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.

users/crumb/files.nix

@@ -1,46 +1,81 @@
 { config, lib, pkgs, ... }:
 
 let mutableSymlink = config.lib.file.mkOutOfStoreSymlink;
-in {
-  # TODO: Move to programs/age.nix.
-  xdg.configFile."sops/age/keys.txt".source =
-    mutableSymlink "/persist/vault/${config.home.username}/keys/melbourne";
+in lib.mkMerge [
+  {
+    # TODO: Move to programs/age.nix.
+    xdg.configFile."sops/age/keys.txt".source =
+      mutableSymlink "/persist/vault/${config.home.username}/keys/melbourne";
+  }
 
-  sydnix.impermanence = {
-    enable = true;
-    directories = [
-      "Music"
-      "Pictures"
-      "Documents"
-      "Videos"
-      "src"
-      "org"
-      ".ssh"
-      {
-        directory = ".local/share/Steam";
-        method = "symlink";
-      }
-      ".passage"
-    ];
-  };
+  {
+    ### Some basic impermanence setup.
 
-  # TODO: sydnix.syncthing module
-  services.syncthing = {
-    enable = true;
-    settings = {
-      overrideDevices = true;
-      overrideFolders = true;
-      devices = {
-        "guix-rebound".id =
-          "Q5B6LIV-5HQMWWV-XFQL5IT-PHP7PVE-XFWUVHK-F6WJ42C-OPMR4M7-GFNK3AG";
-      };
-      folders = {
-        "org" = {
-          path = "~/org";
-          devices = [ "guix-rebound" ];
-          ignorePerms = true;
+    sydnix.impermanence = {
+      enable = true;
+      directories = [
+        "Music"
+        "Pictures"
+        "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"
+        {
+          directory = ".local/share/Steam";
+          method = "symlink";
+        }
+        ".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 = {
+        overrideDevices = true;
+        overrideFolders = true;
+        devices = {
+          "guix-rebound".id =
+            "Q5B6LIV-5HQMWWV-XFQL5IT-PHP7PVE-XFWUVHK-F6WJ42C-OPMR4M7-GFNK3AG";
+        };
+        folders = {
+          "org" = {
+            path = "~/org";
+            devices = [ "guix-rebound" ];
+            ignorePerms = true;
+          };
         };
       };
     };
-  };
-}
+  }
+]