PostgreSQL异常重启postmaster.pid处理

main>PostmasterMain->CreateDataDirLockFile->CreateLockFile：：
  for (ntries = 0;; ntries++){
    // O_EXCL若存在则返回失败，errno是17
    fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
    if (fd >= 0)
      break;  
    if ((errno != EEXIST && errno != EACCES) || ntries > 100)//重试100次
    could not create lock file
    fd = open(filename, O_RDONLY, pg_file_create_mode);
    ...
    if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
    ...
    close(fd);
    //0表示检查进程是否存在。ESRCH：没有这个进程；EPERM：操作不允许
    if (kill(other_pid, 0) == 0 ||(errno != ESRCH && errno != EPERM)){
      lock file \"%s\" already exists
    }
    if (unlink(filename) < 0)//删除，然后循环再试创建
      ...
    }
    //此时创建成功
    if (write(fd, buffer, strlen(buffer)) != strlen(buffer))//写入pid内容
      ...
    if (pg_fsync(fd) != 0)
      ...
    if (close(fd) != 0)
      ...