When using `lib.optionals`, the return value of both branches of the
condition get set as a value to the option.
When using `lib.mkIf`, only the positive condition gets set as a value
to the option.
This small distinction is important when dealing with precedence. For
example here, we wanted to set a boot.grub.devices default value with
lib.mkDefault, and that was getting overridden with the empty value of
`lib.optional (cfg.device != "") cfg.device`.
See https://github.com/nix-community/srvos/pull/491#discussion_r1738827651
The general conclusion is that using `lib.mkIf` is preferable to
`lib.optional` or `lib.optionals` when setting values in the NixOS
module system.
An ancient comment says to unset this variable after 16.03. Considering
we've just gotten past 24.05, I think it's safe to remove this finally.
Tests still pass after this change.
The current kubernetes module only allows you to set a single DNS
resolver for the kubelet. Historically, this has not mattered as the
value was passed to a cli argument as a string and as per the kubelet's
configuration parsing mechanism, multiple values could be passed as a
comma-delimited string. However, recently, the module was refactored to
make configure kubernetes components via configuration files rather than
the deprecated command-line arguments. These files more strongly-typed
than CLI arguments and to pass multiple values, one must define a list
in the file. When this change was made, an incorrect assumption was made
that only a single DNS server could be specified and forced a
single-item list into this configuration file. We need to introduce a
breaking change to the module in order to allow the user to supply their
own list with however many dns resolvers they wish to use.
Every now and then, kubernetes adds new configuration parameters to the
kubelet configuration. Since this is defined using a nix attrset which
is then converted to json/yaml, it would be nice to have an escape hatch
similar to the extraOpts one that exists for additional CLI arguments.
The typical use case would be to configure new settings before they are
officially supported in the nixos module.
