Add post-deploy migrate to cleanup projects in pending delete state

There are many projects in `pending_delete` state, this post-deploy migration
cleans them up.

The script is based on https://gitlab.com/gitlab-org/gitlab-ce/snippets/1648654
and https://gitlab.com/gitlab-org/gitlab-ce/snippets/1611429. The use of these
scripts were described in
https://gitlab.com/gitlab-com/infrastructure/issues/888.

changelogs/unreleased/tc-clean-pending-delete-projects.yml

+---
+title: Add post-deploy migration to clean up projects in `pending_delete` state
+merge_request: 11044
+author:

db/post_migrate/20170502101023_clean_up_pending_delete_projects.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class CleanUpPendingDeleteProjects < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    admin = User.find_by(admin: true)
+    return unless admin
+
+    Project.unscoped.where(pending_delete: true).each { |project| delete_project(project, admin) }
+  end
+
+  def down
+    # noop
+  end
+
+  private
+
+  def delete_project(project, user)
+    project.team.truncate
+
+    unlink_fork(project) if project.forked?
+
+    [:events, :issues, :merge_requests, :labels, :milestones, :notes, :snippets].each do |thing|
+      project.send(thing).delete_all
+    end
+
+    # Override Project#remove_pages for this instance so it doesn't do anything
+    def project.remove_pages
+    end
+
+    project.destroy!
+  end
+
+  def unlink_fork(project)
+    merge_requests = project.forked_from_project.merge_requests.opened.from_project(project)
+
+    merge_requests.update_all(state: 'closed')
+
+    project.forked_project_link.destroy
+  end
+end

spec/migrations/clean_up_pending_delete_projects_spec.rb

+# encoding: utf-8
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20170502101023_clean_up_pending_delete_projects.rb')
+
+describe CleanUpPendingDeleteProjects do
+  let(:migration) { described_class.new }
+  let!(:admin) { create(:admin) }
+  let!(:project) { create(:empty_project, pending_delete: true) }
+
+  describe '#up' do
+    it 'only cleans up pending delete projects' do
+      create(:empty_project)
+
+      expect do
+        migration.up
+      end.to change { Project.unscoped.count }.by(-1)
+    end
+
+    it "truncates the project's team" do
+      project.add_master(admin)
+
+      expect_any_instance_of(ProjectTeam).to receive(:truncate)
+
+      migration.up
+    end
+
+    it 'calls Project#destroy!' do
+      expect_any_instance_of(Project).to receive(:destroy!)
+
+      migration.up
+    end
+
+    it 'does not do anything in Project#remove_pages method' do
+      expect(Gitlab::PagesTransfer).not_to receive(:new)
+
+      migration.up
+    end
+
+    context 'project not a fork of another project' do
+      it "doesn't call unlink_fork" do
+        expect(migration).not_to receive(:unlink_fork)
+
+        migration.up
+      end
+    end
+
+    context 'project forked from another' do
+      let!(:parent_project) { create(:empty_project) }
+
+      before do
+        create(:forked_project_link, forked_to_project: project, forked_from_project: parent_project)
+      end
+
+      it 'closes open merge requests' do
+        project.update_attribute(:pending_delete, false) # needed to create the MR
+        merge_request = create(:merge_request, source_project: project, target_project: parent_project)
+        project.update_attribute(:pending_delete, true)
+
+        migration.up
+
+        expect(merge_request.reload).to be_closed
+      end
+
+      it 'destroys the link' do
+        migration.up
+
+        expect(parent_project.forked_project_links).to be_empty
+      end
+    end
+  end
+end