create DB struct

This is step one in detaching the Database layer from Headscale (h). The
ultimate goal is to have all function that does database operations in
its own package, and keep the business logic and writing separate.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/util/file.go

@@ -0,0 +1,43 @@
+package util
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"github.com/spf13/viper"
+)
+
+const (
+	Base8     = 8
+	Base10    = 10
+	BitSize16 = 16
+	BitSize32 = 32
+	BitSize64 = 64
+)
+
+func AbsolutePathFromConfigPath(path string) string {
+	// If a relative path is provided, prefix it with the directory where
+	// the config file was found.
+	if (path != "") && !strings.HasPrefix(path, string(os.PathSeparator)) {
+		dir, _ := filepath.Split(viper.ConfigFileUsed())
+		if dir != "" {
+			path = filepath.Join(dir, path)
+		}
+	}
+
+	return path
+}
+
+func GetFileMode(key string) fs.FileMode {
+	modeStr := viper.GetString(key)
+
+	mode, err := strconv.ParseUint(modeStr, Base8, BitSize64)
+	if err != nil {
+		return PermissionFallback
+	}
+
+	return fs.FileMode(mode)
+}