Track state change events for Epics

Instead of creating system notes, we should use resource state events
just like we do with issues and merge requests

app/models/resource_state_event.rb

@@ -19,3 +19,5 @@ class ResourceStateEvent < ResourceEvent
     issue || merge_request
   end
 end
+
+ResourceStateEvent.prepend_if_ee('EE::ResourceStateEvent')

app/services/resource_events/change_state_service.rb

@@ -13,14 +13,14 @@ module ResourceEvents
 
       ResourceStateEvent.create(
         user: user,
-        issue: issue,
-        merge_request: merge_request,
+        resource.class.underscore => resource,
         source_commit: commit_id_of(mentionable_source),
         source_merge_request_id: merge_request_id_of(mentionable_source),
         state: ResourceStateEvent.states[state],
         close_after_error_tracking_resolve: close_after_error_tracking_resolve,
         close_auto_resolve_prometheus_alert: close_auto_resolve_prometheus_alert,
-        created_at: Time.zone.now)
+        created_at: Time.zone.now
+      )
 
       resource.expire_note_etag_cache
     end
@@ -56,17 +56,5 @@ module ResourceEvents
 
       mentionable_source.id
     end
-
-    def issue
-      return unless resource.is_a?(Issue)
-
-      resource
-    end
-
-    def merge_request
-      return unless resource.is_a?(MergeRequest)
-
-      resource
-    end
   end
 end

ee/app/models/ee/epic.rb

@@ -12,6 +12,7 @@ module EE
       include Referable
       include Awardable
       include LabelEventable
+      include StateEventable
       include UsageStatistics
       include FromUnion
       include EpicTreeSorting

ee/app/models/ee/resource_state_event.rb

+# frozen_string_literal: true
+
+module EE
+  module ResourceStateEvent
+    extend ActiveSupport::Concern
+    extend ::Gitlab::Utils::Override
+
+    prepended do
+      belongs_to :epic
+    end
+
+    class_methods do
+      def issuable_attrs
+        %i(epic).freeze + super
+      end
+    end
+
+    override :issuable
+    def issuable
+      epic || super
+    end
+  end
+end

ee/changelogs/unreleased/enable-state-event-tracking-for-epics.yml

+---
+title: Track state changes using resource state events for Epics
+merge_request: 42088
+author:
+type: fixed

ee/spec/models/ee/resource_state_event_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ResourceStateEvent do
+  subject { build(:resource_state_event) }
+
+  it { is_expected.to belong_to(:epic) }
+
+  describe 'validations' do
+    describe 'Issuable validation' do
+      it 'is valid if only epic is set' do
+        subject.attributes = { epic: build_stubbed(:epic), issue: nil, merge_request: nil }
+
+        expect(subject).to be_valid
+      end
+
+      it 'is invalid if an epic and an issue is set' do
+        subject.attributes = { epic: build_stubbed(:epic), issue: build_stubbed(:issue), merge_request: nil }
+
+        expect(subject).not_to be_valid
+      end
+    end
+  end
+end

ee/spec/services/epics/close_service_spec.rb

@@ -2,9 +2,9 @@
 require 'spec_helper'
 
 RSpec.describe Epics::CloseService do
-  let(:group) { create(:group, :internal) }
-  let(:user) { create(:user) }
-  let(:epic) { create(:epic, group: group) }
+  let_it_be(:group) { create(:group, :internal) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:epic, reload: true) { create(:epic, group: group) }
 
   describe '#execute' do
     subject { described_class.new(group, user) }
@@ -25,7 +25,7 @@ RSpec.describe Epics::CloseService do
       end
 
       context 'when a user has permissions to update the epic' do
-        before do
+        before_all do
           group.add_maintainer(user)
         end
 
@@ -42,13 +42,33 @@ RSpec.describe Epics::CloseService do
             expect { subject.execute(epic) }.to change { epic.closed_at }
           end
 
-          it 'creates a system note about epic close' do
-            expect { subject.execute(epic) }.to change { epic.notes.count }.by(1)
+          context 'when state event tracking is enabled' do
+            before do
+              stub_feature_flags(track_resource_state_change_events: true)
+            end
 
-            note = epic.notes.last
+            it 'creates a resource state event' do
+              expect { subject.execute(epic) }.to change { epic.resource_state_events.count }.by(1)
 
