{ config, lib, pkgs, ... }:

with lib;

let

  inherit (pkgs) mysql gzip;

  cfg = config.services.mysqlBackup;
  defaultUser = "mysqlbackup";

  backupScript = ''
    set -o pipefail
    failed=""
    ${concatMapStringsSep "\n" backupDatabaseScript cfg.databases}
    if [ -n "$failed" ]; then
      echo "Backup of database(s) failed:$failed"
      exit 1
    fi
  '';
  backupDatabaseScript = db: ''
    dest="${cfg.location}/${db}.gz"
    if ${mysql}/bin/mysqldump ${if cfg.singleTransaction then "--single-transaction" else ""} ${db} | ${gzip}/bin/gzip -c > $dest.tmp; then
      mv $dest.tmp $dest
      echo "Backed up to $dest"
    else
      echo "Failed to back up to $dest"
      rm -f $dest.tmp
      failed="$failed ${db}"
    fi
  '';

in

{
  options = {

    services.mysqlBackup = {

      enable = mkOption {
        default = false;
        description = ''
          Whether to enable MySQL backups.
        '';
      };

      calendar = mkOption {
        type = types.str;
        default = "01:15:00";
        description = ''
          Configured when to run the backup service systemd unit (DayOfWeek Year-Month-Day Hour:Minute:Second).
        '';
      };

      user = mkOption {
        default = defaultUser;
        description = ''
          User to be used to perform backup.
        '';
      };

      databases = mkOption {
        default = [];
        description = ''
          List of database names to dump.
        '';
      };

      location = mkOption {
        default = "/var/backup/mysql";
        description = ''
          Location to put the gzipped MySQL database dumps.
        '';
      };

      singleTransaction = mkOption {
        default = false;
        description = ''
          Whether to create database dump in a single transaction
        '';
      };
    };

  };

  config = mkIf cfg.enable {
    users.users = optionalAttrs (cfg.user == defaultUser) (singleton
      { name = defaultUser;
        isSystemUser = true;
        createHome = false;
        home = cfg.location;
        group = "nogroup";
      });

    services.mysql.ensureUsers = [{
      name = cfg.user;
      ensurePermissions = with lib;
        let
          privs = "SELECT, SHOW VIEW, TRIGGER, LOCK TABLES";
          grant = db: nameValuePair "${db}.*" privs;
        in
          listToAttrs (map grant cfg.databases);
    }];

    systemd = {
      timers."mysql-backup" = {
        description = "Mysql backup timer";
        wantedBy = [ "timers.target" ];
        timerConfig = {
          OnCalendar = cfg.calendar;
          AccuracySec = "5m";
          Unit = "mysql-backup.service";
        };
      };
      services."mysql-backup" = {
        description = "Mysql backup service";
        enable = true;
        serviceConfig = {
          User = cfg.user;
          PermissionsStartOnly = true;
        };
        preStart = ''
          mkdir -m 0700 -p ${cfg.location}
          chown -R ${cfg.user} ${cfg.location}
        '';
        script = backupScript;
      };
    };
  };

}