nixos/syncthing: Use API to merge / override configurations

If one sets either of `override{Device,folder}s` to false, the jq `*`
operator doesn't merge well the devices and folders, creating duplicate
IDs for folders as observed in NixOS#230146. This PR makes the script iterate
via a Bash for loop the devices and folders IDs and merges the keys
using upstream's `curl -X PATCH` support for single objects.

Hence this commit fixes NixOS#230146.

nixos/modules/services/networking/syncthing.nix

@@ -13,6 +13,7 @@ let
   devices = mapAttrsToList (_: device: device // {
     deviceID = device.id;
   }) cfg.settings.devices;
+  devicesIDs = mapAttrsToList (name: device: device.id) cfg.settings.devices;
 
   folders = mapAttrsToList (_: folder: folder //
     throwIf (folder?rescanInterval || folder?watch || folder?watchDelay) ''
@@ -28,12 +29,13 @@ let
   }) (filterAttrs (_: folder:
     folder.enable
   ) cfg.settings.folders);
+  foldersIDs = builtins.map (folder: folder.id) folders;
 
-  updateConfig = pkgs.writers.writeDash "merge-syncthing-config" ''
+  jq = "${pkgs.jq}/bin/jq";
+  updateConfig = pkgs.writers.writeDash "merge-syncthing-config" (''
     set -efu
 
     # be careful not to leak secrets in the filesystem or in process listings
-
     umask 0077
 
     # get the api key by parsing the config.xml
@@ -51,25 +53,71 @@ let
             --retry 1000 --retry-delay 1 --retry-all-errors \
             "$@"
     }
-
-    # query the old config
-    old_cfg=$(curl ${cfg.guiAddress}/rest/config)
-
-    # generate the new config by merging with the NixOS config options
-    new_cfg=$(printf '%s\n' "$old_cfg" | ${pkgs.jq}/bin/jq -c ${escapeShellArg ''. * ${builtins.toJSON cleanedConfig} * {
-        "devices": ('${escapeShellArg (builtins.toJSON devices)}'${optionalString (cfg.settings.devices == {} || ! cfg.overrideDevices) " + .devices"}),
-        "folders": ('${escapeShellArg (builtins.toJSON folders)}'${optionalString (cfg.settings.folders == {} || ! cfg.overrideFolders) " + .folders"})
-    }''})
-
-    # send the new config
-    curl -X PUT -d "$new_cfg" ${cfg.guiAddress}/rest/config
-
+  '' +
+  /* Syncthing's rest API for the folders and devices is identical. Hence we
+  iterate the cfg.settings.{devices,folder} Nix variables using the
+  conf_type which is either "devices" or "folders": */
+  (lib.concatStringsSep "\n" (map (conf_type: let
+    new_conf_IDs = (concatStringsSep " " {
+      devices = devicesIDs;
+      folders = foldersIDs;
+    }.${conf_type});
+    override = {
+      devices = cfg.overrideDevices;
+      folders = cfg.overrideFolders;
+    }.${conf_type};
+    conf = {
+      inherit devices folders;
+    }.${conf_type};
+    # The API's id naming is slightly different for devices and folders
+    GET_IdAttrName = {
+      devices = "deviceID";
+      folders = "id";
+    }.${conf_type};
+    # All URLs and curl commands are based on: https://docs.syncthing.net/rest/config.html
+    baseAddress = "${cfg.guiAddress}/rest/config/${conf_type}";
+  in (
+    (lib.optionalString (new_conf_IDs != []) ''
+      # We iterate the IDs list, and run a curl -X POST command for each, that
+      # should update that device/folder only.
+      for id in ${new_conf_IDs}; do
+        new_cfg=$(echo ${escapeShellArg (builtins.toJSON conf)} | ${jq} '.[] | select(.${GET_IdAttrName} == "'$id'")')
+        # Quoting https://docs.syncthing.net/rest/config.html: > PUT takes an
+        # array and POST a single object. In both cases if a given folder/device
+        # already exists, it’s replaced, otherwise a new one is added.
+        curl -d "$new_cfg" -X POST ${baseAddress}
+      done
+      '') +
+      /* If we need to override devices/folders, we iterate all currently configured
+      IDs, via another `curl -X GET`, and we delete all IDs that are not part of
+      the Nix configured list of IDs
+      */
+      (optionalString override ''
+        old_conf_${conf_type}_ids="$(curl -X GET ${baseAddress} | ${jq} --raw-output '.[].${GET_IdAttrName}')"
+        for id in ''${old_conf_${conf_type}_ids}; do
+          if echo ${new_conf_IDs} | grep -q $id; then
+            continue
+          else
+            curl -X DELETE ${baseAddress}/$id
+          fi
+        done
+      '')
+  )) ["devices" "folders"])) +
+  /* Now we update the subOption "options", "gui" and "ldap" one by one, in
+  order to not override the folders and devices we handled in the previous
+  Nix Loop. */
+  (lib.concatStringsSep "\n" (map (subOption:
+    optionalString (builtins.hasAttr subOption cleanedConfig) ''
+      curl -X PUT -d ${escapeShellArg (builtins.toJSON cleanedConfig.${subOption})} \
+        ${cfg.guiAddress}/rest/config/${subOption}
+    '') [ "options" "gui" "ldap" ])
+  ) + ''
     # restart Syncthing if required
     if curl ${cfg.guiAddress}/rest/config/restart-required |
-       ${pkgs.jq}/bin/jq -e .requiresRestart > /dev/null; then
+       ${jq} -e .requiresRestart > /dev/null; then
         curl -X POST ${cfg.guiAddress}/rest/system/restart
     fi
-  '';
+  '');
 in {
   ###### interface
   options = {