Invalidate ETag cache when note changes

app/models/note.rb

@@ -85,6 +85,7 @@ class Note < ActiveRecord::Base
   before_validation :nullify_blank_type, :nullify_blank_line_code
   before_validation :set_discussion_id
   after_save :keep_around_commit, unless: :for_personal_snippet?
+  after_save :expire_etag_cache
 
   class << self
     def model_name
@@ -272,4 +273,16 @@ class Note < ActiveRecord::Base
       self.class.build_discussion_id(noteable_type, noteable_id || commit_id)
     end
   end
+
+  def expire_etag_cache
+    return unless for_issue?
+
+    key = Gitlab::Routing.url_helpers.namespace_project_noteable_notes_path(
+      noteable.project.namespace,
+      noteable.project,
+      target_type: noteable_type.underscore,
+      target_id: noteable.id
+    )
+    Gitlab::EtagCaching::Store.new.touch(key)
+  end
 end

spec/models/note_spec.rb

@@ -387,4 +387,16 @@ describe Note, models: true do
       end
     end
   end
+
+  describe 'expiring ETag cache' do
+    let(:note) { build(:note_on_issue) }
+
+    it "expires cache for note's issue when note is saved" do
+      expect_any_instance_of(Gitlab::EtagCaching::Store)
+        .to receive(:touch)
+        .with("/#{note.project.namespace.to_param}/#{note.project.to_param}/noteable/issue/#{note.noteable.id}/notes")
+
+      note.save!
+    end
+  end
 end