Write a test mail relay

Provide a simple Python script that would linsten on a given port and print to
standard output any message received via SMTP on that port.

This script will then be used to automatically test gpg-mailgate with different
scenarios (unknown recipient key, RSA key, elliptic curve key).

test/relay.py

@@ -0,0 +1,68 @@
+#!/usr/local/bin/python2
+#
+# This quick-and-dirty script supports only the happy case of SMTP session,
+# i.e. what gpg-mailgate/gpg-lacre needs to deliver encrypted email.
+#
+# It listens on the port given as the only command-line argument and consumes a
+# message, then prints it to standard output.  The goal is to be able to
+# compare that output with expected clear-text or encrypted message body.
+#
+
+import sys
+import socket
+
+
+BUFFER_SIZE = 4096
+EOM = "\r\n.\r\n"
+LAST_LINE = -3
+
+
+def welcome(msg):
+	return "220 %s\r\n" % (msg)
+
+def ok(msg = "OK"):
+	return "250 %s\r\n" % (msg)
+
+def provide_message():
+	return "354 Enter a message, ending it with a '.' on a line by itself\r\n"
+
+def receive_and_confirm(session):
+	session.recv(BUFFER_SIZE)
+	session.sendall(ok())
+
+def serve(port):
+	s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+	s.bind(('', port))
+	s.listen(1)
+
+	(conn, addr) = s.accept()
+	conn.sendall(welcome("TEST SERVER"))
+
+	receive_and_confirm(conn)				# Ignore HELO/EHLO
+	receive_and_confirm(conn)				# Ignore sender address
+	receive_and_confirm(conn)				# Ignore recipient address
+
+	data = conn.recv(BUFFER_SIZE)
+	conn.sendall(provide_message())
+
+	# Consume until we get <CR><LF>.<CR><LF>, the end-of-message marker.
+	message = ''
+	while not message.endswith(EOM):
+		message += conn.recv(BUFFER_SIZE)
+	conn.sendall(ok("OK, id=test"))
+
+	# Trim EOM marker as we're only interested in the message body.
+	return message[:-len(EOM)]
+
+def error(msg):
+	print "ERROR: %s" % (msg)
+	sys.exit(1)
+
+
+if len(sys.argv) < 2:
+	error("Usage: relay.py PORT_NUMBER")
+
+port = int(sys.argv[1])
+body = serve(port)
+
+print body