nixos/bees: use the upstream service file

Instead of duplicating the options from the upstream service file and letting
them get out of sync, use the file directly and only configure the needed
overrides. In particular, the upstream improvements include the mounts not
being globally visible any more, so they can't be used for bypassing nosuid and
the like, and the custom cleanup script that performed the unmount becomes
unnecessary.

nixos/modules/services/misc/bees.nix

···
       '';
     };
   };
-  config = {
+  config = lib.mkIf (cfg.filesystems != { }) {
+    systemd.packages = [ pkgs.bees ];
     systemd.services = lib.mapAttrs' (
       name: fs:
       lib.nameValuePair "beesd@${name}" {
-        description = "Block-level BTRFS deduplication for %i";
-        after = [ "sysinit.target" ];
-
-        serviceConfig =
-          let
-            configOpts = [
-              fs.spec
-              "verbosity=${toString fs.verbosity}"
-              "idxSizeMB=${toString fs.hashTableSizeMB}"
-              "workDir=${fs.workDir}"
+        overrideStrategy = "asDropin";
+        serviceConfig = {
+          ExecStart =
+            let
+              configOpts = [
+                fs.spec
+                "verbosity=${toString fs.verbosity}"
+                "idxSizeMB=${toString fs.hashTableSizeMB}"
+                "workDir=${fs.workDir}"
+              ];
+              configOptsStr = lib.escapeShellArgs configOpts;
+            in
+            [
+              ""
+              "${pkgs.bees}/bin/bees-service-wrapper run ${configOptsStr} -- --no-timestamps ${lib.escapeShellArgs fs.extraOptions}"
             ];
-            configOptsStr = lib.escapeShellArgs configOpts;
-          in
-          {
-            # Values from https://github.com/Zygo/bees/blob/v0.6.5/scripts/beesd@.service.in
-            ExecStart = "${pkgs.bees}/bin/bees-service-wrapper run ${configOptsStr} -- --no-timestamps ${lib.escapeShellArgs fs.extraOptions}";
-            ExecStopPost = "${pkgs.bees}/bin/bees-service-wrapper cleanup ${configOptsStr}";
-            CPUAccounting = true;
-            CPUSchedulingPolicy = "batch";
-            CPUWeight = 12;
-            IOSchedulingClass = "idle";
-            IOSchedulingPriority = 7;
-            IOWeight = 10;
-            KillMode = "control-group";
-            KillSignal = "SIGTERM";
-            MemoryAccounting = true;
-            Nice = 19;
-            Restart = "on-abnormal";
-            StartupCPUWeight = 25;
-            StartupIOWeight = 25;
-            SyslogIdentifier = "beesd"; # would otherwise be "bees-service-wrapper"
-          };
+          SyslogIdentifier = "beesd"; # would otherwise be "bees-service-wrapper"
+        };
         unitConfig.RequiresMountsFor = lib.mkIf (lib.hasPrefix "/" fs.spec) fs.spec;
         wantedBy = [ "multi-user.target" ];
       }