nixos/resilio: support secret files

When using the declarative shared folder configuration for resilio sync
it is now possible to pass a path from which to read the secret should
be read at runtime. The path will not be added to the nix store.

The 'secret' parameter to specify the secret directly is still
supported. This option will still store the secret in the nix store.

This commit follows the pattern described in this issue, for upstream
programs that do not provide support for setting a password using a
file: https://github.com/NixOS/nixpkgs/issues/24288

nixos/doc/manual/from_md/release-notes/rl-2305.section.xml

@@ -121,6 +121,13 @@
           <link xlink:href="https://search.nixos.org/packages?channel=unstable&amp;show=utm&amp;from=0&amp;size=1&amp;sort=relevance&amp;type=packages&amp;query=utm">package</link>.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          Resilio sync secret keys can now be provided using a secrets
+          file at runtime, preventing these secrets from ending up in
+          the Nix store.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
 </section>

nixos/doc/manual/release-notes/rl-2305.section.md

@@ -40,3 +40,5 @@ In addition to numerous new and upgraded packages, this release has the followin
 - The module for the application firewall `opensnitch` got the ability to configure rules. Available as [services.opensnitch.rules](#opt-services.opensnitch.rules)
 
 - A new `virtualisation.rosetta` module was added to allow running `x86_64` binaries through [Rosetta](https://developer.apple.com/documentation/apple-silicon/about-the-rosetta-translation-environment) inside virtualised NixOS guests on Apple silicon. This feature works by default with the [UTM](https://docs.getutm.app/) virtualisation [package](https://search.nixos.org/packages?channel=unstable&show=utm&from=0&size=1&sort=relevance&type=packages&query=utm).
+
+- Resilio sync secret keys can now be provided using a secrets file at runtime, preventing these secrets from ending up in the Nix store.

nixos/modules/services/networking/resilio.nix

@@ -8,7 +8,6 @@ let
   resilioSync = pkgs.resilio-sync;
 
   sharedFoldersRecord = map (entry: {
-    secret = entry.secret;
     dir = entry.directory;
 
     use_relay_server = entry.useRelayServer;
@@ -40,6 +39,31 @@ let
     shared_folders = sharedFoldersRecord;
   }));
 
+  sharedFoldersSecretFiles = map (entry: {
+    dir = entry.directory;
+    secretFile = if builtins.hasAttr "secret" entry then
+      toString (pkgs.writeTextFile {
+        name = "secret-file";
+        text = entry.secret;
+      })
+    else
+      entry.secretFile;
+  }) cfg.sharedFolders;
+
+  runConfigPath = "/run/rslsync/config.json";
+
+  createConfig = pkgs.writeShellScriptBin "create-resilio-config" ''
+    ${pkgs.jq}/bin/jq \
+      '.shared_folders |= map(.secret = $ARGS.named[.dir])' \
+      ${
+        lib.concatMapStringsSep " \\\n  "
+        (entry: ''--arg '${entry.dir}' "$(cat '${entry.secretFile}')"'')
+        sharedFoldersSecretFiles
+      } \
+      <${configFile} \
+      >${runConfigPath}
+  '';
+
 in
 {
   options = {
@@ -186,7 +210,7 @@ in
         default = [];
         type = types.listOf (types.attrsOf types.anything);
         example =
-          [ { secret         = "AHMYFPCQAHBM7LQPFXQ7WV6Y42IGUXJ5Y";
+          [ { secretFile     = "/run/resilio-secret";
               directory      = "/home/user/sync_test";
               useRelayServer = true;
               useTracker     = true;
@@ -202,9 +226,6 @@ in
         description = lib.mdDoc ''
           Shared folder list. If enabled, web UI must be
           disabled. Secrets can be generated using `rslsync --generate-secret`.
-          Note that this secret will be
-          put inside the Nix store, so it is realistically not very
-          secret.
 
           If you would like to be able to modify the contents of this
           directories, it is recommended that you make your user a
@@ -256,8 +277,10 @@ in
         Restart   = "on-abort";
         UMask     = "0002";
         User      = "rslsync";
+        RuntimeDirectory = "rslsync";
+        ExecStartPre = "${createConfig}/bin/create-resilio-config";
         ExecStart = ''
-          ${resilioSync}/bin/rslsync --nodaemon --config ${configFile}
+          ${resilioSync}/bin/rslsync --nodaemon --config ${runConfigPath}
         '';
       };
     };