-            expect(note.note).to eq('closed')
-            expect(note.system_note_metadata.action).to eq('closed')
+              event = epic.resource_state_events.last
+
+              expect(event.state).to eq('closed')
+            end
+          end
+
+          context 'when state event tracking is disabled' do
+            before do
+              stub_feature_flags(track_resource_state_change_events: false)
+            end
+
+            it 'creates a system note about epic close' do
+              expect { subject.execute(epic) }.to change { epic.notes.count }.by(1)
+
+              note = epic.notes.last
+
+              expect(note.note).to eq('closed')
+              expect(note.system_note_metadata.action).to eq('closed')
+            end
           end
 
           it 'notifies the subscribers' do
@@ -82,8 +102,8 @@ RSpec.describe Epics::CloseService do
             expect { subject.execute(epic) }.not_to change { epic.closed_by }
           end
 
-          it 'does not create a system note' do
-            expect { subject.execute(epic) }.not_to change { epic.notes.count }
+          it 'does not create a resource state event' do
+            expect { subject.execute(epic) }.not_to change { epic.resource_state_events.count }
           end
 
           it 'does not send any emails' do

ee/spec/services/epics/reopen_service_spec.rb

@@ -2,9 +2,9 @@
 require 'spec_helper'
 
 RSpec.describe Epics::ReopenService do
-  let(:group) { create(:group, :internal) }
-  let(:user) { create(:user) }
-  let(:epic) { create(:epic, group: group, state: :closed, closed_at: Date.today, closed_by: user) }
+  let_it_be(:group) { create(:group, :internal) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:epic, reload: true) { create(:epic, group: group, state: :closed, closed_at: Date.today, closed_by: user) }
 
   describe '#execute' do
     subject { described_class.new(group, user) }
@@ -25,7 +25,7 @@ RSpec.describe Epics::ReopenService do
       end
 
       context 'when a user has permissions to update the epic' do
-        before do
+        before_all do
           group.add_maintainer(user)
         end
 
@@ -42,13 +42,33 @@ RSpec.describe Epics::ReopenService do
             expect { subject.execute(epic) }.to change { epic.closed_at }.to(nil)
           end
 
-          it 'creates a system note about epic reopen' do
-            expect { subject.execute(epic) }.to change { epic.notes.count }.by(1)
+          context 'when state event tracking is enabled' do
+            before do
+              stub_feature_flags(track_resource_state_change_events: true)
+            end
 
-            note = epic.notes.last
+            it 'creates a resource state event' do
+              expect { subject.execute(epic) }.to change { epic.resource_state_events.count }.by(1)
 
-            expect(note.note).to eq('opened')
-            expect(note.system_note_metadata.action).to eq('opened')
+              event = epic.resource_state_events.last
+
+              expect(event.state).to eq('opened')
+            end
+          end
+
+          context 'when state event tracking is disabled' do
+            before do
+              stub_feature_flags(track_resource_state_change_events: false)
+            end
+
+            it 'creates a system note about epic reopen' do
+              expect { subject.execute(epic) }.to change { epic.notes.count }.by(1)
+
+              note = epic.notes.last
+
+              expect(note.note).to eq('opened')
+              expect(note.system_note_metadata.action).to eq('opened')
+            end
           end
 
           it 'notifies the subscribers' do
@@ -82,8 +102,8 @@ RSpec.describe Epics::ReopenService do
             expect { subject.execute(epic) }.not_to change { epic.closed_by }
           end
 
-          it 'does not create a system note' do
-            expect { subject.execute(epic) }.not_to change { epic.notes.count }
+          it 'does not create a resource state event' do
+            expect { subject.execute(epic) }.not_to change { epic.resource_state_events.count }
           end
 
           it 'does not send any emails' do

spec/lib/gitlab/import_export/all_models.yml

@@ -688,6 +688,7 @@ epic:
 - due_date_sourcing_epic
 - events
 - resource_label_events
+- resource_state_events
 - user_mentions
 - note_authors
 - boards_epic_user_preferences

spec/models/resource_state_event_spec.rb

@@ -11,4 +11,32 @@ RSpec.describe ResourceStateEvent, type: :model do
   it_behaves_like 'a resource event'
   it_behaves_like 'a resource event for issues'
   it_behaves_like 'a resource event for merge requests'
+
+  describe 'validations' do
+    describe 'Issuable validation' do
+      it 'is valid if an issue is set' do
+        subject.attributes = { issue: build_stubbed(:issue), merge_request: nil }
+
+        expect(subject).to be_valid
+      end
+
+      it 'is valid if a merge request is set' do
+        subject.attributes = { issue: nil, merge_request: build_stubbed(:merge_request) }
+
+        expect(subject).to be_valid
+      end
+
+      it 'is invalid if both issue and merge request are set' do
+        subject.attributes = { issue: build_stubbed(:issue), merge_request: build_stubbed(:merge_request) }
+
+        expect(subject).not_to be_valid
+      end
+
+      it 'is invalid if there is no issuable set' do
+        subject.attributes = { issue: nil, merge_request: nil }
+
+        expect(subject).not_to be_valid
+      end
+    end
+  end
 end