Function LockOrUnlock

src/leveldb/util/env_posix.cc:452–461

};

int LockOrUnlock(int fd, bool lock) {
  errno = 0;
  struct ::flock file_lock_info;
  std::memset(&file_lock_info, 0, sizeof(file_lock_info));
  file_lock_info.l_type = (lock ? F_WRLCK : F_UNLCK);
  file_lock_info.l_whence = SEEK_SET;
  file_lock_info.l_start = 0;
  file_lock_info.l_len = 0;  // Lock/unlock entire file.
  return ::fcntl(fd, F_SETLK, &file_lock_info);
}

// Instances are thread-safe because they are immutable.
class PosixFileLock : public FileLock {