Use the open_timeout kwarg where available

Author: mohamedhafez

lib/net/smtp.rb

@@ -263,6 +263,7 @@ def initialize(address, port = nil, tls: false, starttls: :auto, tls_verify: tru
       @tls_verify = tls_verify
       @tls_hostname = tls_hostname
       @ssl_context_params = ssl_context_params
+      @tcpsocket_supports_open_timeout = nil
     end
 
     # If +true+, verify th server's certificate.
@@ -659,18 +660,44 @@ def finish
 
     private
 
-    def tcp_socket(address, port)
-      TCPSocket.open address, port
+    def tcp_socket(address, port, open_timeout: nil)
+      if open_timeout
+        TCPSocket.open(address, port, nil, nil, open_timeout: open_timeout)
+      else
+        TCPSocket.open(address, port)
+      end
+    end
+
+    def timeouted_connect
+      if @tcpsocket_supports_open_timeout == nil || @tcpsocket_supports_open_timeout == true
+        # Try to use built-in open_timeout in TCPSocket.open if:
+        #   - The current Ruby runtime is known to support it, or
+        #   - It is unknown whether the current Ruby runtime supports it (so we'll try).
+        begin
+          sock = tcp_socket(@address, @port, open_timeout: @open_timeout)
+          @tcpsocket_supports_open_timeout = true
+          return sock
+        rescue Errno::ETIMEDOUT => e
+          raise Net::OpenTimeout.new(e) # for compatibility with previous versions
+        rescue ArgumentError => e
+          raise unless e.message.include?('unknown keyword: :open_timeout') || e.message.include?('wrong number of arguments (given 5, expected 2..4)')
+          @tcpsocket_supports_open_timeout = false
+        end
+      end
+
+      # This Ruby runtime is known not to support `TCPSocket(open_timeout:)`.
+      # Directly fall back to Timeout.timeout to avoid performance penalty incured by rescue.
+      Timeout.timeout(@open_timeout, Net::OpenTimeout) do
+        tcp_socket(@address, @port)
+      end
     end
 
     def do_start(helo_domain, user, secret, authtype)
       raise IOError, 'SMTP session already started' if @started
       if user || secret || authtype
         check_auth_args authtype, user, secret
       end
-      s = Timeout.timeout(@open_timeout, Net::OpenTimeout) do
-        tcp_socket(@address, @port)
-      end
+      s = timeouted_connect
       logging "Connection opened: #{@address}:#{@port}"
       @socket = new_internet_message_io(tls? ? tlsconnect(s, @ssl_context_tls) : s)
       check_response critical { recv_response() }