Clear all connections just before db:test:purge

For some reason connections are obtained during db:test:prepare,
specifically with config/initializers/1_postgresql_only.rb, and
config/initializers/postgres_partitioning.rb, and not released when we
come to the step where we purge the test db. Hence we force all
connections to be released first.

lib/tasks/gitlab/db.rake

@@ -170,6 +170,13 @@ namespace :gitlab do
       Rake::Task['gitlab:db:create_dynamic_partitions'].invoke
     end
 
+    desc "Clear all connections"
+    task :clear_all_connections do
+      ActiveRecord::Base.clear_all_connections!
+    end
+
+    Rake::Task['db:test:purge'].enhance(['gitlab:db:clear_all_connections'])
+
     # During testing, db:test:load restores the database schema from scratch
     # which does not include dynamic partitions. We cannot rely on application
     # initializers here as the application can continue to run while

spec/tasks/gitlab/db_rake_spec.rb

@@ -20,6 +20,14 @@ RSpec.describe 'gitlab:db namespace rake task', :silence_stdout do
     allow(Rake::Task['db:seed_fu']).to receive(:invoke).and_return(true)
   end
 
+  describe 'clear_all_connections' do
+    it 'calls clear_all_connections!' do
+      expect(ActiveRecord::Base).to receive(:clear_all_connections!)
+
+      run_rake_task('gitlab:db:clear_all_connections')
+    end
+  end
+
   describe 'mark_migration_complete' do
     context 'with a single database' do
       let(:main_model) { ActiveRecord::Base }