Fixes issues described in #208242 for this part of the nixpkgs tree.
There are no behavioral changes in this, it only adjusts the code so
that it is easier to understand.
these changes were generated with nixq 0.0.2, by running
nixq ">> lib.mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
nixq ">> mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
nixq ">> Inherit >> mdDoc[remove]" --batchmode nixos/**.nix
two mentions of the mdDoc function remain in nixos/, both of which
are inside of comments.
Since lib.mdDoc is already defined as just id, this commit is a no-op as
far as Nix (and the built manual) is concerned.
Otherwise, `postfix_up{path="/var/lib/postfix/queue/public/showq"}` will
always be `0` indicating an postfix outage because this is a unix domain
socket that cannot be connected to:
2021/12/03 14:50:46 Failed to scrape showq socket: dial unix /var/lib/postfix/queue/public/showq: socket: address family not supported by protocol
The postfix exporter needs to access postfix's `queue/public/` directory
to read the `showq` socket inside. Instead of making the public
directory world accessible, this sets the postfix exporter's group to
`postdrop` by default, when the postfix service is enabled.
