fix(syncthing): Persist Syncthing state

Previously, the device ID would be lost and consequently regenerated on boot.
This means that previous connections will recognise it as a new, different
device.

README.org

@@ -179,6 +179,8 @@ sydnix-cli is a command-line utility written in Clojure wrapping various sydnix-
 
 ** Emacs from scratch
 
+*** TODO [#A] [[id:6141dc5c-2232-4bc0-9464-410c21135c86][Strategies]]
+
 *** TODO Completions w/ [[https://github.com/minad/corfu][Corfu]]
 
 *** TODO Overlay org-mode links with the domain name
@@ -231,10 +233,6 @@ If a comment immediately proceeds a sexp, open the new sexp /before/ the comment
 
 *** DONE Evil operators like ~=~ should not move point
 
-*** TODO Fix ~syd-forward-up-sexp~
-
-*** TODO Fix defun text object
-
 *** TODO ESC should immediately close minibuffer, even whilst in insert state
 
 *** DONE Daemon

users/crumb/default.nix

@@ -18,6 +18,8 @@
       ./files.nix
     ];
 
+    xdg.enable = true;
+
     sydnix = {
       sops = {
         enable = true;

users/crumb/files.nix

@@ -1,10 +1,15 @@
 { config, lib, pkgs, ... }:
 
 let mutableSymlink = config.lib.file.mkOutOfStoreSymlink;
-in {
+in lib.mkMerge [
+  {
     # TODO: Move to programs/age.nix.
     xdg.configFile."sops/age/keys.txt".source =
       mutableSymlink "/persist/vault/${config.home.username}/keys/melbourne";
+  }
+
+  {
+    ### Some basic impermanence setup.
 
     sydnix.impermanence = {
       enable = true;
@@ -14,6 +19,9 @@ in {
         "Documents"
         "Videos"
         "src"
+        # Should "org" be declared in emacs.nix?  I don't think so.  I
+        # conjecture that my org files are extremely valuable with or without
+        # Emacs.
         "org"
         ".ssh"
         {
@@ -23,8 +31,34 @@ in {
         ".passage"
       ];
     };
+  }
+
+  {
+    ### Syncthing
 
     # TODO: sydnix.syncthing module
+    sydnix.impermanence.directories =
+      let xdg-state-dir = 
+            "${config.home.stateHome
+              or "${config.home.homeDirectory}/.local/state"}";
+      in [
+        # HACK: This relies on behaviour that is not explicitly defined by the
+        # Syncthing Home-manager module.  We have to trust that $XDG_STATE_DIR,
+        # at the time of syncthing.service's start, does not differ from this
+        # value defined in Nix.  Perhaps a PR to home-manager adding an option
+        # `services.syncthing.stateDir` would be good.
+        #
+        # If Syncthing ever breaks, make sure they didn't start using a different path:
+        # https://github.com/nix-community/home-manager/blob/master/modules/services/syncthing.nix
+        #
+        # Evidence for using this path is found at: (permalink)
+        # https://github.com/nix-community/home-manager/blob/6d3163aea47fdb1fe19744e91306a2ea4f602292/modules/services/syncthing.nix#L624
+        
+        # Hack aside, this directory must be persisted to, at least
+        #  1. Preserve the device ID.
+        "${xdg-state-dir}/syncthing"
+      ];
+
     services.syncthing = {
       enable = true;
       settings = {
@@ -44,3 +78,4 @@ in {
       };
     };
   }
+]