Add rake task to remove Redis namespacing

In Sidekiq 7, `redis_namespace` is no longer supported. We therefore
need to run a migration at the point of upgrading to remove the
namespace from existing Redis keys.

If we don't run the migration, already queued jobs won't get processed.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * Switch from using `redis` gem to `redis-client`
 * BREAKING: Remove `redis-namespace` dependency and support for Redis namespaces
+  * Run the `redis_namespace:remove_namespace` rake task immediately after upgrading to to this version, to retain existing queued jobs.
 * BREAKING: Upgrade Sidekiq to version 7.0, follow these steps to upgrade:
   1. `Sidekiq::Worker` has been deprecated in Sidekiq 7. Replace all instances of `Sidekiq::Worker` with `Sidekiq::Job`, then rename/move your workers to be `app/sidekiq/MyJob.rb` instead of `app/workers/MyWorker.rb`.
   1. Remove the requirement for Sidekiq strict arguments from `config/initializers/sidekiq.rb`. This was added to include Sidekiq 7 strict arguments behaviour in Sidekiq 6, but is no longer needed to be explictly required, since this is now the default behaviour.

lib/govuk_sidekiq/railtie.rb

@@ -7,5 +7,10 @@ class Railtie < Rails::Railtie
         { url: ENV.fetch("REDIS_URL", "redis://127.0.0.1:6379") },
       )
     end
+
+    rake_tasks do
+      path = File.expand_path(__dir__)
+      Dir.glob("#{path}/tasks/**/*.rake").each { |f| load f }
+    end
   end
 end

lib/govuk_sidekiq/tasks/redis_namespace.rake

@@ -0,0 +1,15 @@
+namespace :redis_namespace do
+  desc "Remove application specific namespacing for all Redis keys"
+  task :remove_namespace, :environment do
+    namespace = ENV["GOVUK_APP_NAME"]
+
+    redis = RedisClient.new(url: ENV["REDIS_URL"])
+
+    namespaced_keys = redis.call("KEYS", "#{namespace}:*")
+
+    namespaced_keys.each do |key|
+      new_key = key.gsub(/^#{namespace}:/, "")
+      redis.call("RENAME", key, new_key)
+    end
+  end
+end