email_log: Inherit EmailLogBackEnd from smtp.EmailBackend.

EmailLogBackend used to create a new EmailMessage and copy
only certain values from the original EmailMultiAlternatives
object. This resulted in the loss of information and made
it harder to test PRs like
https://github.com/zulip/zulip/pull/17121.

So instead of creating a new EmailMessage, tweak and send the existing
EmailMultiAlternatives object.

zerver/tests/test_email_log.py

@@ -10,7 +10,7 @@ from zproject.email_backends import get_forward_address
 class EmailLogTest(ZulipTestCase):
     def test_generate_and_clear_email_log(self) -> None:
         with self.settings(EMAIL_BACKEND='zproject.email_backends.EmailLogBackEnd'), \
-                mock.patch('zproject.email_backends.EmailLogBackEnd.send_email_smtp'), \
+                mock.patch('zproject.email_backends.EmailBackend.send_messages'), \
                 self.assertLogs(level="INFO") as m, \
                 self.settings(DEVELOPMENT_LOG_EMAILS=True):
             result = self.client_get('/emails/generate/')
@@ -35,7 +35,7 @@ class EmailLogTest(ZulipTestCase):
         self.assertEqual(get_forward_address(), forward_address)
 
         with self.settings(EMAIL_BACKEND='zproject.email_backends.EmailLogBackEnd'):
-            with mock.patch('zproject.email_backends.EmailLogBackEnd.send_email_smtp'):
+            with mock.patch('zproject.email_backends.EmailBackend.send_messages'):
                 result = self.client_get('/emails/generate/')
                 self.assertEqual(result.status_code, 302)
                 self.assertIn('emails', result['Location'])

zproject/email_backends.py

@@ -1,12 +1,10 @@
 import configparser
 import logging
-import smtplib
-from email.message import EmailMessage
 from typing import List
 
 from django.conf import settings
 from django.core.mail import EmailMultiAlternatives
-from django.core.mail.backends.base import BaseEmailBackend
+from django.core.mail.backends.smtp import EmailBackend
 from django.template import loader
 
 
@@ -29,35 +27,7 @@ def set_forward_address(forward_address: str) -> None:
     with open(settings.FORWARD_ADDRESS_CONFIG_FILE, "w") as cfgfile:
         config.write(cfgfile)
 
-class EmailLogBackEnd(BaseEmailBackend):
-    def send_email_smtp(self, email: EmailMultiAlternatives) -> None:
-        from_email = email.from_email
-        to = get_forward_address()
-
-        msg = EmailMessage()
-        msg['Subject'] = email.subject
-        msg['From'] = from_email
-        msg['To'] = to
-
-        text = email.body
-        html = email.alternatives[0][0]
-
-        # Here, we replace the email addresses used in development
-        # with chat.zulip.org, so that web email providers like Gmail
-        # will be able to fetch the illustrations used in the emails.
-        localhost_email_images_base_uri = settings.ROOT_DOMAIN_URI + '/static/images/emails'
-        czo_email_images_base_uri = 'https://chat.zulip.org/static/images/emails'
-        html = html.replace(localhost_email_images_base_uri, czo_email_images_base_uri)
-
-        msg.add_alternative(text, subtype="plain")
-        msg.add_alternative(html, subtype="html")
-
-        smtp = smtplib.SMTP(settings.EMAIL_HOST, settings.EMAIL_PORT)
-        smtp.starttls()
-        smtp.login(settings.EMAIL_HOST_USER, settings.EMAIL_HOST_PASSWORD)
-        smtp.send_message(msg)
-        smtp.quit()
-
+class EmailLogBackEnd(EmailBackend):
     def log_email(self, email: EmailMultiAlternatives) -> None:
         """Used in development to record sent emails in a nice HTML log"""
         html_message = 'Missing HTML message'
@@ -86,12 +56,30 @@ class EmailLogBackEnd(BaseEmailBackend):
         with open(settings.EMAIL_CONTENT_LOG_PATH, "w+") as f:
             f.write(new_email + previous_emails)
 
+    @staticmethod
+    def prepare_email_messages_for_forwarding(email_messages: List[EmailMultiAlternatives]) -> None:
+        localhost_email_images_base_uri = settings.ROOT_DOMAIN_URI + '/static/images/emails'
+        czo_email_images_base_uri = 'https://chat.zulip.org/static/images/emails'
+
+        for email_message in email_messages:
+            html_alternative = list(email_message.alternatives[0])
+            # Here, we replace the email addresses used in development
+            # with chat.zulip.org, so that web email providers like Gmail
+            # will be able to fetch the illustrations used in the emails.
+            html_alternative[0] = html_alternative[0].replace(localhost_email_images_base_uri, czo_email_images_base_uri)
+            email_message.alternatives[0] = tuple(html_alternative)
+
+            email_message.to = [get_forward_address()]
+
     def send_messages(self, email_messages: List[EmailMultiAlternatives]) -> int:
-        for email in email_messages:
-            if get_forward_address():
-                self.send_email_smtp(email)
-            if settings.DEVELOPMENT_LOG_EMAILS:
+        num_sent = len(email_messages)
+        if get_forward_address():
+            self.prepare_email_messages_for_forwarding(email_messages)
+            num_sent = super().send_messages(email_messages)
+
+        if settings.DEVELOPMENT_LOG_EMAILS:
+            for email in email_messages:
                 self.log_email(email)
                 email_log_url = settings.ROOT_DOMAIN_URI + "/emails"
                 logging.info("Emails sent in development are available at %s", email_log_url)
-        return len(email_messages)
+        return num_sent