FIX: serialize Flags instead of PostActionType (#28362)

### Why?
Before, all flags were static. Therefore, they were stored in class variables and serialized by SiteSerializer. Recently, we added an option for admins to add their own flags or disable existing flags. Therefore, the class variable had to be dropped because it was unsafe for a multisite environment. However, it started causing performance problems. 

### Solution
When a new Flag system is used, instead of using PostActionType, we can serialize Flags and use fragment cache for performance reasons. 

At the same time, we are still supporting deprecated `replace_flags` API call. When it is used, we fall back to the old solution and the admin cannot add custom flags. In a couple of months, we will be able to drop that API function and clean that code properly. However, because it may still be used, redis cache was introduced to improve performance.

To test backward compatibility you can add this code to any plugin
```ruby
  replace_flags do |flag_settings|
    flag_settings.add(
      4,
      :inappropriate,
      topic_type: true,
      notify_type: true,
      auto_action_type: true,
    )
    flag_settings.add(1001, :trolling, topic_type: true, notify_type: true, auto_action_type: true)
  end
```

lib/guardian/post_guardian.rb

@@ -37,15 +37,18 @@ module PostGuardian
     end
 
     taken = opts[:taken_actions].try(:keys).to_a
+    post_action_type_view = opts[:post_action_type_view] || PostActionTypeView.new
     is_flag =
       if (opts[:notify_flag_types] && opts[:additional_message_types])
         opts[:notify_flag_types][action_key] || opts[:additional_message_types][action_key]
       else
-        PostActionType.notify_flag_types[action_key] ||
-          PostActionType.additional_message_types[action_key]
+        post_action_type_view.notify_flag_types[action_key] ||
+          post_action_type_view.additional_message_types[action_key]
       end
-    already_taken_this_action = taken.any? && taken.include?(PostActionType.types[action_key])
-    already_did_flagging = taken.any? && (taken & PostActionType.notify_flag_types.values).any?
+    already_taken_this_action =
+      taken.any? && taken.include?(post_action_type_view.types[action_key])
+    already_did_flagging =
+      taken.any? && (taken & post_action_type_view.notify_flag_types.values).any?
 
     result =
       if authenticated? && post
@@ -61,7 +64,9 @@ module PostGuardian
         # post made by staff, but we don't allow staff flags
         return false if is_flag && (!SiteSetting.allow_flagging_staff?) && post&.user&.staff?
 
-        return false if is_flag && PostActionType.disabled_flag_types.keys.include?(action_key)
+        if is_flag && post_action_type_view.disabled_flag_types.keys.include?(action_key)
+          return false
+        end
 
         if action_key == :notify_user &&
              !@user.in_any_groups?(SiteSetting.personal_message_enabled_groups_map)
@@ -111,12 +116,13 @@ module PostGuardian
     return true if is_admin?
     return false unless topic
 
-    type_symbol = PostActionType.types[post_action_type_id]
+    post_action_type_view = PostActionTypeView.new
+    type_symbol = post_action_type_view.types[post_action_type_id]
 
     return false if type_symbol == :bookmark
     return false if type_symbol == :notify_user && !is_moderator?
 
-    return can_see_flags?(topic) if PostActionType.is_flag?(type_symbol)
+    return can_see_flags?(topic) if post_action_type_view.is_flag?(type_symbol)
 
     true
   end

lib/post_action_creator.rb

@@ -57,7 +57,8 @@ class PostActionCreator
 
     @post = post
     @post_action_type_id = post_action_type_id
-    @post_action_name = PostActionType.types[@post_action_type_id]
+    @post_action_type_view = PostActionTypeView.new
+    @post_action_name = @post_action_type_view.types[@post_action_type_id]
 
     @is_warning = is_warning
     @take_action = take_action && guardian.is_staff?
@@ -96,7 +97,7 @@ class PostActionCreator
     if !post_can_act? || (@queue_for_review && !guardian.is_staff?)
       result.forbidden = true
 
-      if taken_actions&.keys&.include?(PostActionType.types[@post_action_name])
+      if taken_actions&.keys&.include?(@post_action_type_view.types[@post_action_name])
         result.add_error(I18n.t("action_already_performed"))
       else
         result.add_error(I18n.t("invalid_access"))
@@ -115,7 +116,7 @@ class PostActionCreator
 
     # create meta topic / post if needed
     if @message.present? &&
-         (PostActionType.additional_message_types.keys | %i[spam illegal]).include?(
+         (@post_action_type_view.additional_message_types.keys | %i[spam illegal]).include?(
            @post_action_name,
          )
       creator = create_message_creator
@@ -170,11 +171,11 @@ class PostActionCreator
   private
 
   def flagging_post?
-    PostActionType.notify_flag_type_ids.include?(@post_action_type_id)
+    @post_action_type_view.notify_flag_type_ids.include?(@post_action_type_id)
   end
 
   def cannot_flag_again?(reviewable)
-    return false if @post_action_type_id == PostActionType.types[:notify_moderators]
+    return false if @post_action_type_id == @post_action_type_view.types[:notify_moderators]
     flag_type_already_used =
       reviewable.reviewable_scores.any? do |rs|
         rs.reviewable_score_type == @post_action_type_id && !rs.pending?
@@ -233,7 +234,8 @@ class PostActionCreator
     return if @post.hidden?
     return if !@created_by.staff? && @post.user&.staff?
 
