Fix restic#1724 check the validity and permission of the mount point …

…before trying to mount

cmd/restic/cmd_mount.go

@@ -3,7 +3,9 @@
 package main
 
 import (
+	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -157,6 +159,32 @@ func umount(mountpoint string) error {
 	return systemFuse.Unmount(mountpoint)
 }
 
+func checkMountPointValidity(mountpoint string) error {
+	// check if the mountpoint exist and if it's a directory
+	info, err := os.Stat(mountpoint)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return errors.Fatalf("the mount path doesn't exist: %q", mountpoint)
+		}
+		return errors.Fatalf("the mount path is not a directory or doesn't exist %q", mountpoint)
+	}
+	if !info.IsDir() {
+		return errors.Fatalf("the mount path is not a directory %q", mountpoint)
+	}
+	// create a temporary file to test permission whether it's writable directory.
+	tmpFile := filepath.Join(mountpoint, fmt.Sprintf("%d", time.Now().UnixNano()))
+	f, err := os.Create(tmpFile)
+	if err != nil {
+		// check with IsPermission(err) ?
+		return errors.Fatalf("the mount path is not writable directory: %q", mountpoint)
+	}
+	// fixme: check for error ?
+	f.Close()
+	os.Remove(tmpFile)
+
+	return nil
+}
+
 func runMount(opts MountOptions, gopts GlobalOptions, args []string) error {
 	if opts.SnapshotTemplate == "" {
 		return errors.Fatal("snapshot template string cannot be empty")
@@ -172,6 +200,12 @@ func runMount(opts MountOptions, gopts GlobalOptions, args []string) error {
 
 	mountpoint := args[0]
 
+	// avoid unnecessary work if the mount path doesn't exist or is not writable
+	err := checkMountPointValidity(mountpoint)
+	if err != nil {
+		return err
+	}
+
 	AddCleanupHandler(func() error {
 		debug.Log("running umount cleanup handler for mount at %v", mountpoint)
 		err := umount(mountpoint)