Fix cross-database modification when resetting CI minutes

Move the recalculation of additional minutes, which touches
`namespaces` in a different database, to an after commit block.

Changelog: fixed

ee/app/models/ci/minutes/namespace_monthly_usage.rb

@@ -6,6 +6,7 @@ module Ci
     # This class ensures that we keep 1 record per namespace per month.
     class NamespaceMonthlyUsage < Ci::ApplicationRecord
       include Ci::NamespacedModelName
+      include AfterCommitQueue
 
       belongs_to :namespace
 
@@ -29,10 +30,17 @@ module Ci
         current_usage = unsafe_find_current(namespace_id)
         return current_usage if current_usage
 
-        current_month.for_namespace(namespace_id).create!.tap do
-          Namespace.find_by_id(namespace_id).try do |namespace|
-            Ci::Minutes::Limit.new(namespace).recalculate_remaining_purchased_minutes!
+        current_month.for_namespace(namespace_id).new.tap do |new_usage|
+          # TODO: Remove in https://gitlab.com/gitlab-org/gitlab/-/issues/350617
+          # Avoid cross-database modifications in transaction since
+          # recalculation of purchased minutes touches `namespaces` table.
+          new_usage.run_after_commit do
+            Namespace.find_by_id(namespace_id).try do |namespace|
+              Ci::Minutes::Limit.new(namespace).recalculate_remaining_purchased_minutes!
+            end
           end
+
+          new_usage.save!
         end
       rescue ActiveRecord::RecordNotUnique
         unsafe_find_current(namespace_id)

ee/spec/models/ci/minutes/namespace_monthly_usage_spec.rb

@@ -133,6 +133,33 @@ RSpec.describe Ci::Minutes::NamespaceMonthlyUsage do
         it_behaves_like 'attempts recalculation of additional minutes'
       end
 
+      context 'when inside a transaction in ci database' do
+        let!(:previous_usage) do
+          create(:ci_namespace_monthly_usage,
+            namespace: namespace,
+            date: described_class.beginning_of_month(3.months.ago))
+        end
+
+        let(:project) { create(:project, namespace: namespace) }
+        let(:pipeline) { create(:ci_pipeline, project: project) }
+        let(:build) { create(:ci_build, :created, pipeline: pipeline) }
+
+        before do
+          namespace.clear_memoization(:ci_minutes_quota)
+          create(:ci_runner, :instance_type)
+        end
+
+        subject do
+          pipeline.transaction do
+            pipeline.touch
+            Ci::Minutes::NamespaceMonthlyUsage.find_or_create_current(namespace_id: namespace.id)
+          end
+        end
+
+        it_behaves_like 'creates usage record'
+        it_behaves_like 'attempts recalculation of additional minutes'
+      end
+
       context 'when last known usage is more than 1 month ago' do
         let!(:previous_usage) do
           create(:ci_namespace_monthly_usage,