FIX: serialize Flags instead of PostActionType (#28259)

### 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
```

spec/lib/post_action_type_view_spec.rb

@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+RSpec.describe PostActionTypeView do
+  let(:post_action_type_view) { PostActionTypeView.new }
+
+  it "returns correct types" do
+    expect(post_action_type_view.flag_types).to eq(
+      {
+        illegal: 10,
+        inappropriate: 4,
+        notify_moderators: 7,
+        notify_user: 6,
+        off_topic: 3,
+        spam: 8,
+      },
+    )
+    expect(post_action_type_view.public_types).to eq({ like: 2 })
+
+    expect(post_action_type_view.notify_flag_types).to eq(
+      { illegal: 10, inappropriate: 4, notify_moderators: 7, off_topic: 3, spam: 8 },
+    )
+
+    expect(post_action_type_view.topic_flag_types).to eq(
+      { illegal: 10, inappropriate: 4, notify_moderators: 7, spam: 8 },
+    )
+
+    expect(post_action_type_view.additional_message_types).to eq(
+      { illegal: 10, notify_moderators: 7, notify_user: 6 },
+    )
+    expect(post_action_type_view.score_types).to eq({ needs_approval: 9 })
+
+    flag = Fabricate(:flag, name: "custom", enabled: false)
+    expect(PostActionTypeView.new.disabled_flag_types).to eq({ custom: flag.id })
+    flag.destroy!
+  end
+
+  it "defines names of flags" do
+    expect(post_action_type_view.names).to eq(
+      {
+        6 => "notify_user",
+        3 => "off_topic",
+        4 => "inappropriate",
+        8 => "spam",
+        10 => "illegal",
+        7 => "notify_moderators",
+        9 => "needs_approval",
+        2 => "like",
+      },
+    )
+  end
+
+  it "defines descriptions of flags" do
+    flag = Fabricate(:flag, enabled: false, description: "custom flag description")
+    expect(post_action_type_view.descriptions[flag.id]).to eq("custom flag description")
+    flag.destroy!
+  end
+
+  it "defines where flags can be applies to" do
+    expect(post_action_type_view.applies_to).to eq(
+      {
+        6 => %w[Post Chat::Message],
+        3 => %w[Post Chat::Message],
+        4 => %w[Post Topic Chat::Message],
+        8 => %w[Post Topic Chat::Message],
+        10 => %w[Post Topic Chat::Message],
+        7 => %w[Post Topic Chat::Message],
+        9 => [],
+        2 => ["Post"],
+      },
+    )
+  end
+
+  it "defines is post action type is a flag" do
+    expect(post_action_type_view.is_flag?(:like)).to be false
+    expect(post_action_type_view.is_flag?(:off_topic)).to be true
+  end
+end

spec/models/flag_spec.rb

@@ -70,4 +70,30 @@ RSpec.describe Flag, type: :model do
       %i[notify_user off_topic inappropriate spam illegal notify_moderators needs_approval],
     )
   end
+
+  describe ".used_flag_ids" do
+    fab!(:post_action) { Fabricate(:post_action, post_action_type_id: PostActionType.types[:like]) }
+    fab!(:post_action_2) do
+      Fabricate(:post_action, post_action_type_id: PostActionType.types[:like])
+    end
+    fab!(:post_action_3) do
+      Fabricate(:post_action, post_action_type_id: PostActionType.types[:off_topic])
+    end
+    fab!(:reviewable_score) do
+      Fabricate(:reviewable_score, reviewable_score_type: PostActionType.types[:off_topic])
+    end
+    fab!(:reviewable_score_2) do
+      Fabricate(:reviewable_score, reviewable_score_type: PostActionType.types[:illegal])
+    end
+
+    it "returns an array of unique flag ids" do
+      expect(Flag.used_flag_ids).to eq(
+        [
+          PostActionType.types[:like],
+          PostActionType.types[:off_topic],
+          PostActionType.types[:illegal],
+        ],
+      )
+    end
+  end
 end

spec/models/post_action_type_spec.rb

@@ -4,17 +4,15 @@ RSpec.describe PostActionType do
   describe "Callbacks" do
     describe "#expiry_cache" do
       it "should clear the cache on save" do
-        cache = ApplicationSerializer.fragment_cache
-
-        cache["post_action_types_#{I18n.locale}"] = "test"
-        cache["post_action_flag_types_#{I18n.locale}"] = "test2"
+        Discourse.cache.write("post_action_types_#{I18n.locale}", "test")
+        Discourse.cache.write("post_action_flag_types_#{I18n.locale}", "test2")
 
         PostActionType.new(name_key: "some_key").save!
