Add post-migration to drain all Geo related redis queues

Since gitlab-org/gitlab-ee!2644 is merged, some redis queues are no
longer used. This post-migration drains these queues.

Closes gitlab-org/gitlab-ee#3373.

changelogs/unreleased-ee/tc-geo-drain-queues.yml

+---
+title: Add post-migration to drain all Geo related redis queues
+merge_request: 3289
+author:
+type: fixed

db/post_migrate/20171103152048_geo_drain_redis_queues.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class GeoDrainRedisQueues < ActiveRecord::Migration
+  DOWNTIME = false
+  GEO_NAMESPACE = 'geo:gitlab'.freeze
+
+  disable_ddl_transaction!
+
+  def up
+    Gitlab::Redis::SharedState.with do |redis|
+      # Delete everything from Geo namespace
+      matched_keys = redis.scan_each(match: "#{GEO_NAMESPACE}*").to_a
+      redis.del(*matched_keys) if matched_keys.any?
+
+      # Delete the keys related to GeoBulkNotifyWorker (which is removed in gitlab-org/gitlab-ee!2644)
+      redis.del('resque:gitlab:cron_job:geo_bulk_notify_worker')
+      redis.del('resque:gitlab:cron_job:geo_bulk_notify_worker:enqueued')
+    end
+  end
+
+  def down
+    # noop
+  end
+end

spec/migrations/geo_drain_redis_queues_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20171103152048_geo_drain_redis_queues')
+
+describe GeoDrainRedisQueues, :clean_gitlab_redis_shared_state do
+  subject(:migration) { described_class.new }
+
+  def exists_in_redis?(name)
+    Gitlab::Redis::SharedState.with do |redis|
+      redis.exists(name)
+    end
+  end
+
+  def set_in_redis(name, value)
+    expect(
+      Gitlab::Redis::SharedState.with do |redis|
+        redis.set(name, value)
+      end
+    ).to eq("OK")
+  end
+
+  describe '#up' do
+    it 'deletes everything in the "geo:gitlab" namespace' do
+      set_in_redis('geo:gitlab:foo', 'bar')
+      set_in_redis('bar:gitlab:foo', 'bar')
+
+      migration.up
+
+      expect(exists_in_redis?('geo:gitlab:foo')).to be_falsy
+      expect(exists_in_redis?('bar:gitlab:foo')).to be_truthy
+    end
+
+    it 'deletes "resque:gitlab:cron_job:geo_bulk_notify_worker"' do
+      set_in_redis('resque:gitlab:cron_job:geo_bulk_notify_worker', 'bar')
+      set_in_redis('resque:gitlab:cron_job:geo_bulk_notify_worker:enqueued', 'bar')
+      set_in_redis('resque:gitlab:cron_job:other_worker', 'bar')
+
+      migration.up
+
+      expect(exists_in_redis?('resque:gitlab:cron_job:geo_bulk_notify_worker:enqueued')).to be_falsy
+      expect(exists_in_redis?('resque:gitlab:cron_job:geo_bulk_notify_worker')).to be_falsy
+      expect(exists_in_redis?('resque:gitlab:cron_job:other_worker')).to be_truthy
+    end
+  end
+end