Fix timeouts when destroying a project with many notes

Generally, we like to use foreign key associations to destroy dependent
records. Where this is possible, it's faster and uses fewer resources
than doing it in GitLab.

In 028db84c , the notes association was
converted from `dependent: :destroy` to using foreign keys. However,
when destroying a specific project with > 60K notes, we're experiencing
statement timeouts in the staging environment. Additionally, notes have
a deprecated `attachments` uploader that requires Ruby intervention to
avoid orphaning files on disk - there are at least some notes on
GitLab.com that still use this deprecated uploader.

Since 2017, we've introduced the BatchDestroyDependentAssociations
concern, which takes care of processing large numbers of dependent
records in batches. Re-introducing `dependent: :destroy` on notes will
automatically make use of this code.

Changelog: fixed

app/models/note.rb

@@ -96,7 +96,9 @@ class Note < ApplicationRecord
 
   validate :does_not_exceed_notes_limit?, on: :create, unless: [:system?, :importing?]
 
-  # @deprecated attachments are handler by the MarkdownUploader
+  # @deprecated attachments are handled by the Upload model.
+  #
+  # https://gitlab.com/gitlab-org/gitlab/-/issues/20830
   mount_uploader :attachment, AttachmentUploader
 
   # Scopes

app/models/project.rb

@@ -260,7 +260,15 @@ class Project < ApplicationRecord
   has_many :events
   has_many :milestones
   has_many :iterations
-  has_many :notes
+
+  # Projects with a very large number of notes may time out destroying them
+  # through the foreign key. Additionally, the deprecated attachment uploader
+  # for notes requires us to use dependent: :destroy to avoid orphaning uploaded
+  # files.
+  #
+  # https://gitlab.com/gitlab-org/gitlab/-/issues/207222
+  has_many :notes, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
+
   has_many :snippets, class_name: 'ProjectSnippet'
   has_many :hooks, class_name: 'ProjectHook'
   has_many :protected_branches

spec/models/note_spec.rb

@@ -1522,4 +1522,16 @@ RSpec.describe Note do
       it { is_expected.to be_truthy }
     end
   end
+
+  describe '#attachment' do
+    it 'is cleaned up correctly when project is destroyed' do
+      note = create(:note_on_issue, :with_attachment)
+
+      attachment = note.attachment
+
+      note.project.destroy!
+
+      expect(attachment).not_to be_exist
+    end
+  end
 end

spec/models/project_spec.rb

@@ -28,7 +28,7 @@ RSpec.describe Project, factory_default: :keep do
     it { is_expected.to have_many(:project_members).dependent(:delete_all) }
     it { is_expected.to have_many(:users).through(:project_members) }
     it { is_expected.to have_many(:requesters).dependent(:delete_all) }
-    it { is_expected.to have_many(:notes) }
+    it { is_expected.to have_many(:notes).dependent(:destroy) }
     it { is_expected.to have_many(:snippets).class_name('ProjectSnippet') }
     it { is_expected.to have_many(:deploy_keys_projects) }
     it { is_expected.to have_many(:deploy_keys) }