Update snap_pac.py to Solve the problem that pacman cannot run normal…

[rankaiyx]

…ly after rollback due to the pacman lock file in the snapshot.

I now regularly use snapper-rollback with Arch Linux's recommended btrfs setup to elegantly and efficiently restore my system.

However, after rolling back to pre/post snapshots created by snap-pac, pacman stops working. My old solutions don't work with snapper-rollback's elegant approach.

The core problem:
Pacman's lock mechanism doesn't work well with snapshot hooks. The current sequence is:

Pacman locks database 🔒

Pre-snapshot hook runs 📸

Post-snapshot hook runs 📸

Pacman unlocks database 🔓

This works for most hooks, but fails for system snapshots. For snapshots, the ideal sequence should be:

Pre-snapshot hook 📸

Pacman locks 🔒

Pacman unlocks 🔓

Post-snapshot hook 📸

My solution:
By deleting db.lck in snapshots, we achieve the same result as having the ideal sequence. This keeps pacman working after rollbacks while maintaining snapper-rollback's smooth operation.

before:
➜ ~ sudo snapper list
...
19 │ single │ │ Sat 07 Jun 2025 08:04:42 PM │ root │ │ test2 │
20 │ pre │ │ Sat 07 Jun 2025 08:04:51 PM │ root │ number │ pacman -S iperf3 │
21 │ post │ 20 │ Sat 07 Jun 2025 08:04:52 PM │ root │ number │ iperf3 lksctp-tools │
22 │ single │ │ Sat 07 Jun 2025 08:04:58 PM │ root │ │ test3 │
➜ ~ sudo snapper status 19..20
c..... /home/abc/.zsh_history
+..... /var/lib/pacman/db.lck
➜ ~ sudo snapper status 21..22
c..... /home/abc/.zsh_history
-..... /var/lib/pacman/db.lck

new:
➜ ~ sudo snapper list
...
26 │ single │ │ Sun 08 Jun 2025 01:17:27 AM │ root │ │ test6 │
27 │ pre │ │ Sun 08 Jun 2025 01:17:37 AM │ root │ number │ pacman -S htop │
28 │ post │ 27 │ Sun 08 Jun 2025 01:17:37 AM │ root │ number │ htop │
29 │ single │ │ Sun 08 Jun 2025 01:17:55 AM │ root │ │ test7 │
➜ ~ sudo snapper status 26..27
c..... /home/abc/.zsh_history
➜ ~ sudo snapper status 28..29
c..... /home/abc/.zsh_history

my btrfs layout:
├── @
├── @snapshots
└── @var_log
from https://wiki.archlinux.org/title/Snapper

[rankaiyx]

There are still some unexplained issues. Maybe modifying files locked by snapper isn't a good idea. I'll try handling the rolled-back system within snapper-rollback instead.

I found a method with almost no side effects, tested it repeatedly, and it works well.