Replace Timeout.timeout with socket timeout

Timeout.timeout is inefficient since it spins up a new thread for each invocation, use Socket.tcp's connect_timeout option instead

Author: mohamedhafez

lib/net/smtp.rb

@@ -575,7 +575,12 @@ def finish
  private
 
  def tcp_socket(address, port)
- TCPSocket.open address, port
+ begin
+ Socket.tcp address, port, nil, nil, connect_timeout: @open_timeout
+ rescue Errno::ETIMEDOUT #raise Net:OpenTimeout instead for compatibility with previous versions
+ raise Net::OpenTimeout, "Timeout to open TCP connection to "\
+ "#{address}:#{port} (exceeds #{@open_timeout} seconds)"
+ end
  end
 
  def do_start(helo_domain, user, secret, authtype)
@@ -584,9 +589,7 @@ def do_start(helo_domain, user, secret, authtype)
  check_auth_method(authtype || DEFAULT_AUTH_TYPE)
  check_auth_args user, secret
  end
- s = Timeout.timeout(@open_timeout, Net::OpenTimeout) do
- tcp_socket(@address, @port)
- end
+ s = tcp_socket(@address, @port)
  logging "Connection opened: #{@address}:#{@port}"
  @socket = new_internet_message_io(tls? ? tlsconnect(s, @ssl_context_tls) : s)
  check_response critical { recv_response() }