-    not_auto_action_flag_type = !PostActionType.auto_action_flag_types.include?(@post_action_name)
+    not_auto_action_flag_type =
+      !@post_action_type_view.auto_action_flag_types.include?(@post_action_name)
     return if not_auto_action_flag_type && !@queue_for_review
 
     if @queue_for_review
@@ -304,14 +306,14 @@ class PostActionCreator
 
     if post_action
       case @post_action_type_id
-      when *PostActionType.notify_flag_type_ids
+      when *@post_action_type_view.notify_flag_type_ids
         DiscourseEvent.trigger(:flag_created, post_action, self)
-      when PostActionType.types[:like]
+      when @post_action_type_view.types[:like]
         DiscourseEvent.trigger(:like_created, post_action, self)
       end
     end
 
-    if @post_action_type_id == PostActionType.types[:like]
+    if @post_action_type_id == @post_action_type_view.types[:like]
       GivenDailyLike.increment_for(@created_by.id)
     end
 
@@ -381,7 +383,7 @@ class PostActionCreator
         target: @post,
         topic: @post.topic,
         reviewable_by_moderator: true,
-        potential_spam: @post_action_type_id == PostActionType.types[:spam],
+        potential_spam: @post_action_type_id == @post_action_type_view.types[:spam],
         payload: {
           targets_topic: @targets_topic,
         },

lib/post_action_destroyer.rb

@@ -17,6 +17,10 @@ class PostActionDestroyer
     new(destroyed_by, post, PostActionType.types[action_key], opts).perform
   end
 
+  def post_action_type_view
+    @post_action_type_view ||= PostActionTypeView.new
+  end
+
   def perform
     result = DestroyResult.new
 
@@ -50,14 +54,14 @@ class PostActionDestroyer
 
     post_action.remove_act!(@destroyed_by)
     post_action.post.unhide! if post_action.staff_took_action
-    if @post_action_type_id == PostActionType.types[:like]
+    if @post_action_type_id == post_action_type_view.types[:like]
       GivenDailyLike.decrement_for(@destroyed_by.id)
     end
 
     case @post_action_type_id
-    when *PostActionType.notify_flag_type_ids
+    when *post_action_type_view.notify_flag_type_ids
       DiscourseEvent.trigger(:flag_destroyed, post_action, self)
-    when PostActionType.types[:like]
+    when post_action_type_view.types[:like]
       DiscourseEvent.trigger(:like_destroyed, post_action, self)
     end
 
@@ -78,7 +82,7 @@ class PostActionDestroyer
   end
 
   def notify_subscribers
-    name = PostActionType.types[@post_action_type_id]
+    name = post_action_type_view.types[@post_action_type_id]
     if name == :like
       @post.publish_change_to_clients!(
         :unliked,

