feat: make paperless have only one master node to enable syncing

modules/paperless/default.nix

@@ -32,7 +32,7 @@ in {
       description = "Directory to save data in";
       type = types.str;
       example = "/home/pub_solar/Paperless";
-      default = "/home/${psCfg.user.name}/Paperless";
+      default = "${xdg.dataHome}/Paperless";
     };
 
     consumptionDir = mkOption {
@@ -42,6 +42,23 @@ in {
       default = "/var/lib/paperless/consume";
     };
 
+    sync = {
+      enable = mkEnableOption ''
+          You can use this option to sync several paperless instances, for example via nextcloud.
+          It will sync the media directory and database, automatically merging sqlite dbs via dump and import.
+          Logs, the classification model, and other files are left unsynced.
+        '';
+
+      masterNode = mkEnableOption "If this node is the master node, it will only export paperless data, otherwise it will only import";
+
+      directory = mkOption {
+        description = "Directory to sync with.";
+        type = types.str;
+        example = "/home/pub_solar/Nextcloud/Paperless";
+        default = "/home/${psCfg.user.name}/Paperless";
+      };
+    };
+
     scannerDefaultDevice = mkOption {
       description = ''
         The scanner device. To find this, use `scanimage -L`.
@@ -91,5 +108,41 @@ in {
         SCANNER_OUTPUT_DIR = cfg.consumptionDir;
       };
     };
+
+    systemd = let
+      copy-in = pkgs.writeShellScriptBin "copy-in" ''
+        ${pkgs.systemd}/bin/systemctl stop paperless-web.service paperless-task-queue.service paperless-scheduler.service paperless-consumer.service
+        cp -r ${cfg.sync.directory}/media ${cfg.dataDir}/media
+        cp ${cfg.sync.directory}/db.sqlite3 ${cfg.dataDir}/db.sqlite3
+        ${pkgs.systemd}/bin/systemctl start paperless-web.service paperless-task-queue.service paperless-scheduler.service paperless-consumer.service
+      '';
+      copy-out = pkgs.writeShellScriptBin "copy-out" ''
+        ${pkgs.systemd}/bin/systemctl stop paperless-web.service paperless-task-queue.service paperless-scheduler.service paperless-consumer.service
+        cp -r ${cfg.dataDir}/media ${cfg.sync.directory}/media
+        cp ${cfg.dataDir}/db.sqlite3 ${cfg.sync.directory}/db.sqlite3
+        ${pkgs.systemd}/bin/systemctl start paperless-web.service paperless-task-queue.service paperless-scheduler.service paperless-consumer.service
+      '';
+    in mkIf cfg.sync.enable {
+      services.nextcloud-paperless-autosync = {
+        unitConfig = {
+          Description = "Auto sync paperless to or from Nextcloud";
+          After = "network-online.target";
+        };
+        serviceConfig = {
+          Type = "simple";
+          ExecStart= if cfg.sync.masterNode then "${copy-out}/bin/copy-out" else "${copy-in}/bin/copy-in";
+          TimeoutStopSec = "180";
+          KillMode = "process";
+          KillSignal = "SIGINT";
+        };
+        wantedBy = ["multi-user.target"];
+      };
+
+      timers.nextcloud-paperless-autosync = {
+        unitConfig.Description = "Automatic sync files with Nextcloud when booted up after 5 minutes then rerun every 30 minutes";
+        timerConfig.OnUnitActiveSec = "30min";
+        wantedBy = ["multi-user.target" "timers.target"];
+      };
+    };
   };
 }