Added syncoid service

Sets up ZFS replication. Syncoid modules handles configuration for both
the remote and the target but I'm thinking I should split this up in the
future.

modules/services/syncoid/default.nix

@@ -0,0 +1,80 @@
+{
+  options,
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+with lib; let
+  cfg = config.aa.services.syncoid;
+in {
+  options.aa.services.syncoid = with types; {
+    enable = mkEnableOption "syncoid (ZFS snap replication)";
+    commands = mkOption {
+      type = attrs;
+      default = {};
+      description = "Commands to pass directly to syncoid, see `services.syncoid.commands`";
+    };
+    remoteTargetUser = mkOption {
+      type = str;
+      default = "";
+      description = "The user to use on the target machine.";
+    };
+    remoteTargetDatasets = mkOption {
+      type = listOf str;
+      default = [];
+      description = "Datasets to be used as a remote target (e.g. a NAS's backups dataset)";
+    };
+    remoteTargetPublicKeys = mkOption {
+      type = listOf str;
+      default = [];
+      description = "SSH public keys that the syncoid service's user should trust";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    services.syncoid = {
+      enable = true;
+      commands = mkAliasDefinitions options.aa.services.syncoid.commands;
+    };
+
+    environment.systemPackages = mkIf (cfg.remoteTargetUser != "") [
+      pkgs.lzop
+      pkgs.mbuffer
+    ];
+
+    users = mkIf (cfg.remoteTargetUser != "") {
+      users."${cfg.remoteTargetUser}" = {
+        shell = pkgs.bashInteractive;
+        group = cfg.remoteTargetUser;
+        isSystemUser = true;
+        home = "/var/lib/${cfg.remoteTargetUser}";
+        createHome = true;
+        openssh.authorizedKeys.keys = cfg.remoteTargetPublicKeys;
+      };
+      groups."${cfg.remoteTargetUser}" = {};
+    };
+
+    systemd.services.setup-syncoid-remote = {
+      description = "Permission setup for syncoid remote targets";
+      documentation = ["https://github.com/jimsalterjrs/sanoid/wiki/Syncoid#running-without-root"];
+      wantedBy = ["multi-user.target"];
+      path = [pkgs.zfs];
+
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = "yes";
+      };
+
+      script = ''
+        DATASETS=(${toString cfg.remoteTargetDatasets})
+              for dataset in "''${DATASETS[@]}"; do
+                zfs allow \
+              	-u ${cfg.remoteTargetUser} \
+              	compression,mountpoint,create,mount,receive,rollback,destroy \
+              	"$dataset"
+              done
+      '';
+    };
+  };
+}

modules/system/zfs/default.nix

@@ -16,7 +16,12 @@ in {
   config = mkIf cfg.enable {
     services.zfs = {
       autoScrub.enable = true;
-      autoSnapshot.enable = true;
+      # Still need to set `com.sun:auto-snapshot` to `true` on datasets
+      # zfs set com.sun:auto-snapshot=true pool/dataset
+      autoSnapshot = {
+        enable = true;
+        flags = "-k -p --utc";
+      };
     };
   };
 }