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

let
  cfg = config.nfsServicesStandby;
in
{
  options.nfsServicesStandby = {
    enable = lib.mkEnableOption "NFS services standby" // { default = true; };

    replicationKeys = lib.mkOption {
      type = lib.types.listOf lib.types.str;
      default = [];
      description = ''
        SSH public keys authorized to replicate btrfs snapshots to this standby.
        These keys are restricted to only run 'btrfs receive /persist/services-standby'.

        Get the public key from the NFS server:
          ssh <nfs-server> sudo cat /persist/root/.ssh/btrfs-replication.pub
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    # Allow root SSH login for replication (restricted by command= in authorized_keys)
    # This is configured in common/sshd.nix

    # Restricted SSH keys for btrfs replication
    users.users.root.openssh.authorizedKeys.keys =
      map (key: ''command="btrfs receive /persist/services-standby",restrict ${key}'') cfg.replicationKeys;

    # Mount point for services-standby subvolume
    # This is just declarative documentation - the subvolume must be created manually once:
    #   sudo btrfs subvolume create /persist/services-standby
    # After that, it will persist across reboots (it's under /persist)
    fileSystems."/persist/services-standby" = {
      device = "/persist/services-standby";
      fsType = "none";
      options = [ "bind" ];
      noCheck = true;
    };

    # Cleanup old snapshots on standby (keep last 4 hours for HA failover)
    systemd.services.cleanup-services-standby-snapshots = {
      description = "Cleanup old btrfs snapshots in services-standby";
      path = [ pkgs.btrfs-progs pkgs.findutils ];

      script = ''
        set -euo pipefail
        # Keep last 4 hours of snapshots (48 snapshots at 5min intervals)
        find /persist/services-standby -maxdepth 1 -name 'services@*' -mmin +240 -exec btrfs subvolume delete {} \; || true
      '';

      serviceConfig = {
        Type = "oneshot";
        User = "root";
      };
    };

    systemd.timers.cleanup-services-standby-snapshots = {
      description = "Timer for cleaning up old snapshots on standby";
      wantedBy = [ "timers.target" ];
      timerConfig = {
        OnCalendar = "daily";
        Persistent = true;
      };
    };
  };
}