Add advantech router app updates

updates/advantech.go

@@ -0,0 +1,48 @@
+//go:build !windows
+
+package updates
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/fosrl/newt/logger"
+)
+
+const advantechVersionFile = "/opt/newt/etc/version"
+
+// postUpdateAdvantech performs Advantech Router App-specific post-update steps:
+//   - Writes the new version string to /opt/newt/etc/version so that the
+//     router firmware's package management reflects the installed version.
+//   - Updates the PID file at pidFile (if non-empty) with the current process
+//     PID, in case the re-exec lands with a new PID on platforms where
+//     syscall.Exec behaviour differs.
+func postUpdateAdvantech(newVersion, pidFile string) error {
+	// --- Write version file ---
+	versionDir := filepath.Dir(advantechVersionFile)
+	if err := os.MkdirAll(versionDir, 0755); err != nil {
+		return fmt.Errorf("advantech: failed to create version directory %s: %w", versionDir, err)
+	}
+
+	if err := os.WriteFile(advantechVersionFile, []byte(newVersion+"\n"), 0644); err != nil {
+		return fmt.Errorf("advantech: failed to write version file %s: %w", advantechVersionFile, err)
+	}
+	logger.Debug("postUpdateAdvantech: wrote version %s to %s", newVersion, advantechVersionFile)
+
+	// --- Update PID file ---
+	// syscall.Exec replaces the process image in-place so the PID is preserved.
+	// We update the PID file here anyway so that any race between the old and
+	// new binary is covered (e.g. on platforms that fork before exec).
+	if pidFile != "" {
+		pid := fmt.Sprintf("%d\n", os.Getpid())
+		if err := os.WriteFile(pidFile, []byte(pid), 0644); err != nil {
+			// Non-fatal: log and continue so the update still proceeds.
+			logger.Debug("postUpdateAdvantech: warning: failed to update PID file %s: %v", pidFile, err)
+		} else {
+			logger.Debug("postUpdateAdvantech: updated PID file %s with PID %d", pidFile, os.Getpid())
+		}
+	}
+
+	return nil
+}