Add Redis healthcheck

This was done in response to the work to enable continuous deployment
for local-links-manager. Given local-links-manager uses Redis on it's
own, without sidekiq, similarly to email-alert-frontend, it made sense
to move the recently written healthcheck by @1pretz1 to govuk-app-config
so it could be used by both applications.

Co-authored-by: Peter Hartshorn
<[email protected]>

Trello: https://trello.com/c/uAx6eKAQ/2276-activate-continuous-deployment-for-local-links-manager

lib/govuk_app_config/govuk_healthcheck.rb

@@ -2,6 +2,7 @@
 require "govuk_app_config/govuk_healthcheck/active_record"
 require "govuk_app_config/govuk_healthcheck/mongoid"
 require "govuk_app_config/govuk_healthcheck/rails_cache"
+require "govuk_app_config/govuk_healthcheck/redis_connection"
 require "govuk_app_config/govuk_healthcheck/sidekiq_redis"
 require "govuk_app_config/govuk_healthcheck/threshold_check"
 require "govuk_app_config/govuk_healthcheck/sidekiq_queue_check"

lib/govuk_app_config/govuk_healthcheck/redis_connection.rb

@@ -0,0 +1,21 @@
+module GovukHealthcheck
+  class RedisConnection
+    def name
+      :redis_connection
+    end
+
+    def status
+      client = Redis.new
+
+      client.set("healthcheck", "val")
+      client.get("healthcheck")
+      client.del("healthcheck")
+
+      client.close
+
+      GovukHealthcheck::OK
+    rescue StandardError
+      GovukHealthcheck::CRITICAL
+    end
+  end
+end

spec/lib/govuk_healthcheck/redis_connection_spec.rb

@@ -0,0 +1,30 @@
+require "spec_helper"
+require "govuk_app_config/govuk_healthcheck"
+require_relative "shared_interface"
+
+RSpec.describe GovukHealthcheck::RedisConnection do
+  let(:redis) { double(:redis) }
+  let(:redis_client) { double(:redis_client, set: "OK", get: "val", del: 1, close: nil) }
+
+  before do
+    stub_const("Redis", redis)
+    allow(redis).to receive(:new).and_return(redis_client)
+  end
+
+  context "when the database is connected" do
+    before { allow(redis_client).to receive(:set) }
+    it_behaves_like "a healthcheck"
+    it "returns OK" do
+      expect(redis_client).to receive(:set).with(anything, anything)
+      expect(subject.status).to eq(GovukHealthcheck::OK)
+    end
+  end
+
+  context "when the database is not connected" do
+    before { allow(redis_client).to receive(:set).with(anything, anything).and_raise("error") }
+    it_behaves_like "a healthcheck"
+    it "returns CRITICAL" do
+      expect(subject.status).to eq(GovukHealthcheck::CRITICAL)
+    end
+  end
+end