2 files changed, 50 insertions, 0 deletions
diff --git a/app/controllers/activitypub/followers_synchronizations_controller.rb b/app/controllers/activitypub/followers_synchronizations_controller.rb
new file mode 100644
index 000000000..525031105
--- /dev/null
+++ b/app/controllers/activitypub/followers_synchronizations_controller.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class ActivityPub::FollowersSynchronizationsController < ActivityPub::BaseController
+  include SignatureVerification
+  include AccountOwnedConcern
+
+  before_action :require_signature!
+  before_action :set_items
+  before_action :set_cache_headers
+
+  def show
+    expires_in 0, public: false
+    render json: collection_presenter,
+           serializer: ActivityPub::CollectionSerializer,
+           adapter: ActivityPub::Adapter,
+           content_type: 'application/activity+json'
+  end
+
+  private
+
+  def uri_prefix
+    signed_request_account.uri[/http(s?):\/\/[^\/]+\//]
+  end
+
+  def set_items
+    @items = @account.followers.where(Account.arel_table[:uri].matches(uri_prefix + '%', false, true)).pluck(:uri)
+  end
+
+  def collection_presenter
+    ActivityPub::CollectionPresenter.new(
+      id: account_followers_synchronization_url(@account),
+      type: :ordered,
+      items: @items
+    )
+  end
+end
diff --git a/app/controllers/activitypub/inboxes_controller.rb b/app/controllers/activitypub/inboxes_controller.rb
index 0a561e7f0..fdb60d590 100644
--- a/app/controllers/activitypub/inboxes_controller.rb
+++ b/app/controllers/activitypub/inboxes_controller.rb
@@ -11,6 +11,7 @@ class ActivityPub::InboxesController < ActivityPub::BaseController
 
   def create
     upgrade_account
+    process_collection_synchronization
     process_payload
     head 202
   end
@@ -52,6 +53,19 @@ class ActivityPub::InboxesController < ActivityPub::BaseController
     DeliveryFailureTracker.reset!(signed_request_account.inbox_url)
   end
 
+  def process_collection_synchronization
+    raw_params = request.headers['Collection-Synchronization']
+    return if raw_params.blank? || ENV['DISABLE_FOLLOWERS_SYNCHRONIZATION'] == 'true'
+
+    # Re-using the syntax for signature parameters
+    tree   = SignatureParamsParser.new.parse(raw_params)
+    params = SignatureParamsTransformer.new.apply(tree)
+
+    ActivityPub::PrepareFollowersSynchronizationService.new.call(signed_request_account, params)
+  rescue Parslet::ParseFailed
+    Rails.logger.warn 'Error parsing Collection-Synchronization header'
+  end
+
   def process_payload
     ActivityPub::ProcessingWorker.perform_async(signed_request_account.id, body, @account&.id)
   end