mjhjmtu

/var/lib/dpkg/lock is file that holding a lock when "A package manager is working". But how this system works? I have /var/lib/dpkg/lock everytime when I have Linux working. When I use one of package manager for dpkg I have it without any change. So I can't see it in action.

I don't know for certain, but this is most likely implemented via flock(). The flock() system call creates an advisory lock on a file. If another application tries to attain a lock on the file, the kernel will block until the original lock is gone, or return EWOULDBLOCK if the LOCK_NB option is given. This locking mechanism would allow the lock file to be used without deleting and re-creating it.

Update: Checked the source and verified that it is advisory locking, but it doesn't use flock() directly. fcntl is used:

enquiry.c:

 if (modstatdb_is_locked())
 puts(_(
"Another process has locked the database for writing, and might currently ben"
"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;

file.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"

From the fcntl manpage:

 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).

As I see, dpkg locks the file /var/lib/dpkg/lock using lockf(3), which in turn uses fcntl(2).

$ sudo strace dpkg -r somepackage 2>&1 |
> grep F_SETLKW
fcntl64(5, F_SETLKW64, l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0) = 0

This means that you cannot lock the file from the shell using flock(1), because this calls flock(2), which in many systems doesn't interact with fnctl locks.

$ sudo strace flock -x /var/lib/dpkg/lock 2>&1 |
> grep 'flock('
flock(3, LOCK_EX)

You can however lock the file with the with-lock-ex program or this Python script, both of which acquire a compatible lock.