限制程序只有一个下载进程的改动

我的小linux服务器每分钟检查一次网盘里是否有新文件，如果有就开始下载，这样就有一个问题：如果正在下的文件很大，比如说电影，下载时又启动了一个新进程。 因此需要增加进程检查的功能。

      def syncdown(self, remotedir = '', localdir = '', deletelocal = False):
            ''' Usage: syncdown [remotedir] [localdir] [deletelocal] - \
  sync down from the remote direcotry to the local directory
   remotedir - the remote directory at Baidu Yun (after app's direcotry) to sync from. \
  if not specified, it defaults to the root directory
     localdir - the local directory to sync to if not specified, it defaults to the current directory.
     deletelocal - delete local files that are not inside Baidu Yun direcotry, default is False
            '''
            pid = str(os.getpid())   #新增
            pidfile = "/var/bypy.pid"  #新增
            if os.path.isfile(pidfile):  #新增
                    print "%s already exists, exiting" % pidfile #新增
                    sys.exit() #新增
            else: #新增
                    file(pidfile, 'w').write(pid)  #新增

            result = ENoError
            rpath = get_pcs_path(remotedir)

. . . .

                            elif os.path.isdir(p):
                                    subresult = removedir(p, self.Verbose)
                                    if subresult != ENoError:
                                            result = subresult

                os.unlink(pidfile)  #新增
                return result

`

只改了syncdown