code: def _GetSourceFileSystem(self, source_path_spec, resolver_context=None):
      
      if not source_path_spec:
        raise RuntimeError('Missing source.')
  
      file_system = path_spec_resolver.Resolver.OpenFileSystem(
          source_path_spec, resolver_context=resolver_context)
  
      type_indicator = source_path_spec.type_indicator
      if path_spec_factory.Factory.IsSystemLevelTypeIndicator(type_indicator):
        mount_point = source_path_spec
      else:
        mount_point = source_path_spec.parent
  
      return file_system, mount_point
docs: Retrieves the file system of the source.
  
      Args:
        source_path_spec (dfvfs.PathSpec): source path specification of the file
            system.
        resolver_context (dfvfs.Context): resolver context.
  
      Returns:
        tuple: containing:
  
          dfvfs.FileSystem: file system.
          dfvfs.PathSpec: mount point path specification that refers
              to the base location of the file system.
  
      Raises:
        RuntimeError: if source path specification is not set.