/var/lib/dpkg/lock 如何工作？

Question

/var/lib/dpkg/lock 是在“包管理器正在工作”时持有锁的文件。但是这个系统是如何运作的呢？每次运行 Linux 时，我都有 /var/lib/dpkg/lock 。当我使用 dpkg 的包管理器之一时，我没有任何更改。所以我看不到它在行动。

Answer 1

我不确定，但这很可能是通过flock(). 该flock()系统调用创建一个文件的咨询锁。如果另一个应用程序试图获得对文件的锁定，内核将阻塞直到原始锁定消失，或者EWOULDBLOCK如果LOCK_NB给出选项则返回。这种锁定机制将允许使用锁定文件而无需删除和重新创建它。

更新：检查源并验证它是建议锁定，但它不flock()直接使用。fcntl用来：

查询.c：

        if (modstatdb_is_locked())
          puts(_(
"Another process has locked the database for writing, and might currently be\n"
"modifying it, some of the following problems might just be due to that.\n"));
        head_running = true;
      }

dbmodify.c:

modstatdb_is_locked(void)
{
  int lockfd;
  bool locked;

  if (dblockfd == -1) {
    lockfd = open(lockfile, O_RDONLY);
    if (lockfd == -1)
      ohshite(_("unable to open lock file %s for testing"), lockfile);
  } else {
    lockfd = dblockfd;
  }

  locked = file_is_locked(lockfd, lockfile);

  /* We only close the file if there was no lock open, otherwise we would
   * release the existing lock on close. */
  if (dblockfd == -1)
    close(lockfd);

  return locked;
}

文件.c：

file_is_locked(int lockfd, const char *filename)
{
    struct flock fl;

    file_lock_setup(&fl, F_WRLCK);

    if (fcntl(lockfd, F_GETLK, &fl) == -1)
        ohshit(_("unable to check file '%s' lock status"), filename);

    if (fl.l_type == F_WRLCK && fl.l_pid != getpid())
        return true;
    else
        return false;
}

dpkg.h:

#define LOCKFILE          "lock"

从fcntl联机帮助页：

   Advisory locking
       F_GETLK,  F_SETLK  and  F_SETLKW  are  used to acquire, release, and test for the existence of record locks (also known as file-segment or file-region locks).  The third
       argument, lock, is a pointer to a structure that has at least the following fields (in unspecified order).