nix: reload daemon when nix.conf changes

modules/nix/default.nix

@@ -54,6 +54,16 @@ in
       '';
     };
 
+    nix.useDaemon = mkOption {
+      type = types.bool;
+      default = false;
+      description = "
+        If set, Nix will use the daemon to perform operations.
+        Use this instead of services.nix-daemon.enable if you don't wan't the
+        daemon service to be managed for you.
+      ";
+    };
+
     nix.maxJobs = mkOption {
       type = types.int;
       default = 1;
@@ -346,6 +356,14 @@ in
         NIX_CURRENT_LOAD = "/run/nix/current-load";
       };
 
+    environment.extraInit = mkIf cfg.useDaemon ''
+      # Set up secure multi-user builds: non-root users build through the
+      # Nix daemon.
+      if [ "$USER" != root -o ! -w /nix/var/nix/db ]; then
+          export NIX_REMOTE=daemon
+      fi
+    '';
+
     # Set up the environment variables for running Nix.
     environment.variables = cfg.envVars //
       { NIX_PATH = concatStringsSep ":" cfg.nixPath;
@@ -357,5 +375,12 @@ in
       fi
     '';
 
+    system.activationScripts.nix-daemon.text = mkIf cfg.useDaemon ''
+      if ! diff /etc/nix/nix.conf /run/current-system/etc/nix/nix.conf &> /dev/null; then
+        echo "reloading nix-daemon..." >&2
+        pkill -HUP nix-daemon
+      fi
+    '';
+
   };
 }

modules/services/nix-daemon.nix

@@ -11,7 +11,7 @@ in
     services.nix-daemon.enable = mkOption {
       type = types.bool;
       default = false;
-      description = "Whether to activate system at boot time.";
+      description = "Whether to enable the nix-daemon service.";
     };
 
     services.nix-daemon.logFile = mkOption {
@@ -35,13 +35,7 @@ in
 
   config = mkIf cfg.enable {
 
-    environment.extraInit = ''
-      # Set up secure multi-user builds: non-root users build through the
-      # Nix daemon.
-      if [ "$USER" != root -o ! -w /nix/var/nix/db ]; then
-          export NIX_REMOTE=daemon
-      fi
-    '';
+    nix.useDaemon = true;
 
     launchd.daemons.nix-daemon = {
       command = "${config.nix.package}/bin/nix-daemon";

modules/system/activation-scripts.nix

@@ -56,6 +56,7 @@ in
       ${cfg.activationScripts.applications.text}
       ${cfg.activationScripts.etc.text}
       ${cfg.activationScripts.launchd.text}
+      ${cfg.activationScripts.nix-daemon.text}
       ${cfg.activationScripts.time.text}
       ${cfg.activationScripts.networking.text}