nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/services/backup/default.nix

@@ -0,0 +1,106 @@
+# Backups using Backblaze B2 and `restic`
+{ config, pkgs, lib, ... }:
+let
+  cfg = config.my.services.backup;
+
+  excludeArg = with builtins; with pkgs; "--exclude-file=" +
+    (writeText "excludes.txt" (concatStringsSep "\n" cfg.exclude));
+in
+{
+  options.my.services.backup = with lib; {
+    enable = mkEnableOption "Enable backups for this host";
+
+    repository = mkOption {
+      type = types.str;
+      example = "/mnt/backup-hdd";
+      description = "The repository to back up to";
+    };
+
+    passwordFile = mkOption {
+      type = types.str;
+      example = "/var/lib/restic/password.txt";
+      description = "Read the repository's password from this path";
+    };
+
+    credentialsFile = mkOption {
+      type = types.str;
+      example = "/var/lib/restic/creds.env";
+      description = ''
+        Credential file as an 'EnvironmentFile' (see `systemd.exec(5)`)
+      '';
+    };
+
+    paths = mkOption {
+      type = with types; listOf str;
+      default = [ ];
+      example = [
+        "/var/lib"
+        "/home"
+      ];
+      description = "Paths to backup";
+    };
+
+    exclude = mkOption {
+      type = with types; listOf str;
+      default = [ ];
+      example = [
+        # very large paths
+        "/var/lib/docker"
+        "/var/lib/systemd"
+        "/var/lib/libvirt"
+
+        # temporary files created by `cargo` and `go build`
+        "**/target"
+        "/home/*/go/bin"
+        "/home/*/go/pkg"
+      ];
+      description = "Paths to exclude from backup";
+    };
+
+    pruneOpts = mkOption {
+      type = with types; listOf str;
+      default = [
+        "--keep-last 10"
+        "--keep-hourly 24"
+        "--keep-daily 7"
+        "--keep-weekly 5"
+        "--keep-monthly 12"
+        "--keep-yearly 100"
+      ];
+      example = [ "--keep-last 5" "--keep-weekly 2" ];
+      description = ''
+        List of options to give to the `forget` subcommand after a backup.
+      '';
+    };
+
+    timerConfig = mkOption {
+      # NOTE: I do not know how to cleanly set the type
+      default = {
+        OnCalendar = "daily";
+      };
+      example = {
+        OnCalendar = "00:05";
+        RandomizedDelaySec = "5h";
+      };
+      description = ''
+        When to run the backup. See man systemd.timer for details.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.restic.backups.backblaze = {
+      # Take care of included and excluded files
+      paths = cfg.paths;
+      extraBackupArgs = [ "--verbose=2" ]
+        ++ lib.optional (builtins.length cfg.exclude != 0) excludeArg
+      ;
+      # Take care of creating the repository if it doesn't exist
+      initialize = true;
+      # give B2 API key securely
+      environmentFile = cfg.credentialsFile;
+
+      inherit (cfg) passwordFile pruneOpts timerConfig repository;
+    };
+  };
+}