAllowsOtherDistros(t *testing.T) {
+	origGreeterIsNixOSFn := greeterIsNixOSFn
+	greeterIsNixOSFn = func() bool { return false }
+	t.Cleanup(func() {
+		greeterIsNixOSFn = origGreeterIsNixOSFn
+	})
+
+	if err := rejectNixOSGreeterMutation(&cobra.Command{Use: "sync"}); err != nil {
+		t.Fatalf("expected non-NixOS greeter command to be allowed, got %v", err)
+	}
+}