# Grafana Server
#
# Scope: Beautiful plots of time series data retrieved from Prometheus
# See https://christine.website/blog/prometheus-grafana-loki-nixos-2020-11-20

{ config, lib, ... }:

let
  cfg = config.services.private-storage.monitoring.grafana;
  grafanaAuth = if (cfg.googleOAuthClientID == "") then {
                  anonymous.enable = true;
                } else {
                  google.enable = true;
                  # Grafana considers it "sign up" to let in a user it has
                  # never seen before.
                  google.allowSignUp = true;
                  google.clientSecretFile = cfg.googleOAuthClientSecretFile;
                  google.clientId = cfg.googleOAuthClientID;
                };

in {
  options.services.private-storage.monitoring.grafana = {
    domains = lib.mkOption
    { type = lib.types.listOf lib.types.str;
      example = [ "grafana.grid.private.storage" ];
      description = "The domain names at which the server is reachable.";
    };
    prometheusUrl = lib.mkOption
    { type = lib.types.str;
      example = "http://prometheus:9090/";
      default = "http://localhost:9090/";
      description = "The URL of the Prometheus host to access";
    };
    lokiUrl = lib.mkOption
    { type = lib.types.str;
      example = "http://loki:3100/";
      default = "http://localhost:3100/";
      description = "The URL of the Loki host to access";
    };
    letsEncryptAdminEmail = lib.mkOption
    { type = lib.types.str;
      description = ''
        An email address to give to Let's Encrypt as an
        operational contact for the service's TLS certificate.
      '';
    };
    googleOAuthClientID = lib.mkOption
    { type = lib.types.str;
      example = "grafana-staging-345678";
      default = "replace-by-your-client-id-or-set-empty-string-for-anonymous-access";
      description = "The GSuite OAuth2 SSO Client ID.  Empty string turns SSO auth off and anonymous (free for all) access on.";
    };
    googleOAuthClientSecretFile = lib.mkOption
    { type = lib.types.path;
      example = /var/secret/monitoring-gsuite-client-secret;
      default = /run/keys/grafana-google-sso.secret;
      description = "The path to the GSuite SSO secret file.";
    };
    adminPasswordFile = lib.mkOption
    { type = lib.types.path;
      example = "/var/secret/monitoring-admin-password";
      default = /run/keys/grafana-admin.password;
      description = "A file containing the password for the Grafana Admin account.";
    };
    enableSlackAlert = lib.mkOption
    { type = lib.types.bool;
      default = false;
      description = ''
        Enables the slack alerter. Expects a file that contains
        the secret Slack Web Hook URL in grafanaSlackUrlFile (see below).
      '';
    };
    grafanaSlackUrlFile = lib.mkOption
    { type = lib.types.path;
      default = /run/keys/grafana-slack-url;
      description = ''
        Where to find the file that containts the slack URL.
      '';
    };
  };

  config =
    let
      # We'll refer to this collection of domains by the first domain in the list.
      domain = builtins.head cfg.domains;

    in {
    # Port 80 for ACME ssl retrieval only. 443 for nginx -> grafana.
    networking.firewall.allowedTCPPorts = [ 80 443 ];

    services.grafana = {
      enable = true;
      inherit domain; 
      port = 2342;
      addr = "127.0.0.1";

      # No phoning home
      analytics.reporting.enable = false;

      # Force Grafana to believe it is reachable via https on the default port
      # number because that's where the nginx that forwards traffic to it is
      # listening.  Grafana's own server listens on an internal address that
      # doesn't matter to anyone except our nginx instance.
      rootUrl = "https://%(domain)s/";

      extraOptions = {
        # Defend against DNS rebinding attacks.
        SERVER_ENFORCE_DOMAIN = "true";
        # Same time zone for all users by default
        DATE_FORMATS_DEFAULT_TIMEZONE = "UTC";
      };

      auth = {
        anonymous.org_role = "Admin";
        anonymous.org_name = "Main Org.";
      } // grafanaAuth;

      # Give users that come through GSuite SSO the highest possible privileges:
      users.autoAssignOrgRole = "Editor";

      # Read the admin password from a file in our secrets folder:
      security.adminPasswordFile = cfg.adminPasswordFile;

      provision = {
        enable = true;
        # See https://grafana.com/docs/grafana/latest/administration/provisioning/#datasources
        datasources = [{
          name = "Prometheus";
          type = "prometheus";
          access = "proxy";
          url = cfg.prometheusUrl;
          isDefault = true;
        } {
          name = "Loki";
          type = "loki";
          access = "proxy";
          url = cfg.lokiUrl;
        }];
        # See https://grafana.com/docs/grafana/latest/administration/provisioning/#dashboards
        dashboards = [{
          name = "provisioned";
          options.path = ./grafana-dashboards;
        }];
        # See https://grafana.com/docs/grafana/latest/administration/provisioning/#example-alert-notification-channels-config-file
        notifiers = [ ] ++ (lib.optionals (cfg.enableSlackAlert) [{
          uid = "slack-notifier-1";
          name = "Slack";
          type = "slack";
          is_default = true;
          send_reminder = false;
          settings = {
            username = "${domain}";
            uploadImage = true;
          };
          secure_settings = {
            # `$__file{}` reads the value from the named file.
            # See https://grafana.com/docs/grafana/latest/administration/configuration/#file-provider
            url = "$__file{${toString cfg.grafanaSlackUrlFile}}";
          };
        }]);
      };
    };

    # nginx reverse proxy
    security.acme.email = cfg.letsEncryptAdminEmail;
    security.acme.acceptTerms = true;
    services.nginx = {
      enable = true;

      recommendedGzipSettings = true;
      recommendedOptimisation = true;
      recommendedProxySettings = true;
      recommendedTlsSettings = true;

      # Only allow PFS-enabled ciphers with AES256:
      sslCiphers = "AES256+EECDH:AES256+EDH:!aNULL";

      virtualHosts."${domain}" = {
        serverAliases = builtins.tail cfg.domains;
        enableACME = true;
        forceSSL = true;
        locations."/" = {
          proxyPass = "http://127.0.0.1:${toString config.services.grafana.port}";
          proxyWebsockets = true;
        };
        locations."/metrics" = {
          # Only allow our monitoringvpn subnet
          # And localhost since we're the monitoring server currently
          extraConfig = ''
            allow ${config.grid.monitoringvpnIPv4}/24;
            allow 127.0.0.1;
            allow ::1;
            deny all;
          '';
          proxyPass = "http://127.0.0.1:${toString config.services.grafana.port}";
        };
      };
    };

    # Let Grafana read from keys, if necessary.
    users.users.grafana.extraGroups = [ "keys" ];
  };
}