docs: def is_pid(value):
      """
      This function checks whether file path
      that is specified at "pid_file" option eixsts,
      whether write permission to the file path.
  
      Return the following value:
      case1: exists path and write permission
          is_pid('/tmp')
              '/tmp/hogehoge.pid'
  
      case2: non-exists path and write permission
          is_pid('/tmp/hogehoge')
              '/tmp/hogehoge'
          In this case, hogehoge doesn't exist.
          but hogehoge is considered as a file.
          Thus, create pid file named 'hogehoge'.
  
      case3: complete non-exists path
          is_pid('/tmp/hogehoge/fugafuga')
              IOError: [Error 2] No such file or directory.
          The last part of given path is only considered pid_file's name.
          In this case, "fugafuga" is considered pid_file's name.
  
      In any case, check whether given path exists before checking permission.
  
      Notes: Even if given relative path, works fine.
             But, if don't use as much as possible if good.
  
             Recommended giving the absolute path including the pid file name.
      """
  
      value = os.path.expanduser(value)
      value = os.path.expandvars(value)
      value = os.path.abspath(value)
  
      pid_file = 'blackbird.pid'
  
      if os.path.exists(value):
  
          if os.path.isdir(value):
  
              if os.access(value, os.W_OK):
                  return os.path.join(value, pid_file)
  
              else:
                  err_message = ('{path}: Permission denied.'
                                 ''.format(path=value)
                                 )
                  raise validate.VdtValueError(err_message)
  
          else:
              err_message = 'pid file already exists.'
              raise AlreadyLocked(err_message)
  
      else:
          directory = os.path.split(value)[0]
  
          if os.path.isdir(directory):
  
              if os.access(directory, os.W_OK):
                  return value
  
              else:
                  err_message = ('{directory}: Permission denied.'
                                 ''.format(directory=directory)
                                 )
                  raise validate.VdtValueError(err_message)
  
          else:
  
              if os.path.exists(directory):
                  err_message = ('{directory} is file.'
                                 ''.format(directory=directory)
                                 )
                  raise validate.VdtTypeError(err_message)
  
              else:
                  err_message = ('{directory}: No such file or directory.'
                                 ''.format(directory=directory)
                                 )
                  raise validate.VdtValueError(err_message)