Wrap some upgrading in function to trigger defer

This will ensure all file handles are closed before attempting to
rename or remove directories.

snapshot/upgrader.go

@@ -89,6 +89,8 @@ func Upgrade(old, new string, logger *log.Logger) error {
 		return fmt.Errorf("failed to write new snapshot meta file: %s", err)
 	}
 
+	// Ensure all file handles are closed before any dircetory is moved or removed.
+	if err := func() error {
 		// Write SQLite data into generation directory, as the base SQLite file.
 		newSqliteBasePath := filepath.Join(newGenerationDir, baseSqliteFile)
 		newSqliteFd, err := os.Create(newSqliteBasePath)
@@ -123,6 +125,10 @@ func Upgrade(old, new string, logger *log.Logger) error {
 		if !db.IsValidSQLiteFile(newSqliteBasePath) {
 			return fmt.Errorf("migrated SQLite file %s is not valid", newSqliteBasePath)
 		}
+		return nil
+	}(); err != nil {
+		return err
+	}
 
 	// Move the upgraded snapshot directory into place.
 	if err := os.Rename(newTmpDir, new); err != nil {