modules/consul-service-registry: fix bugs, add external registration mode

modules/consul-service-registry/default.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 
@@ -7,28 +7,51 @@ let
 
   consul = "${config.services.consul.package}/bin/consul";
 
-  consulServiceDefinition = submodule ({ name, ... }: {
+  consulServiceDefinition = types.submodule ({ name, ... }: {
+    options = {
       unit = mkOption {
         description = "Which systemd service to attach to.";
         default = name;
         type = types.str;
       };
+      mode = mkOption {
+        description = "How to attach command executions to the service.";
+        type = types.enum [ "direct" "external" ];
+        default = "direct";
+      };
       definition = mkOption {
         description = "Consul service definition.";
         type = types.attrs;
       };
+    };
   });
 
   attachToService = name: conf: let
     serviceJson = pkgs.writeText "consul-service-${name}.json" (builtins.toJSON conf.definition);
   in {
-    name = conf.unit;
+    name = {
+      direct = conf.unit;
+      external = "register-consul-svc-${conf.unit}";
+    }.${conf.mode};
     value = {
+      direct = {
         serviceConfig = {
           ExecStartPost = "${consul} services register ${serviceJson}";
           ExecStopPre = "${consul} services deregister ${serviceJson}";
         };
       };
+      external = {
+        after = [ "${conf.unit}.service" ];
+        wantedBy = [ "${conf.unit}.service" ];
+        unitConfig.BindsTo = "${conf.unit}.service";
+        serviceConfig = {
+          Type = "oneshot";
+          RemainAfterExit = true;
+          ExecStart = "${consul} services register ${serviceJson}";
+          ExecStop = "${consul} services deregister ${serviceJson}";
+        };
+      };
+    }.${conf.mode};
   };
 in