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nixos/restic: correct location of cache directory #99579

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@eliasp eliasp commented Oct 4, 2020

Motivation for this change

By default, restic determines the location of the cache based on the XDG
base dir specification, which is ~/.cache/restic when the environment
variable $XDG_CACHE_HOME isn't set.
As restic is executed as root by default, this resulted in the cache being
written to /root/.cache/restic, which is not quite right for a system
service and also meant, multiple backup services would use the same cache
directory - potentially causing issues with locking, data corruption,
etc.

The goal was to ensure, restic uses the correct cache location for a
system service - one cache per backup specification, using /var/cache
as the base directory for it.

systemd sets the environment variable $CACHE_DIRECTORY once
CacheDirectory= is defined, but restic doesn't change its behavior
based on the presence of this environment variable.
Instead, the specifier [1] %C can be used to point restic explicitly
towards the correct cache location using the --cache-dir argument.

Furthermore, the CacheDirectoryMode= was set to 0700, as the default
of 0755 is far too open in this case, as the cache might contain
sensitive data.

[1] https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Specifiers

Things done
  • Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
  • Built on platform(s)
    • NixOS
    • macOS
    • other Linux distributions
  • Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • Tested execution of all binary files (usually in ./result/bin/)
  • Determined the impact on package closure size (by running nix path-info -S before and after)
  • Ensured that relevant documentation is up to date
  • Fits CONTRIBUTING.md.
By default, restic determines the location of the cache based on the XDG
base dir specification, which is `~/.cache/restic` when the environment
variable `$XDG_CACHE_HOME` isn't set.
As restic is executed as root by default, this resulted in the cache being
written to `/root/.cache/restic`, which is not quite right for a system
service and also meant, multiple backup services would use the same cache
directory - potentially causing issues with locking, data corruption,
etc.

The goal was to ensure, restic uses the correct cache location for a
system service - one cache per backup specification, using `/var/cache`
as the base directory for it.

systemd sets the environment variable `$CACHE_DIRECTORY` once
`CacheDirectory=` is defined, but restic doesn't change its behavior
based on the presence of this environment variable.
Instead, the specifier [1] `%C` can be used to point restic explicitly
towards the correct cache location using the `--cache-dir` argument.

Furthermore, the `CacheDirectoryMode=` was set to `0700`, as the default
of `0755` is far too open in this case, as the cache might contain
sensitive data.

[1] https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Specifiers
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