systemd: make the unit option type more robust

This should allow more sensible merging behavior. In particular, with
this change it is possible to use, for example, `mkForce` for greater
control of merging.

Fixes #543

modules/misc/news.nix

@@ -1019,6 +1019,36 @@ in
           A new module is available: 'services.xcape'.
         '';
       }
+
+      {
+        time = "2019-04-11T22:50:10+00:00";
+        condition = hostPlatform.isLinux;
+        message = ''
+          The type used for the systemd unit options under
+
+              systemd.user.services, systemd.user.sockets, etc.
+
+          has been changed to offer more robust merging of configurations.
+
+          If you don't override values within systemd units then you are not
+          affected by this change. Unfortunately, if you do override unit values
+          you may encounter errors due to this change.
+
+          In particular, if you get an error saying that a "unique option" is
+          "defined multiple times" then you need to use 'lib.mkForce'. For
+          example,
+
+              systemd.user.services.foo.Service.ExecStart = "/foo/bar";
+
+          becomes
+
+              systemd.user.services.foo.Service.ExecStart = lib.mkForce "/foo/bar";
+
+          We had to make this change because the old merging was causing too
+          many confusing situations for people. Apologies for potentially
+          breaking your configuration!
+        '';
+      }
     ];
   };
 }

modules/systemd.nix

@@ -58,8 +58,13 @@ let
 
   servicesStartTimeoutMs = builtins.toString cfg.servicesStartTimeoutMs;
 
-  attrsRecursivelyMerged = types.attrs // {
-    merge = loc: foldl' (res: def: recursiveUpdate res def.value) {};
+  unitType = unitKind: with types;
+    let
+      primitive = either bool (either int str);
+    in
+      attrsOf (attrsOf (attrsOf (either primitive (listOf primitive))))
+      // {
+        description = "systemd ${unitKind} unit configuration";
       };
 
   unitDescription = type: ''
@@ -78,6 +83,7 @@ let
     {
       Unit = {
         Description = "Example description";
+        Documentation = [ "man:example(1)" "man:example(5)" ];
       };
 
       ${type} = {
@@ -113,35 +119,35 @@ in
 
       services = mkOption {
         default = {};
-        type = attrsRecursivelyMerged;
+        type = unitType "service";
         description = unitDescription "service";
         example = unitExample "Service";
       };
 
       sockets = mkOption {
         default = {};
-        type = attrsRecursivelyMerged;
+        type = unitType "socket";
         description = unitDescription "socket";
         example = unitExample "Socket";
       };
 
       targets = mkOption {
         default = {};
-        type = attrsRecursivelyMerged;
+        type = unitType "target";
         description = unitDescription "target";
         example = unitExample "Target";
       };
 
       timers = mkOption {
         default = {};
-        type = attrsRecursivelyMerged;
+        type = unitType "timer";
         description = unitDescription "timer";
         example = unitExample "Timer";
       };
 
       paths = mkOption {
         default = {};
-        type = attrsRecursivelyMerged;
+        type = unitType "path";
         description = unitDescription "path";
         example = unitExample "Path";
       };