Handle cache invalidation events on Geo secondary

Expires the cache of the given key when the secondary node
gets a Geo::CacheInvalidationEvent.

ee/lib/gitlab/geo/log_cursor/events/cache_invalidation_event.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Geo
+    module LogCursor
+      module Events
+        class CacheInvalidationEvent
+          include BaseEvent
+
+          def process
+            result = expire_cache_for_event_key
+            log_cache_invalidation_event(result)
+          end
+
+          private
+
+          def expire_cache_for_event_key
+            Rails.cache.delete(event.key)
+          end
+
+          def log_cache_invalidation_event(expired)
+            logger.event_info(
+              created_at,
+              'Cache invalidation',
+              cache_key: event.key,
+              cache_expired: expired,
+              skippable: skippable?
+            )
+          end
+
+          def skippable?
+            false
+          end
+        end
+      end
+    end
+  end
+end

ee/spec/lib/gitlab/geo/log_cursor/events/cache_invalidation_event_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Geo::LogCursor::Events::CacheInvalidationEvent, :postgresql, :clean_gitlab_redis_shared_state do
+  let(:logger) { Gitlab::Geo::LogCursor::Logger.new(described_class, Logger::INFO) }
+  let(:event_log) { create(:geo_event_log, :cache_invalidation_event) }
+  let!(:event_log_state) { create(:geo_event_log_state, event_id: event_log.id - 1) }
+  let(:cache_invalidation_event) { event_log.cache_invalidation_event }
+  let(:cache_key) { cache_invalidation_event.key }
+
+  subject { described_class.new(cache_invalidation_event, Time.now, logger) }
+
+  around do |example|
+    Sidekiq::Testing.fake! { example.run }
+  end
+
+  describe '#process' do
+    it 'expires the cache of the given key' do
+      expect(Rails.cache).to receive(:delete).with(cache_key).and_call_original
+
+      subject.process
+    end
+
+    it 'logs an info event' do
+      data = {
+        class: described_class.name,
+        message: 'Cache invalidation',
+        cache_key: cache_key,
+        cache_expired: false,
+        skippable: false
+      }
+
+      expect(::Gitlab::Logger)
+        .to receive(:info)
+        .with(hash_including(data))
+
+      subject.process
+    end
+  end
+end