{ config, pkgs, ... }:

{
  # https://nixos.wiki/wiki/Docker#Installation

  # TODO: rootless Docker. https://nixos.wiki/wiki/Docker#Rootless_docker
  # TODO: run as systemd services. https://nixos.wiki/wiki/Docker#docker_containers_as_systemd_services

  virtualisation = {
    docker = {
      enable = true;

      # As we use btrfs, we enable the according storageDriver option.
      storageDriver = "btrfs";

      # Run `docker system prune -f` every week.
      autoPrune.enable = true;
      autoPrune.dates = "weekly";
      # https://docs.docker.com/engine/reference/commandline/system_prune/#options
      autoPrune.flags = [
        "--all"
        "--volumes"
        # https://docs.docker.com/engine/reference/commandline/system_prune/#filter
        # https://pkg.go.dev/maze.io/x/duration#ParseDuration
        "--filter until=7d"
      ];
    };
  };

  # Monitor unhealthy Docker containers.
  systemd.timers."docker-health" = {
    wantedBy = [ "timers.target" ];
    partOf = [ "docker-health.service" ];
    timerConfig = {
      OnBootSec = "0m";
      OnUnitInactiveSec = "3m";

      AccuracySec = "15s";
      RandomizedDelaySec = "15s";
    };
  };
  systemd.services."docker-health" = {
    serviceConfig = {
      Type = "oneshot";
      PrivateTmp = true;
      # `docker` requires root access.
      User = "root";
      Nice = 19;
      IOSchedulingClass = "idle";
    };
    path = with pkgs; [
      docker
    ];
    # If there are no unhealthy Docker containers, the output of `docker ps -f health=unhealthy` is just one line:
    #   CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
    # We filter this line with `grep -v`.
    # As a result, grep returns exit code 1 if there are no unhealthy containers (as not a single line is printed).
    # Thus, we prefix the whole command with `!`.
    # Lastly, we redirect stdout to stderr with `1>&2` so that unhealthy containers are written to stderr.
    script = ''
      set -eu -o pipefail
      ! sudo docker ps -f health=unhealthy | grep -v 'CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES' 1>&2
    '';
  };
}