Fix incompatibility with Twisted < 21. (#10713)

Turns out that the functionality added in #10546 to skip TLS was incompatible
with older Twisted versions, so we need to be a bit more inventive.

Also, add a test to (hopefully) not break this in future. Sadly, testing TLS is
really hard.

tests/handlers/test_send_email.py

@@ -0,0 +1,112 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+from typing import List, Tuple
+
+from zope.interface import implementer
+
+from twisted.internet import defer
+from twisted.internet.address import IPv4Address
+from twisted.internet.defer import ensureDeferred
+from twisted.mail import interfaces, smtp
+
+from tests.server import FakeTransport
+from tests.unittest import HomeserverTestCase
+
+
+@implementer(interfaces.IMessageDelivery)
+class _DummyMessageDelivery:
+    def __init__(self):
+        # (recipient, message) tuples
+        self.messages: List[Tuple[smtp.Address, bytes]] = []
+
+    def receivedHeader(self, helo, origin, recipients):
+        return None
+
+    def validateFrom(self, helo, origin):
+        return origin
+
+    def record_message(self, recipient: smtp.Address, message: bytes):
+        self.messages.append((recipient, message))
+
+    def validateTo(self, user: smtp.User):
+        return lambda: _DummyMessage(self, user)
+
+
+@implementer(interfaces.IMessageSMTP)
+class _DummyMessage:
+    """IMessageSMTP implementation which saves the message delivered to it
+    to the _DummyMessageDelivery object.
+    """
+
+    def __init__(self, delivery: _DummyMessageDelivery, user: smtp.User):
+        self._delivery = delivery
+        self._user = user
+        self._buffer: List[bytes] = []
+
+    def lineReceived(self, line):
+        self._buffer.append(line)
+
+    def eomReceived(self):
+        message = b"\n".join(self._buffer) + b"\n"
+        self._delivery.record_message(self._user.dest, message)
+        return defer.succeed(b"saved")
+
+    def connectionLost(self):
+        pass
+
+
+class SendEmailHandlerTestCase(HomeserverTestCase):
+    def test_send_email(self):
+        """Happy-path test that we can send email to a non-TLS server."""
+        h = self.hs.get_send_email_handler()
+        d = ensureDeferred(
+            h.send_email(
+                "foo@bar.com", "test subject", "Tests", "HTML content", "Text content"
+            )
+        )
+        # there should be an attempt to connect to localhost:25
+        self.assertEqual(len(self.reactor.tcpClients), 1)
+        (host, port, client_factory, _timeout, _bindAddress) = self.reactor.tcpClients[
+            0
+        ]
+        self.assertEqual(host, "localhost")
+        self.assertEqual(port, 25)
+
+        # wire it up to an SMTP server
+        message_delivery = _DummyMessageDelivery()
+        server_protocol = smtp.ESMTP()
+        server_protocol.delivery = message_delivery
+        # make sure that the server uses the test reactor to set timeouts
+        server_protocol.callLater = self.reactor.callLater  # type: ignore[assignment]
+
+        client_protocol = client_factory.buildProtocol(None)
+        client_protocol.makeConnection(FakeTransport(server_protocol, self.reactor))
+        server_protocol.makeConnection(
+            FakeTransport(
+                client_protocol,
+                self.reactor,
+                peer_address=IPv4Address("TCP", "127.0.0.1", 1234),
+            )
+        )
+
+        # the message should now get delivered
+        self.get_success(d, by=0.1)
+
+        # check it arrived
+        self.assertEqual(len(message_delivery.messages), 1)
+        user, msg = message_delivery.messages.pop()
+        self.assertEqual(str(user), "foo@bar.com")
+        self.assertIn(b"Subject: test subject", msg)