Fix incorrectly cached content after editing

lib/pleroma/activity/html.ex

@@ -8,6 +8,40 @@ defmodule Pleroma.Activity.HTML do
 
   @cachex Pleroma.Config.get([:cachex, :provider], Cachex)
 
+  # We store a list of cache keys related to an activity in a
+  # separate cache, scrubber_management_cache. It has the same
+  # size as scrubber_cache (see application.ex). Every time we add
+  # a cache to scrubber_cache, we update scrubber_management_cache.
+  #
+  # The most recent write of a certain key in the management cache
+  # is the same as the most recent write of any record related to that
+  # key in the main cache.
+  # Assuming LRW ( https://hexdocs.pm/cachex/Cachex.Policy.LRW.html ),
+  # this means when the management cache is evicted by cachex, all
+  # related records in the main cache will also have been evicted.
+
+  defp get_cache_keys_for(activity_id) do
+    with {:ok, list} when is_list(list) <- @cachex.get(:scrubber_management_cache, activity_id) do
+      list
+    else
+      _ -> []
+    end
+  end
+
+  defp add_cache_key_for(activity_id, additional_key) do
+    current = get_cache_keys_for(activity_id)
+
+    unless additional_key in current do
+      @cachex.put(:scrubber_management_cache, activity_id, [additional_key | current])
+    end
+  end
+
+  def invalidate_cache_for(activity_id) do
+    keys = get_cache_keys_for(activity_id)
+    Enum.map(keys, &@cachex.del(:scrubber_cache, &1))
+    @cachex.del(:scrubber_management_cache, activity_id)
+  end
+
   def get_cached_scrubbed_html_for_activity(
         content,
         scrubbers,
@@ -19,6 +53,8 @@ defmodule Pleroma.Activity.HTML do
 
     @cachex.fetch!(:scrubber_cache, key, fn _key ->
       object = Object.normalize(activity, fetch: false)
+
+      add_cache_key_for(activity.id, key)
       HTML.ensure_scrubbed_html(content, scrubbers, object.data["fake"] || false, callback)
     end)
   end