virtcontainers/persist: introduce rootless fs driver

Rootless fs driver inherits from FS and may overwrite its methods. All files
and directories created by this driver are under a path accessible for the
current user, typically this path is defined by the environment variable
`XDG_RUNTIME_DIR`, if this variable is not defined, the default path
`/run/user/$UID` is used instead, where $UID is the current user ID.

fixes #2416

Signed-off-by: Julio Montes <julio.montes@intel.com>

virtcontainers/persist/fs/rootlessfs.go

@@ -0,0 +1,48 @@
+// Copyright (c) 2020 Intel Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package fs
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	persistapi "github.com/kata-containers/runtime/virtcontainers/persist/api"
+)
+
+// default xdg runtime directory just in case XDG_RUNTIME_DIR is not set
+var defaultXdgRuntimeDir = fmt.Sprintf("/run/user/%d", os.Getuid())
+
+type RootlessFS struct {
+	// inherit from FS. Overwrite if needed.
+	*FS
+}
+
+func RootlessInit() (persistapi.PersistDriver, error) {
+	driver, err := Init()
+	if err != nil {
+		return nil, fmt.Errorf("Could not create Rootless FS driver: %v", err)
+	}
+
+	fsDriver, ok := driver.(*FS)
+	if !ok {
+		return nil, fmt.Errorf("Could not create Rootless FS driver")
+	}
+
+	// XDG_RUNTIME_DIR defines the base directory relative to
+	// which user-specific non-essential runtime files are stored.
+	rootlessDir := os.Getenv("XDG_RUNTIME_DIR")
+	if rootlessDir == "" {
+		rootlessDir = defaultXdgRuntimeDir
+		fsLog.WithField("default-runtime-dir", defaultXdgRuntimeDir).
+			Warnf("XDG_RUNTIME_DIR variable is not set. Using default runtime directory")
+	}
+
+	fsDriver.storageRootPath = filepath.Join(rootlessDir, fsDriver.storageRootPath)
+	fsDriver.driverName = "rootlessfs"
+
+	return &RootlessFS{fsDriver}, nil
+}