feat: new background work process

This removes all previous dependencies on RabbitMQ and the need to run separate cron and requeueing processes.

lib/worker/jobs/process_queued_messages_job.rb

@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+module Worker
+  module Jobs
+    class ProcessQueuedMessagesJob < BaseJob
+
+      def call
+        @lock_time = Time.current
+        @locker = Postal.locker_name_with_suffix(SecureRandom.hex(8))
+
+        find_ip_addresses
+        lock_message_for_processing
+        obtain_locked_messages
+        process_messages
+        @messages_to_process
+      end
+
+      private
+
+      # Returns an array of IP address IDs that are present on the host that is
+      # running this job.
+      #
+      # @return [Array<Integer>]
+      def find_ip_addresses
+        ip_addresses = { 4 => [], 6 => [] }
+        Socket.ip_address_list.each do |address|
+          next if local_ip?(address.ip_address)
+
+          ip_addresses[address.ipv4? ? 4 : 6] << address.ip_address
+        end
+        @ip_addresses = IPAddress.where(ipv4: ip_addresses[4]).or(IPAddress.where(ipv6: ip_addresses[6])).pluck(:id)
+      end
+
+      # Is the given IP address a local address?
+      #
+      # @param [String] ip
+      # @return [Boolean]
+      def local_ip?(ip)
+        !!(ip =~ /\A(127\.|fe80:|::)/)
+      end
+
+      # Obtain a queued message from the database for processing
+      #
+      # @return [void]
+      def lock_message_for_processing
+        QueuedMessage.where(ip_address_id: [nil, @ip_addresses])
+                     .where(locked_by: nil, locked_at: nil)
+                     .ready_with_delayed_retry
+                     .limit(1)
+                     .update_all(locked_by: @locker, locked_at: @lock_time)
+      end
+
+      # Get a full list of all messages which we can process (i.e. those which have just
+      # been locked by us for processing)
+      #
+      # @return [void]
+      def obtain_locked_messages
+        @messages_to_process = QueuedMessage.where(locked_by: @locker, locked_at: @lock_time)
+      end
+
+      # Process the messages we obtained from the database
+      #
+      # @return [void]
+      def process_messages
+        @messages_to_process.each do |message|
+          work_completed!
+          UnqueueMessageService.new(queued_message: message, logger: logger).call
+        end
+      end
+
+    end
+  end
+end