[4.0.x] Refs #32074 -- Removed usage of deprecated asyncore and smtpd modules.

asyncore and smtpd modules were deprecated in Python 3.10.
Backport of 569a33579c from main

docs/internals/contributing/writing-code/unit-tests.txt

@@ -273,6 +273,7 @@ Running all the tests
 If you want to run the full suite of tests, you'll need to install a number of
 dependencies:
 
+*  aiosmtpd_
 *  argon2-cffi_ 19.1.0+
 *  asgiref_ 3.3.2+ (required)
 *  bcrypt_
@@ -322,6 +323,7 @@ associated tests will be skipped.
 To run some of the autoreload tests, you'll need to install the Watchman_
 service.
 
+.. _aiosmtpd: https://pypi.org/project/aiosmtpd/
 .. _argon2-cffi: https://pypi.org/project/argon2-cffi/
 .. _asgiref: https://pypi.org/project/asgiref/
 .. _bcrypt: https://pypi.org/project/bcrypt/

tests/mail/tests.py

@@ -1,11 +1,9 @@
-import asyncore
 import mimetypes
 import os
 import shutil
-import smtpd
+import socket
 import sys
 import tempfile
-import threading
 from email import charset, message_from_binary_file, message_from_bytes
 from email.header import Header
 from email.mime.text import MIMEText
@@ -14,7 +12,7 @@ from io import StringIO
 from pathlib import Path
 from smtplib import SMTP, SMTPException
 from ssl import SSLError
-from unittest import mock
+from unittest import mock, skipUnless
 
 from django.core import mail
 from django.core.mail import (
@@ -27,6 +25,12 @@ from django.test import SimpleTestCase, override_settings
 from django.test.utils import requires_tz_support
 from django.utils.translation import gettext_lazy
 
+try:
+    from aiosmtpd.controller import Controller
+    HAS_AIOSMTPD = True
+except ImportError:
+    HAS_AIOSMTPD = False
+
 
 class HeadersCheckMixin:
 
@@ -1336,109 +1340,78 @@ class ConsoleBackendTests(BaseEmailBackendTests, SimpleTestCase):
         self.assertIn(b'\nDate: ', message)
 
 
-class FakeSMTPChannel(smtpd.SMTPChannel):
-
-    def collect_incoming_data(self, data):
-        try:
-            smtpd.SMTPChannel.collect_incoming_data(self, data)
-        except UnicodeDecodeError:
-            # Ignore decode error in SSL/TLS connection tests as the test only
-            # cares whether the connection attempt was made.
-            pass
-
-
-class FakeSMTPServer(smtpd.SMTPServer, threading.Thread):
-    """
-    Asyncore SMTP server wrapped into a thread. Based on DummyFTPServer from:
-    http://svn.python.org/view/python/branches/py3k/Lib/test/test_ftplib.py?revision=86061&view=markup
-    """
-    channel_class = FakeSMTPChannel
-
+class SMTPHandler:
     def __init__(self, *args, **kwargs):
-        threading.Thread.__init__(self)
-        smtpd.SMTPServer.__init__(self, *args, decode_data=True, **kwargs)
-        self._sink = []
-        self.active = False
-        self.active_lock = threading.Lock()
-        self.sink_lock = threading.Lock()
+        self.mailbox = []
 
-    def process_message(self, peer, mailfrom, rcpttos, data):
-        data = data.encode()
-        m = message_from_bytes(data)
-        maddr = parseaddr(m.get('from'))[1]
+    async def handle_DATA(self, server, session, envelope):
+        data = envelope.content
+        mail_from = envelope.mail_from
 
-        if mailfrom != maddr:
-            # According to the spec, mailfrom does not necessarily match the
+        message = message_from_bytes(data.rstrip())
+        message_addr = parseaddr(message.get('from'))[1]
+        if mail_from != message_addr:
+            # According to the spec, mail_from does not necessarily match the
             # From header - this is the case where the local part isn't
             # encoded, so try to correct that.
-            lp, domain = mailfrom.split('@', 1)
+            lp, domain = mail_from.split('@', 1)
             lp = Header(lp, 'utf-8').encode()
-            mailfrom = '@'.join([lp, domain])
+            mail_from = '@'.join([lp, domain])
 
