Ensure project backfilling has finished

As we have follow up backfilling migrations coming up that
depend on project namespaces being backfilled, we need to
ensure backfilling has finalized first.

Changelog: added

db/post_migrate/20220322071127_finalize_project_namespaces_backfill.rb

+# frozen_string_literal: true
+
+# This migration acts as a gate-keeper for other migrations related to project namespace back-filling
+# so that other migrations that depend on project namespace back-filling cannot be run unless project namespace
+# back-filling has finalized successfully.
+class FinalizeProjectNamespacesBackfill < Gitlab::Database::Migration[1.0]
+  MIGRATION = 'ProjectNamespaces::BackfillProjectNamespaces'
+
+  def up
+    ensure_batched_background_migration_is_finished(
+      job_class_name: MIGRATION,
+      table_name: :projects,
+      column_name: :id,
+      job_arguments: [nil, 'up']
+    )
+  end
+
+  def down
+    # noop
+  end
+end

db/schema_migrations/20220322071127

+c7d84b6d92566d66f69732071fe7fc6c4e5ce32475e0d2c42413c9f005aed5b0
\ No newline at end of file

spec/migrations/finalize_project_namespaces_backfill_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!
+
+RSpec.describe FinalizeProjectNamespacesBackfill, :migration do
+  let(:batched_migrations) { table(:batched_background_migrations) }
+
+  let_it_be(:migration) { described_class::MIGRATION }
+
+  describe '#up' do
+    shared_examples 'raises migration not finished exception' do
+      it 'raises exception' do
+        expect { migrate! }.to raise_error(/Expected batched background migration for the given configuration to be marked as 'finished'/)
+      end
+    end
+
+    context 'when project namespace backfilling migration is missing' do
+      it 'warns migration not found' do
+        expect(Gitlab::AppLogger)
+          .to receive(:warn).with(/Could not find batched background migration for the given configuration:/)
+
+        migrate!
+      end
+    end
+
+    context 'with backfilling migration present' do
+      let!(:project_namespace_backfill) do
+        batched_migrations.create!(
+          job_class_name: 'ProjectNamespaces::BackfillProjectNamespaces',
+          table_name: :projects,
+          column_name: :id,
+          job_arguments: [nil, 'up'],
+          interval: 2.minutes,
+          min_value: 1,
+          max_value: 2,
+          batch_size: 1000,
+          sub_batch_size: 200,
+          status: 3 # finished
+        )
+      end
+
+      context 'when project namespace backfilling migration finished successfully' do
+        it 'does not raise exception' do
+          expect { migrate! }.not_to raise_error(/Expected batched background migration for the given configuration to be marked as 'finished'/)
+        end
+      end
+
+      context 'when project namespace backfilling migration is paused' do
+        using RSpec::Parameterized::TableSyntax
+
+        where(:status, :description) do
+          0 | 'paused'
+          1 | 'active'
+          4 | 'failed'
+          5 | 'finalizing'
+        end
+
+        with_them do
+          before do
+            project_namespace_backfill.update!(status: status)
+          end
+
+          it_behaves_like 'raises migration not finished exception'
+        end
+      end
+    end
+  end
+end