Fix threading of private posts received out of order

app/lib/activitypub/activity/create.rb

@@ -88,6 +88,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     end
 
     resolve_thread(@status)
+    fixup_thread(@status)
     fetch_replies(@status)
     distribute
     forward_for_reply
@@ -334,6 +335,10 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     ThreadResolveWorker.perform_async(status.id, in_reply_to_uri, { 'request_id' => @options[:request_id] })
   end
 
+  def fixup_thread(status)
+    ThreadRepair.new(status.uri).reattach_orphaned_children!(status)
+  end
+
   def fetch_replies(status)
     collection = @object['replies']
     return if collection.nil?

app/lib/thread_repair.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+class ThreadRepair
+  include Redisable
+
+  THREAD_FIXUP_WINDOW = 1.hour.to_i
+
+  def initialize(parent_uri)
+    @parent_uri = parent_uri
+  end
+
+  def find_parent(child_id)
+    with_redis do |redis|
+      redis.sadd(redis_key, child_id)
+      redis.expire(redis_key, THREAD_FIXUP_WINDOW)
+
+      parent = ActivityPub::TagManager.instance.uri_to_resource(@parent_uri, Status)
+      redis.srem(redis_key, child_id) if parent.present?
+
+      parent
+    end
+  end
+
+  def reattach_orphaned_children!(parent)
+    with_redis do |redis|
+      redis.sscan_each(redis_key, count: 1000) do |ids|
+        statuses = Status.where(id: ids).to_a
+
+        statuses.each { |status| status.update(thread: parent) }
+
+        # Updated statuses need to be distributed to clients/inserted in TLs
+        DistributionWorker.push_bulk(statuses.filter(&:within_realtime_window?)) do |status|
+          [status.id, { 'skip_notifications' => true }]
+        end
+
+        redis.srem(redis_key, ids)
+      end
+    end
+  end
+
+  def redis_key
+    "thread_repair:#{@parent_uri}"
+  end
+end

app/workers/thread_resolve_worker.rb

@@ -13,6 +13,11 @@ class ThreadResolveWorker
     parent_status = ActivityPub::TagManager.instance.uri_to_resource(parent_url, Status)
     parent_status ||= FetchRemoteStatusService.new.call(parent_url, **options.deep_symbolize_keys)
 
+    # The parent post does not exist or is private, so take note of it so the thread can be
+    # reconstructed if the parent arrives.
+    # This will re-check the database to avoid race conditions
+    parent_status ||= ThreadRepair.new(parent_url).find_parent(child_status_id)
+
     return if parent_status.nil?
 
     child_status.thread = parent_status