class Fluent::Plugin::FileOutput

def find_filepath_available(path_with_placeholder, with_lock: false) # for non-append
  raise "BUG: index placeholder not found in path: #{path_with_placeholder}" unless path_with_placeholder.index('_**')
  i = 0
  dir_path = locked = nil
  while true
    path = path_with_placeholder.sub('_**', "_#{i}")
    i += 1
    next if File.exist?(path)
    if with_lock
      dir_path = path + '.lock'
      locked = Dir.mkdir(dir_path) rescue false
      next unless locked
      # ensure that other worker doesn't create a file (and release lock)
      # between previous File.exist? and Dir.mkdir
      next if File.exist?(path)
    end
    break
  end
  yield path
ensure
  if dir_path && locked && Dir.exist?(dir_path)
    Dir.rmdir(dir_path) rescue nil
  end
end