 
-        expect(cache["post_action_types_#{I18n.locale}"]).to eq(nil)
-        expect(cache["post_action_flag_types_#{I18n.locale}"]).to eq(nil)
+        expect(Discourse.cache.read("post_action_types_#{I18n.locale}")).to eq(nil)
+        expect(Discourse.cache.read("post_action_flag_types_#{I18n.locale}")).to eq(nil)
       ensure
-        ApplicationSerializer.fragment_cache.clear
+        PostActionType.new.expire_cache
       end
     end
   end
@@ -30,7 +28,9 @@ RSpec.describe PostActionType do
   end
 
   describe ".additional_message_types" do
-    before { described_class.stubs(:overridden_by_plugin_or_skipped_db?).returns(overriden) }
+    before do
+      PostActionTypeView.any_instance.stubs(:overridden_by_plugin_or_skipped_db?).returns(overriden)
+    end
 
     context "when overridden by plugin or skipped DB" do
       let(:overriden) { true }

spec/multisite/flags_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe "Custom flags in multisite", type: :multisite do
+  describe "PostACtionType#all_flags" do
+    use_redis_snapshotting
+
+    it "does not share flag definitions between sites" do
+      flag_1 = Flag.create!(name: "test flag 1", position: 99, applies_to: ["Post"])
+
+      test_multisite_connection("second") do
+        flag_2 = Flag.create!(name: "test flag 2", position: 99, applies_to: ["Post"])
+        PostActionType.new.expire_cache
+        expect(PostActionType.all_flags.last).to eq(
+          flag_2.attributes.except("created_at", "updated_at").transform_keys(&:to_sym),
+        )
+      end
+
+      PostActionType.new.expire_cache
+      expect(PostActionType.all_flags.last).to eq(
+        flag_1.attributes.except("created_at", "updated_at").transform_keys(&:to_sym),
+      )
+    end
+  end
+end

spec/requests/api/schemas/json/site_response.json

@@ -362,6 +362,9 @@
             },
             "is_used": {
               "type": "boolean"
+            },
+            "position": {
+              "type": "integer"
             }
           },
           "required": [
@@ -414,6 +417,9 @@
             },
             "is_used": {
               "type": "boolean"
+            },
+            "position": {
+              "type": "integer"
             }
           },
           "required": [

spec/serializers/flag_serializer_spec.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+RSpec.describe FlagSerializer do
+  let(:flag) { Flag.find_by(name: "illegal") }
+
+  context "when system flag" do
+    it "returns translated name" do
+      serialized = described_class.new(flag, used_flag_ids: []).as_json
+      expect(serialized[:flag][:name]).to eq(I18n.t("post_action_types.illegal.title"))
+    end
+
+    it "returns translated description" do
+      serialized = described_class.new(flag, used_flag_ids: []).as_json
+      expect(serialized[:flag][:description]).to eq(I18n.t("post_action_types.illegal.description"))
+    end
+  end
+
+  context "when custom flag" do
+    fab!(:flag) { Fabricate(:flag, name: "custom title", description: "custom description") }
+
+    it "returns translated name" do
+      serialized = described_class.new(flag, used_flag_ids: []).as_json
+      expect(serialized[:flag][:name]).to eq("custom title")
+    end
+
+    it "returns translated description" do
+      serialized = described_class.new(flag, used_flag_ids: []).as_json
+      expect(serialized[:flag][:description]).to eq("custom description")
+    end
+  end
+
+  it "returns is_flag true for flags" do
+    serialized = described_class.new(flag, used_flag_ids: []).as_json
+    expect(serialized[:flag][:is_flag]).to be true
+  end
+
+  it "returns is_flag false for like" do
+    flag = Flag.unscoped.find_by(name: "like")
+    serialized = described_class.new(flag, used_flag_ids: []).as_json
+    expect(serialized[:flag][:is_flag]).to be false
+  end
+
+  it "returns is_used false when not used" do
+    serialized = described_class.new(flag, used_flag_ids: []).as_json
+    expect(serialized[:flag][:is_used]).to be false
+  end
+
+  it "returns is_used true when used" do
+    serialized = described_class.new(flag, used_flag_ids: [flag.id]).as_json
+    expect(serialized[:flag][:is_used]).to be true
+  end
+
+  it "returns applies_to" do
+    serialized = described_class.new(flag, used_flag_ids: []).as_json
+    expect(serialized[:flag][:applies_to]).to eq(%w[Post Topic Chat::Message])
+  end
+end