lib/post_action_type_view.rb

@@ -0,0 +1,144 @@
+# frozen_string_literal: true
+
+class PostActionTypeView
+  ATTRIBUTE_NAMES = %i[
+    id
+    name
+    name_key
+    description
+    notify_type
+    auto_action_type
+    require_message
+    applies_to
+    position
+    enabled
+    score_type
+  ]
+
+  def all_flags
+    @all_flags ||=
+      Discourse
+        .cache
+        .fetch(PostActionType::POST_ACTION_TYPE_ALL_FLAGS_KEY) do
+          Flag
+            .unscoped
+            .order(:position)
+            .pluck(ATTRIBUTE_NAMES)
+            .map { |attributes| ATTRIBUTE_NAMES.zip(attributes).to_h }
+        end
+  end
+
+  def flag_settings
+    @flag_settings ||= PostActionType.flag_settings
+  end
+
+  def types
+    if overridden_by_plugin_or_skipped_db?
+      return Enum.new(like: PostActionType::LIKE_POST_ACTION_ID).merge!(flag_settings.flag_types)
+    end
+    Enum.new(like: PostActionType::LIKE_POST_ACTION_ID).merge(flag_types)
+  end
+
+  def overridden_by_plugin_or_skipped_db?
+    flag_settings.flag_types.present? || GlobalSetting.skip_db?
+  end
+
+  def auto_action_flag_types
+    return flag_settings.auto_action_types if overridden_by_plugin_or_skipped_db?
+    flag_enum(all_flags.select { |flag| flag[:auto_action_type] })
+  end
+
+  def public_types
+    types.except(*flag_types.keys << :notify_user)
+  end
+
+  def public_type_ids
+    Discourse
+      .cache
+      .fetch(PostActionType::POST_ACTION_TYPE_PUBLIC_TYPE_IDS_KEY) { public_types.values }
+  end
+
+  def flag_types_without_additional_message
+    return flag_settings.without_additional_message_types if overridden_by_plugin_or_skipped_db?
+    flag_enum(flags.reject { |flag| flag[:require_message] })
+  end
+
+  def flags
+    all_flags.reject do |flag|
+      flag[:score_type] || flag[:id] == PostActionType::LIKE_POST_ACTION_ID
+    end
+  end
+
+  def flag_types
+    return flag_settings.flag_types if overridden_by_plugin_or_skipped_db?
+    flag_enum(flags)
+  end
+
+  def score_types
+    return flag_settings.flag_types if overridden_by_plugin_or_skipped_db?
+    flag_enum(all_flags.filter { |flag| flag[:score_type] })
+  end
+
+  # flags resulting in mod notifications
+  def notify_flag_type_ids
+    notify_flag_types.values
+  end
+
+  def notify_flag_types
+    return flag_settings.notify_types if overridden_by_plugin_or_skipped_db?
+    flag_enum(all_flags.select { |flag| flag[:notify_type] })
+  end
+
+  def topic_flag_types
+    if overridden_by_plugin_or_skipped_db?
+      flag_settings.topic_flag_types
+    else
+      flag_enum(all_flags.select { |flag| flag[:applies_to].include?("Topic") })
+    end
+  end
+
+  def disabled_flag_types
+    flag_enum(all_flags.reject { |flag| flag[:enabled] })
+  end
+
+  def additional_message_types
+    return flag_settings.additional_message_types if overridden_by_plugin_or_skipped_db?
+    flag_enum(all_flags.select { |flag| flag[:require_message] })
+  end
+
+  def names
+    all_flags.reduce({}) do |acc, f|
+      acc[f[:id]] = f[:name]
+      acc
+    end
+  end
+
+  def descriptions
+    all_flags.reduce({}) do |acc, f|
+      acc[f[:id]] = f[:description]
+      acc
+    end
+  end
+
+  def applies_to
+    all_flags.reduce({}) do |acc, f|
+      acc[f[:id]] = f[:applies_to]
+      acc
+    end
+  end
+
+  def is_flag?(sym)
+    flag_types.valid?(sym)
+  end
+
+  private
+
+  def flag_enum(scope)
+    Enum.new(
+      scope.reduce({}) do |acc, f|
+        acc[f[:name_key].to_sym] = f[:id]
+        acc
+      end,
+    )
+  end
+end

lib/post_destroyer.rb

@@ -348,6 +348,10 @@ class PostDestroyer
     Jobs.enqueue(:feature_topic_users, topic_id: @post.topic_id)
   end
 
+  def post_action_type_view
+    @post_action_type_view ||= PostActionTypeView.new
+  end
+
   def trash_public_post_actions
     if public_post_actions = PostAction.publics.where(post_id: @post.id)
       public_post_actions.each { |pa| permanent? ? pa.destroy! : pa.trash!(@user) }
@@ -357,7 +361,7 @@ class PostDestroyer
       @post.custom_fields["deleted_public_actions"] = public_post_actions.ids
       @post.save_custom_fields
 
-      f = PostActionType.public_types.map { |k, _| ["#{k}_count", 0] }
+      f = post_action_type_view.public_types.map { |k, _| ["#{k}_count", 0] }
       Post.with_deleted.where(id: @post.id).update_all(Hash[*f.flatten])
     end
   end
@@ -387,7 +391,7 @@ class PostDestroyer
     # ReviewableScore#types is a superset of PostActionType#flag_types.
     # If the reviewable score type is not on the latter, it means it's not a flag by a user and
     #  must be an automated flag like `needs_approval`. There's no flag reason for these kind of types.
-    flag_type = PostActionType.flag_types[rs.reviewable_score_type]
+    flag_type = post_action_type_view.flag_types[rs.reviewable_score_type]
     return unless flag_type
 
     notify_responders = options[:notify_responders]

lib/topic_view.rb

@@ -598,17 +598,28 @@ class TopicView
       ReviewableQueuedPost.pending.where(target_created_by: @user, topic: @topic).order(:created_at)
   end
 
+  def post_action_type_view
+    @post_action_type_view ||= PostActionTypeView.new
+  end
+
   def actions_summary
     return @actions_summary unless @actions_summary.nil?
 
     @actions_summary = []
     return @actions_summary unless post = posts&.first
-    PostActionType.topic_flag_types.each do |sym, id|
+    post_action_type_view.topic_flag_types.each do |sym, id|
       @actions_summary << {
         id: id,
         count: 0,
         hidden: false,
-        can_act: @guardian.post_can_act?(post, sym),
+        can_act:
+          @guardian.post_can_act?(
+            post,
+            sym,
+            opts: {
+              post_action_type_view: post_action_type_view,
+            },
+          ),
       }
     end
 
@@ -623,22 +634,6 @@ class TopicView
     @pm_params ||= TopicQuery.new(@user).get_pm_params(topic)
   end
 
-  def flag_types
-    @flag_types ||= PostActionType.types
-  end
-
-  def public_flag_types
-    @public_flag_types ||= PostActionType.public_types
-  end
-
-  def notify_flag_types
-    @notify_flag_types ||= PostActionType.notify_flag_types
-  end
-
-  def additional_message_types
-    @additional_message_types ||= PostActionType.additional_message_types
-  end
-
   def suggested_topics
     if @include_suggested
       @suggested_topics ||=