-        if mailfrom != maddr:
-            return "553 '%s' != '%s'" % (mailfrom, maddr)
-        with self.sink_lock:
-            self._sink.append(m)
+        if mail_from != message_addr:
+            return f"553 '{mail_from}' != '{message_addr}'"
+        self.mailbox.append(message)
+        return '250 OK'
 
-    def get_sink(self):
-        with self.sink_lock:
-            return self._sink[:]
-
-    def flush_sink(self):
-        with self.sink_lock:
-            self._sink[:] = []
-
-    def start(self):
-        assert not self.active
-        self.__flag = threading.Event()
-        threading.Thread.start(self)
-        self.__flag.wait()
-
-    def run(self):
-        self.active = True
-        self.__flag.set()
-        while self.active and asyncore.socket_map:
-            with self.active_lock:
-                asyncore.loop(timeout=0.1, count=1)
-        asyncore.close_all()
-
-    def stop(self):
-        if self.active:
-            self.active = False
-            self.join()
+    def flush_mailbox(self):
+        self.mailbox[:] = []
 
 
+@skipUnless(HAS_AIOSMTPD, 'No aiosmtpd library detected.')
 class SMTPBackendTestsBase(SimpleTestCase):
 
     @classmethod
     def setUpClass(cls):
         super().setUpClass()
-        cls.server = FakeSMTPServer(('127.0.0.1', 0), None)
+        # Find a free port.
+        with socket.socket() as s:
+            s.bind(('127.0.0.1', 0))
+            port = s.getsockname()[1]
+        cls.smtp_handler = SMTPHandler()
+        cls.smtp_controller = Controller(
+            cls.smtp_handler, hostname='127.0.0.1', port=port,
+        )
         cls._settings_override = override_settings(
-            EMAIL_HOST="127.0.0.1",
-            EMAIL_PORT=cls.server.socket.getsockname()[1])
+            EMAIL_HOST=cls.smtp_controller.hostname,
+            EMAIL_PORT=cls.smtp_controller.port,
+        )
         cls._settings_override.enable()
         cls.addClassCleanup(cls._settings_override.disable)
-        cls.server.start()
-        cls.addClassCleanup(cls.server.stop)
+        cls.smtp_controller.start()
+        cls.addClassCleanup(cls.stop_smtp)
+
+    @classmethod
+    def stop_smtp(cls):
+        cls.smtp_controller.stop()
 
 
+@skipUnless(HAS_AIOSMTPD, 'No aiosmtpd library detected.')
 class SMTPBackendTests(BaseEmailBackendTests, SMTPBackendTestsBase):
     email_backend = 'django.core.mail.backends.smtp.EmailBackend'
 
     def setUp(self):
         super().setUp()
-        self.server.flush_sink()
+        self.smtp_handler.flush_mailbox()
 
     def tearDown(self):
-        self.server.flush_sink()
+        self.smtp_handler.flush_mailbox()
         super().tearDown()
 
     def flush_mailbox(self):
-        self.server.flush_sink()
+        self.smtp_handler.flush_mailbox()
 
     def get_mailbox_content(self):
-        return self.server.get_sink()
+        return self.smtp_handler.mailbox
 
     @override_settings(
         EMAIL_HOST_USER="not empty username",
@@ -1657,17 +1630,18 @@ class SMTPBackendTests(BaseEmailBackendTests, SMTPBackendTestsBase):
         self.assertEqual(sent, 0)
 
 
+@skipUnless(HAS_AIOSMTPD, 'No aiosmtpd library detected.')
 class SMTPBackendStoppedServerTests(SMTPBackendTestsBase):
-    """
-    These tests require a separate class, because the FakeSMTPServer is shut
-    down in setUpClass(), and it cannot be restarted ("RuntimeError: threads
-    can only be started once").
-    """
     @classmethod
     def setUpClass(cls):
         super().setUpClass()
         cls.backend = smtp.EmailBackend(username='', password='')
-        cls.server.stop()
+        cls.smtp_controller.stop()
+
+    @classmethod
+    def stop_smtp(cls):
+        # SMTP controller is stopped in setUpClass().
+        pass
 
     def test_server_stopped(self):
         """

tests/requirements/py3.txt

@@ -1,3 +1,4 @@
+aiosmtpd
 asgiref >= 3.3.2
 argon2-cffi >= 16.1.0
 backports.zoneinfo; python_version < '3.9'