modules/programs: enforce that user services don't accidentally override PATH

hosts/common/programs/ntfy-sh.nix

@@ -25,7 +25,6 @@ in
     services.ntfy-sub = {
       description = "listen for push-notifications";
       wantedBy = lib.mkIf cfg.config.autostart [ "default.target" ];
-      path = [ cfg.package ];
       script = ''
         topic=$(cat ~/.config/ntfy-sh/topic)
         ntfy sub "https://ntfy.uninsane.org:2587/$topic"

hosts/common/programs/wob/default.nix

@@ -67,7 +67,6 @@ in
         Restart = "always";
         RestartSec = "20s";
       };
-      path = [ cfg.package ];
       script = ''
         wobsock="$XDG_RUNTIME_DIR/${cfg.config.sock}"
         rm -f "$wobsock" || true

hosts/modules/gui/sxmo/default.nix

@@ -615,11 +615,10 @@ in
             source "$XDG_CONFIG_HOME/sxmo/profile"
             source ${package}/etc/profile.d/sxmo_init.sh
             source "$XDG_CONFIG_HOME/sxmo/profile"
-            export PATH="$XDG_CONFIG_HOME/sxmo/hooks:$PATH"
+            export PATH="$XDG_CONFIG_HOME/sxmo/hooks:$PATH:${lib.makeBinPath sxmoPath}"
           '';
           sxmoService = name: {
             description = "sxmo ${name}";
-            path = sxmoPath;
             script = ''
               ${sxmoEnvSetup}
               exec sxmo_${name}.sh
@@ -644,7 +643,6 @@ in
           # sxmo_wob = sxmoService "wob";
           sxmo-x11-status = sxmoService "status_xsetroot";
 
-          bonsaid.path = sxmoPath;
           bonsaid.script = lib.mkBefore sxmoEnvSetup;
         };
       }

modules/users.nix

@@ -133,12 +133,16 @@ let
             # see: <repo:nix-community/home-manager:modules/systemd.nix>
             cleanName = utils.systemdUtils.lib.mkPathSafeName serviceName;
             generatedUnit = utils.systemdUtils.lib.serviceToUnit serviceName (value // {
-              environment = {
+              environment = lib.throwIf (value.path != []) "user service ${serviceName} specifies unsupported 'path' attribute (${builtins.toString value.path})" {
                 # clear PATH to allow inheriting it from environment.
                 # otherwise, nixos would force it to `systemd.globalEnvironment.PATH`, which is mostly tools like sed/find/etc.
                 # clearing PATH here allows user services to inherit whatever PATH the graphical session sets
                 # (see `dbus-update-activation-environment` call in ~/.config/sway/config),
                 # which is critical to making it so user services can see user *programs*/packages.
+                #
+                # note that systemd provides no way to *append* to the PATH, only to override it (or not).
+                # nor do they intend to ever support that:
+                # - <https://github.com/systemd/systemd/issues/1082>
                 PATH = null;
               } // (value.environment or {});
             });