use TCPServer for rake-task readiness checks

- the Socket we were opening would pass the readiness probe checks
initially but begin failing later
- this replaces it with a TCPServer that runs in a background thread
that can accept multiple readiness probe connections
- addresses cloudfoundry/capi-k8s-release#91

[#175388005](https://www.pivotaltracker.com/story/show/175388005)

Authored-by: Tim Downey <tdowney@vmware.com>

Author: tcdowney

lib/cloud_controller/background_job_environment.rb

@@ -1,4 +1,8 @@
+require 'socket'
+
 class BackgroundJobEnvironment
+  attr_reader :readiness_server
+
   def initialize(config)
     @config = config
     @log_counter = Steno::Sink::Counter.new
@@ -16,21 +20,26 @@ def setup_environment(readiness_port=nil)
     @config.configure_components
 
     if readiness_port && readiness_port > 0
-      open_readiness_port(readiness_port)
+      listen_on_readiness_port(readiness_port)
     end
 
     yield if block_given?
   end
 
   private
 
-  def open_readiness_port(port)
-    # rubocop:disable Style/GlobalVars
-    $socket = Socket.new(Socket::AF_INET, Socket::SOCK_STREAM)
-    sockaddr = Socket.pack_sockaddr_in(port, '127.0.0.1')
-    $socket.bind(sockaddr)
-
-    $socket.listen(READINESS_SOCKET_QUEUE_DEPTH)
-    # rubocop:enable Style/GlobalVars
+  def listen_on_readiness_port(port)
+    @readiness_server = TCPServer.open('0.0.0.0', port)
+
+    Thread.new do
+      loop do
+        Thread.start(@readiness_server.accept) do |c|
+          c.puts 'ok'
+          c.close
+        end
+      end
+    rescue Errno::EBADF
+      Thread.exit
+    end
   end
 end

spec/unit/lib/background_job_environment_spec.rb

@@ -16,7 +16,6 @@
   describe '#setup_environment' do
     before do
       allow(VCAP::CloudController::DB).to receive(:load_models)
-      allow(Thread).to receive(:new).and_yield
       allow(EM).to receive(:run).and_yield
       allow(VCAP::CloudController::ResourcePool).to receive(:new)
     end
@@ -55,7 +54,7 @@ def open_port_count
         expect {
           background_job_environment.setup_environment(9999)
         }.to change { open_port_count }.by(1)
-        expect { TCPSocket.new('127.0.0.1', 9999).close }.not_to raise_error
+        expect { background_job_environment.readiness_server.close }.not_to raise_error
       end
     end
   end