swap-sentinel.nix

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

let
  cfg = config.services.swap-sentinel;
  # Naming it 'swap-sentinel' to ensure uniqueness.
  storagePath = "/var/lib/swap-sentinel";
in {
  options.services.swap-sentinel = {
    enable = lib.mkEnableOption "Dynamic Btrfs swapfile manager";
    minFreeMB = lib.mkOption {
      type = lib.types.int;
      default = 512;
      description = "Threshold of free swap in MB before creating a new file.";
    };
    chunkSizeGB = lib.mkOption {
      type = lib.types.int;
      default = 1;
      description = "Size of each dynamic swap chunk.";
    };
  };

  config = lib.mkIf cfg.enable {
    systemd.services.swap-sentinel = {
      description = "Dynamic Swap Sentinel for Btrfs";
      after = [ "local-fs.target" ];
      wantedBy = [ "multi-user.target" ];

      path = with pkgs; [ 
        bash 
        gawk 
        procps 
        util-linux 
        coreutils 
        btrfs-progs 
      ];

      serviceConfig = {
        Type = "simple";
        ExecStartPre = "${pkgs.bash}/bin/bash -c 'mkdir -p ${storagePath} && chattr +C ${storagePath} || true'";
        ExecStart = let
          script = pkgs.writeShellScript "swap-sentinel-run" ''
            set -euo pipefail
            
            CHUNK_SIZE_MB=$(( ${toString cfg.chunkSizeGB} * 1024 ))
            THRESHOLD=${toString cfg.minFreeMB}
            
            echo "Sentinel started. Monitoring swap on ${storagePath}..."

            while true; do
              # Get current free swap in MB
              FREE_SWAP=$(free -m | awk '/^Swap:/ {print $4}')
              
              if [ "$FREE_SWAP" -lt "$THRESHOLD" ]; then
                TIMESTAMP=$(date +%s)
                SWAPFILE="${storagePath}/extra-$TIMESTAMP.swap"
                
                echo "Low swap ($FREE_SWAP MB). Creating $SWAPFILE..."
                
                # Btrfs specific swapfile creation
                truncate -s 0 "$SWAPFILE"
                chattr +C "$SWAPFILE"
                fallocate -l "$(( ${toString cfg.chunkSizeGB} ))G" "$SWAPFILE"
                chmod 600 "$SWAPFILE"
                mkswap "$SWAPFILE"
                swapon "$SWAPFILE"
              
              elif [ "$FREE_SWAP" -gt "$(( CHUNK_SIZE_MB + THRESHOLD + 512 ))" ]; then
                # Check if we have dynamic files to remove (ignore the 16GB hibernation file)
                OLDEST_DYNAMIC=$(ls -t ${storagePath}/extra-*.swap 2>/dev/null | tail -n 1 || true)
                
                if [ -n "$OLDEST_DYNAMIC" ]; then
                  echo "High swap headroom ($FREE_SWAP MB). Removing $OLDEST_DYNAMIC..."
                  swapoff "$OLDEST_DYNAMIC"
                  rm "$OLDEST_DYNAMIC"
                fi
              fi
              
              sleep 30
            done
          '';
        in "${script}";
        
        Restart = "always";
      };
    };
  };
}