Only sync doom when config changes

modules/applications/emacs/default.nix

@@ -1,6 +1,14 @@
 { config, lib, pkgs, ... }:
 
-{
+let
+  doomSync = (let
+    emacs = pkgs.emacs;
+    git = pkgs.git;
+  in ''
+    export PATH=${emacs}/bin:${git}/bin:$PATH
+    ${config.xdg.configHome}/emacs/bin/doom sync -u -j $((`nproc`/4*3))
+  '');
+in {
   home.packages = [
     pkgs.emacs-all-the-icons-fonts
     pkgs.fontconfig
@@ -21,20 +29,24 @@
     ".config/emacs" = {
       source = fetchGit {
         url = "https://github.com/doomemacs/doomemacs.git";
+        # When updating me, remember to run `doom sync`
         rev = "36e7aaa619342eff61b1daf3ac664f94d5272db7";
       };
+      # We need to use recursive mode here or else doom fails to sync for
+      # some reason related to the permissions on the synced path. I'm not
+      # quite sure of everything that's going on here.
       recursive = true;
+
+      # Because `recursive = true` will cause this to sync every single
+      # activation, we turn this off here.
+      #
+      # There's probably a way we could do better detection of this within
+      # our onChange shell?
+      #
+      # onChange = doomSync;
     };
   };
 
-  home.activation.doomSync = lib.hm.dag.entryAfter [ "writeBoundary" ] (let
-    emacs = pkgs.emacs;
-    git = pkgs.git;
-  in ''
-    export PATH=${emacs}/bin:${git}/bin:$PATH
-    ${config.xdg.configHome}/emacs/bin/doom sync -u -j $((`nproc`/4*3))
-  '');
-
   home.sessionVariables = {
     DOOMLOCALDIR = "${config.xdg.dataHome}/doom";
     EDITOR = "emacs -nw";
@@ -49,5 +61,9 @@
     package = pkgs.emacs;
   };
 
-  xdg.configFile."doom".source = ./doom;
+  xdg.configFile."doom" = {
+    source = ./doom;
+    # Sync doom if we updated the config
+    onChange = doomSync;
+  };
 }