Use flocks to lock the database instead of purely relying on file existence.

With PackageKit or pacman itself, it sometimes occurs that the lock file is left over erroneously. A manual administrative $ rm /var/lib/{pacman,PackageKit/alpm}/db.lck is required. This is also the preferred way to fix this issue as per the Wiki [1][2].

This is not really user-friendly when it happens, and has bothered me personally about five or six times now to a point where I hacked this patch.

The lock file can now preexist when using libalpm, and libalpm now flock()s the file exclusively. Linux ensures internally that no second exclusive lock can be held. However, we still can't get rid of the unlink when unlocking altogether, because existing bash scripts (e.g. makepkg.sh) depend on it. Refactoring those to use flocks would be quite a mess, too. But because the lock file should normally not exist without any lock on it, the scripts would simply say that someone holds the lock. Refreshing the pacman database now fixes the issue on it's own. This is now also user triggerable when using PackageKit :)

[1] https://wiki.archlinux.org/title/Pacman 3.3 "Failed to init transaction (unable to lock database)" error [2] https://wiki.archlinux.org/title/KDE 6.15 Discover stops showing updates from Arch repositories

Signed-off-by: Stephan Brunner s.brunner@stephan-brunner.net