test: cover FakeSmtpServer and initialize

test/test_mailer_smtp.rb

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+require 'minitest/autorun'
+require 'prism'
+require 'socket'
+
+# A minimal but real SMTP server: enough of RFC 5321 (EHLO/MAIL FROM/RCPT
+# TO/DATA/QUIT) for Ruby's Net::SMTP client to complete a full session
+# against it over an actual TCP socket. Used to prove Prism::Mailer's :smtp
+# delivery path really speaks SMTP, not just that it calls into Net::SMTP.
+class FakeSmtpServer
+  attr_reader :commands, :data
+
+  def initialize
+    @server = TCPServer.new('127.0.0.1', 0)
+    @commands = []
+    @data = nil
+    @mutex = Mutex.new
+  end
+
+  def port
+    @server.addr[1]
+  end
+
+  def start
+    @thread = Thread.new { accept_loop }
+    self
+  end
+
+  def stop
+    @server.close
+    @thread&.join(1)
+  end
+
+  private
+
+  def accept_loop
+    loop do
+      client = @server.accept
+      handle(client)
+      client.close
+    end
+  rescue IOError, Errno::EBADF
+    nil
+  end
+
+  def handle(client)
+    client.print "220 fake.smtp ready\r\n"
+    loop do
+      line = client.gets
+      break unless line
+
+      record(line.chomp)
+      break unless dispatch(client, line)
+    end
+  end
+
+  def record(line)
+    @mutex.synchronize { @commands << line }
+  end
+
+  def dispatch(client, line)
+    case line
+    when /\A(EHLO|HELO)/i
+      client.print "250 fake.smtp\r\n"
+    when /\AMAIL FROM/i, /\ARCPT TO/i
+      client.print "250 OK\r\n"
+    when /\ADATA/i
+      client.print "354 go ahead\r\n"
+      read_data(client)
+      client.print "250 OK queued\r\n"
+    when /\AQUIT/i
+      client.print "221 bye\r\n"
+      return false
+    else
+      client.print "500 unrecognized\r\n"
+    end
+    true
+  end
+
+  def read_data(client)
+    body = +''
+    while (line = client.gets)
+      break if line == ".\r\n"
+
+      body << line
+    end
+    @mutex.synchronize { @data = body }
+  end
+end
+
+class MailerSmtpTest < Minitest::Test
+  def setup
+    @previous_method = Prism::Mailer.delivery_method
+    @previous_settings = Prism::Mailer.smtp_settings
+  end
+
+  def teardown
+    Prism::Mailer.delivery_method = @previous_method
+    Prism::Mailer.smtp_settings = @previous_settings
+  end
+
+  def test_smtp_delivery_speaks_real_smtp_to_a_live_server_over_tcp
+    server = FakeSmtpServer.new.start
+    Prism::Mailer.delivery_method = :smtp
+    Prism::Mailer.smtp_settings = { address: '127.0.0.1', port: server.port, domain: 'test.local' }
+
+    message = Prism::Mailer::Message.new
+    message.from = 'sender@example.com'
+    message.to = 'receiver@example.com'
+    message.subject = 'Live SMTP test'
+    message.text_body = 'delivered over a real socket'
+
+    Prism::Mailer.deliver!(message)
+    server.stop
+
+    assert(server.commands.any? { |c| c =~ /\AMAIL FROM:<sender@example\.com>/i })
+    assert(server.commands.any? { |c| c =~ /\ARCPT TO:<receiver@example\.com>/i })
+    assert_includes server.data, 'delivered over a real socket'
+    assert_includes server.data, 'Subject: Live SMTP test'
+  end
+
+  def test_smtp_delivery_raises_when_no_server_is_listening
+    Prism::Mailer.delivery_method = :smtp
+    Prism::Mailer.smtp_settings = { address: '127.0.0.1', port: 1, domain: 'test.local' }
+
+    message = Prism::Mailer::Message.new
+    message.from = 'sender@example.com'
+    message.to = 'receiver@example.com'
+    message.subject = 'Unreachable'
+    message.text_body = 'never sent'
+
+    assert_raises(StandardError) { Prism::Mailer.deliver!(message) }
+  end
+
+  def test_unknown_delivery_method_raises_immediately
+    Prism::Mailer.delivery_method = :carrier_pigeon
+    message = Prism::Mailer::Message.new
+    message.from = 'sender@example.com'
+    message.to = 'receiver@example.com'
+    message.subject = 'x'
+    message.text_body = 'x'
+
+    assert_raises(ArgumentError) { Prism::Mailer.deliver!(message) }
+  end
+end