Tarsnap: add symlink options and a restore service #37897
Conversation
A new option `explicitSymlinks` will set `-H` when creating an archive. This option makes tarsnap follow any symlinks specified explicitly on the commandline, but not any found inside the file tree. A new option `followSymlinks` will set `-L` when creating an archive. This option makes tarsnap follow any symlinks found anywhere in the file tree instead of storing them as-is.
This service will never run automatically, but it encapsulates the necessary logic and configuration to run a restore of the latest archive, and allows to hook more specific logic, such as loading a database dump, via `postStart`.
mkaito
force-pushed
the
tarsnap-symlinks-and-restore
branch
from
4121dcc
to
bde525a
Compare
| description = "Tarsnap restore '${name}'"; | ||
| requires = [ "network-online.target" ]; | ||
|
|
||
| path = [ pkgs.iputils pkgs.tarsnap pkgs.utillinux ]; |
There was a problem hiding this comment.
Nitpick: path = with pkgs; [ iputils tarsnap utillinux ];
|
|
||
| path = [ pkgs.iputils pkgs.tarsnap pkgs.utillinux ]; | ||
|
|
||
| ## |
| ''; | ||
|
|
||
| script = | ||
| let |
There was a problem hiding this comment.
Nitpick: seemingly, indentation is off.
script = let
...
in
|
|
||
| ## | ||
| preStart = '' | ||
| while ! ping -q -c 1 v1-0-0-server.tarsnap.com &> /dev/null; do sleep 3; done |
There was a problem hiding this comment.
Nitpick: should probably be split to multiple lines to fit 80 chars.
There was a problem hiding this comment.
This sits at 88 characters, but only because of indentation. I think leaving it as-is is more readable than splitting it.
| mkdir -p ${cfg.cachedir} | ||
| chmod 0700 ${cfg.cachedir} | ||
|
|
||
| ( flock 9 |
There was a problem hiding this comment.
By the way, why one has to lock and write to files in cache dir? Seems like running tarsnap --fsck should be enough.
There was a problem hiding this comment.
As this module generates one service per archive, all with the same period, they all fire at the same time. The flock dance prevents locking issues while --fsck runs.
I have my doubts about the double flock just to have the unit abort, when you can presumably achieve the same thing with flock -n, but I did not write the original part, and it has no comments to the respect, so I'm unwilling to touch.
The part was introduced in this commit two years ago by @abbradar. Perhaps he is willing to comment on this.
There was a problem hiding this comment.
I've traced back my email exchange with tarsnap developers and my possible thought process, which runs like this:
- Lock
lockfwhich is a locktarsnapalso uses; - Check if
firstrunexists. This is our own lock that is used to force systemd services to order themselves; - If it doesn't exist create and lock it; we need to release our lock on
lockffortarsnapto do its job; - When it's done we relock
lockf(I likely did this for consistency -- it shouldn't be needed);
Then we perform the operation itself, locking firstrun in process.
While tarsnap --fsck still starts lockf isn't locked; that's why we keep firstrun locked to avoid a race condition when other operation can start before it. lockf itself is locked so that we can check if firstrun exists to decide if --fsck is needed. Hence double locking -- this should have been commented in the code.
Perhaps this can be simplified somehow?
EDIT: First message was inconsistent babble because I was thinking while writing :D
There was a problem hiding this comment.
To answer @mkaito -- this locking doesn't abort units, it properly orders them instead. I don't think it's a good idea to make a backup unit abort if another one is already running.
|
Evaluates with |
Motivation for this change
Improvements to the tarsnap module
Things done
The options are:
exclusiveSymlinkswill set-H.followSymlinkswill set-L.The restore service is a mirror of the archive service, but does not ever run
automatically. It serves as a known interface to restore the last archive
(
systemctl start tarsnap-restore-foo), as well as providing a hook viapostStartto run service-specific post-restore logic, such as loadinga database dump.
build-use-sandboxinnix.confon non-NixOS)nix-shell -p nox --run "nox-review wip"./result/bin/)