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snap-pac/README.md
James Barnett 5afd92ddfb
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2017-02-03 09:44:23 -06:00

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snap-pac

Arch Version License

This is a set of pacman hooks and script that causes snapper to automatically take a pre and post snapshot before and after pacman transactions, similar to how YaST does with OpenSuse. This provides a simple way to undo changes to a system after a pacman transaction.

Installation

# pacman -S snap-pac

PGP Key

I have signed the release tarball with my PGP key. You may need to import my public key before installation:

$ gpg --keyserver hkp://pgp.mit.edu --recv-keys 0xE4B5E45AA3B8C5C3

The key's fingerprint is 8535CEF3F3C38EE69555BF67E4B5E45AA3B8C5C3.

Configuration

Configuration is done via the snapper configuration files, with extra variables specific to these pacman hooks. The defaults should be suitable for most users, so you shouldn't need to change anything.

The following are possible settings you can place in each snapper configuration file (e.g., /etc/snapper/configs/root, etc.):

  • PACMAN_PRE_POST - perform pacman pre/post snapshots for this configuration. Default is "no" for all configurations, except for the root configuration which is "yes".
  • PACMAN_CLEANUP_ALGORITHM - snapper algorithm used in cleaning up the pacman pre/post snapshots. Default is "number".
  • PACMAN_PRE_DESCRIPTION - snapper description used for the pacman pre snapshot. Default is the pacman command that called the snapshot.
  • PACMAN_POST_DESCRIPTION - snapper description used for the pacman post snapshot. Default is the pacman command that called the snapshot.

The following setting can be changed in the snapper configuration file /etc/conf.d/snapper:

  • PACMAN_ABORT_ON_FAIL - By default this is set to "no". When set to "yes" this causes pacman to abort a transaction if the snap-pac pre hook fails. This prevents an upgrade/installation/removal from occurring if a pre snapshot cannot be performed.

Usage

Taking snapshots

Use pacman—and AUR helpers—as normal and watch snapper do its thing. No bash scripts for you to call. No bash aliases to setup.

Because these are pacman hooks, it doesn't matter how you call pacman—whether directly, through an AUR helper, or using an alias—snapper will create the snapshots when pacman installs, upgrades, or removes a package. The pacman command used is logged in the snapper description for the snapshots. Additionally the snapshot numbers are output to the screen and to the pacman log for each snapper configuration during the pacman transaction.

Undoing a transaction

To undo changes from a pacman transaction, use snapper undochange. See the snapper manpage and the following example.

If you have severe breakage—like snapper is gone for some reason and you can't get it back—you'll have to resort to more extreme methods, such as taking a snapshot of the pre snapshot and making it the default subvolume or mounting it as /. Most likely you'll need to use a live USB to get into a chroot environment to do any of these things. Snapper has a snapper rollback feature, but your setup has to be properly configured to use it. The exact procedure depends on your specific setup. Be careful.

Example

Installing the nano package as normal:

# pacman -S nano
resolving dependencies...
looking for conflicting packages...

Packages (1) nano-2.5.3-1

Total Installed Size:  2.14 MiB

:: Proceed with installation? [Y/n] Y
(1/1) checking keys in keyring                               [######################################] 100%
(1/1) checking package integrity                             [######################################] 100%
(1/1) loading package files                                  [######################################] 100%
(1/1) checking for file conflicts                            [######################################] 100%
(1/1) checking available disk space                          [######################################] 100%
:: Running pre-transaction hooks...
(1/1) Performing snapper pre snapshots...
    home N/A
    root 1033 ✓
:: Processing package changes...
(1/1) installing nano                                        [######################################] 100%
:: Running post-transaction hooks...
(1/1) Performing snapper post snapshots...
    home N/A
    root 1034 ✓

The snapper snapshot number is given for each snapper configuration that is used (tip: this is also logged in pacman's log). "N/A" means that configuration is not set up for snap-pac (see above).

And here are the snapshots:

# snapper -c root list -t pre-post | tail -n 1
1033  | 1034   | Fri 22 Apr 2016 01:54:13 PM CDT | Fri 22 Apr 2016 01:54:14 PM CDT | pacman -S nano      |

What changed?

# snapper -c root status 1033..1034
+..... /etc/nanorc
c..... /etc/snapper/.snap-pac-pre
+..... /usr/bin/nano
+..... /usr/bin/rnano
+..... /usr/share/doc/nano
+..... /usr/share/doc/nano/faq.html
+..... /usr/share/doc/nano/fr
+..... /usr/share/doc/nano/fr/nano.1.html
+..... /usr/share/doc/nano/fr/nanorc.5.html
+..... /usr/share/doc/nano/fr/rnano.1.html

I truncated the above output, but it continues. See the manpage for snapper to see what each symbol means. You can also do snapper diff in the same way—I'll spare you that one.

To undo the upgrade:

# snapper -c root undochange 1033..1034
create:0 modify:3 delete:100

And nano is now gone, along with all the files it changed:

$ pacman -Qi nano
error: package 'nano' was not found

Troubleshooting

==> ERROR: One or more PGP signatures could not be verified!

See PGP Key on how to import my PGP key.

ERROR: /etc/conf.d/snapper does not exist!

snap-pac reads in the snapper configurations from /etc/conf.d/snapper. It can't do that if the file doesn't exist. I'm not sure what you've done to not have it exist in the default location.

WARNING: No snapper configurations found, so not taking any snapshots!

No snapper configurations were found in /etc/conf.d/snapper. This means you haven't created any configurations yet using snapper create-config. See the snapper manpage on how to do this.

WARNING: No snapper configurations are set up for snapshots to be taken!

Although you seem to have created at least one snapper configuration, none of them are set up for snap-pac's pacman hooks. By default snap-pac will take snapshots for the root configuration and any other configuration which has PACMAN_PRE_POST set to yes in its configuration file. This message means you don't have a snapper configuration named root (or PACMAN_PRE_POST is set to no for it) and no other configuration is set up for snapshots. See the configuration section above.

WARNING: prefile does not exist, so no post snapshot will be taken. If you are initially installing snap-pac, this is normal.

snap-pac saves the pre snapshot's number in a temporary file. Somehow it got removed before the post snapshot could be taken. When you initially install snap-pac the post hook is run, but the pre hook never was, so this message will show up then as well and is safe to ignore in that circumstance.

WARNING: Didn't find pacman running.

The script gets the description from the pacman command that was run. If you get this warning it looks like you may have run the script directly instead of letting it run through pacman's hooks.

ERROR: Unable to use snapper without dbus. Are you in a chroot environment?

Snapper requires dbus. If you chroot into another environment, dbus will not be available, so snapper can't take snapshots. Although snapper can be set up to not need dbus, this is a simple way to check if one is in a chroot and avoid attempting snapshots in it.

"N/A" next to one of the snapper configurations in the hook's output

snap-pac lists all snapper configurations it finds and tells you which ones it took a snapshot of along with the snapshot number. "N/A" means the configuration is not set up for snap-pac, so no snapshot was taken for that snapper configuration. See configuration.

After restoring snapshot from snap-pac, pacman database is locked

The pre/post snaphots are taken while pacman is running, so this is expected. Follow the instructions pacman gives you (e.g., removing the lock file).

Other problems

If you have a problem not listed here, check the open issues and file new issue if your problem is not listed.

FAQ

Does snap-pac backup non-btrfs /boot partitions?

Nope. See here for an example on how to set this up.

License

snap-pac

Copyright (C) 2016 James W. Barnett

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

See also