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 #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 #230146.

nixos/modules/services/networking/syncthing.nix

@@ -12,6 +12,7 @@ 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) ''
@@ -25,12 +26,13 @@ let
         device
     ) folder.devices;
   }) cfg.settings.folders;
+  foldersIDs = mapAttrsToList (name: folder: folder.id) cfg.settings.folders;
 
+  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
@@ -49,21 +51,66 @@ let
             "$@"
     }
 
-    # 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 ''. * ${builtins.toJSON cfg.settings} * {
-        "devices": (${builtins.toJSON devices}${optionalString (cfg.settings.devices == {} || ! cfg.overrideDevices) " + .devices"}),
-        "folders": (${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/${conf_type}";
+    in ''
+    # We iterate the IDs list, and run a curl -X POST command for each, that
+    # should update that device/folder only.
+    IFS=$'\n'; 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
+    ${optionalString override /*
+    If we need to override devices/folders, we iterate all currently configured
+    IDs, via another `curl -X GET`, and we delete all IDs that are not part of
+    the Nix configured list of IDs
+    */ ''
+      old_conf_${conf_type}_ids="$(curl -X GET ${baseAddress} | ${jq} --raw-output '.[].${GET_IdAttrName}')"
+      IFS=$'\n'; for id in ''${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 cfg.settings) ''
+        curl -X PUT -d ${escapeShellArg (builtins.toJSON cfg.settings.${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
   '';