refactor: storage interface

pkg/storage/fs/fs.go

@@ -0,0 +1,27 @@
+// fs implements Storage on filesystem
+package fs
+
+import (
+	"errors"
+	"galaxy/util"
+)
+
+type fs struct {
+	rootPath string
+}
+
+// NewStorage returns on-filesystem implementation of the "galaxy/storage.Storage" with root located at rootPath.
+// rootPath must me a directory and has write access to the current user. If initial checks failed, return nil and non-nil error.
+func NewFS(rootPath string) (*fs, error) {
+	ok, err := util.Writable(rootPath)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, errors.New("user does not have write permissions to the storage root")
+	}
+	s := &fs{
+		rootPath: rootPath,
+	}
+	return s, nil
+}