Add rake task for batched bg migration status

Add a rake task which prints the status of all batched background
migrations to the console, to simplify instructions for finding
migrations which haven't completed.

lib/tasks/gitlab/background_migrations.rake

@@ -2,6 +2,7 @@
 
 namespace :gitlab do
   namespace :background_migrations do
+    desc 'Synchronously finish executing a batched background migration'
     task :finalize, [:job_class_name, :table_name, :column_name, :job_arguments] => :environment do |_, args|
       [:job_class_name, :table_name, :column_name, :job_arguments].each do |argument|
         unless args[argument]
@@ -19,5 +20,23 @@ namespace :gitlab do
 
       puts "Done.".color(:green)
     end
+
+    desc 'Display the status of batched background migrations'
+    task status: :environment do
+      statuses = Gitlab::Database::BackgroundMigration::BatchedMigration.statuses
+      max_status_length = statuses.keys.map(&:length).max
+      format_string = "%-#{max_status_length}s | %s\n"
+
+      Gitlab::Database::BackgroundMigration::BatchedMigration.find_each(batch_size: 100) do |migration|
+        identification_fields = [
+          migration.job_class_name,
+          migration.table_name,
+          migration.column_name,
+          migration.job_arguments.to_json
+        ].join(',')
+
+        printf(format_string, migration.status, identification_fields)
+      end
+    end
   end
 end

spec/factories/gitlab/database/background_migration/batched_migrations.rb

@@ -12,5 +12,13 @@ FactoryBot.define do
     sequence(:job_arguments) { |n| [["column_#{n}"], ["column_#{n}_convert_to_bigint"]] }
     total_tuple_count { 10_000 }
     pause_ms { 100 }
+
+    trait :finished do
+      status { :finished }
+    end
+
+    trait :failed do
+      status { :failed }
+    end
   end
 end

spec/tasks/gitlab/background_migrations_rake_spec.rb

+# frozen_string_literal: true
+
+require 'rake_helper'
+
+RSpec.describe 'gitlab:background_migrations namespace rake tasks' do
+  before do
+    Rake.application.rake_require 'tasks/gitlab/background_migrations'
+  end
+
+  describe 'finalize' do
+    subject(:finalize_task) { run_rake_task('gitlab:background_migrations:finalize', *arguments) }
+
+    context 'without the proper arguments' do
+      let(:arguments) { %w[CopyColumnUsingBackgroundMigrationJob events id] }
+
+      it 'exits without finalizing the migration' do
+        expect(Gitlab::Database::BackgroundMigration::BatchedMigrationRunner).not_to receive(:finalize)
+
+        expect { finalize_task }.to output(/Must specify job_arguments as an argument/).to_stdout
+          .and raise_error(SystemExit) { |error| expect(error.status).to eq(1) }
+      end
+    end
+
+    context 'with the proper arguments' do
+      let(:arguments) { %w[CopyColumnUsingBackgroundMigrationJob events id [["id1"\,"id2"]]] }
+
+      it 'finalizes the matching migration' do
+        expect(Gitlab::Database::BackgroundMigration::BatchedMigrationRunner).to receive(:finalize)
+          .with('CopyColumnUsingBackgroundMigrationJob', 'events', 'id', [%w[id1 id2]])
+
+        expect { finalize_task }.to output(/Done/).to_stdout
+      end
+    end
+  end
+
+  describe 'status' do
+    subject(:status_task) { run_rake_task('gitlab:background_migrations:status') }
+
+    it 'outputs the status of background migrations' do
+      migration1 = create(:batched_background_migration, :finished, job_arguments: [%w[id1 id2]])
+      migration2 = create(:batched_background_migration, :failed, job_arguments: [])
+
+      expect { status_task }.to output(<<~OUTPUT).to_stdout
+        finished   | #{migration1.job_class_name},#{migration1.table_name},#{migration1.column_name},[["id1","id2"]]
+        failed     | #{migration2.job_class_name},#{migration2.table_name},#{migration2.column_name},[]
+      OUTPUT
+    end
+  end
+end