email_mirror: Change default topic name if subject ends up empty.

If the email subject is something like `Fwd:`, it gets stripped to an
empty string, activating the "(no topic)" override. This however leads
to failure if the organization enables the setting forcing every message
to have a topic. Such emails should still go through, so we should just
change the topic value used.

tools/lib/capitalization.py

@@ -73,6 +73,7 @@ IGNORED_PHRASES = [
     r"and",
     r"bot",
     r"e\.g\.",
+    r"email",
     r"enabled",
     r"signups",
     # Placeholders

zerver/lib/email_mirror.py

@@ -409,11 +409,17 @@ def is_forwarded(subject: str) -> bool:
 
 def process_stream_message(to: str, message: EmailMessage) -> None:
     subject_header = message.get("Subject", "")
-    subject = strip_from_subject(subject_header) or "(no topic)"
 
+    subject = strip_from_subject(subject_header)
     # We don't want to reject email messages with disallowed characters in the Subject,
     # so we just remove them to make it a valid Zulip topic name.
-    subject = "".join([char for char in subject if is_character_printable(char)]) or "(no topic)"
+    subject = "".join([char for char in subject if is_character_printable(char)])
+
+    # If the subject gets stripped to the empty string, we need to set some
+    # default value for the message topic. We can't use the usual
+    # "(no topic)" as that value is not permitted if the realm enforces
+    # that all messages must have a topic.
+    subject = subject or _("Email with no subject")
 
     stream, options = decode_stream_email_address(to)
     # Don't remove quotations if message is forwarded, unless otherwise specified:

zerver/tests/test_email_mirror.py

@@ -326,7 +326,7 @@ class TestStreamEmailMessagesSuccess(ZulipTestCase):
 
         self.assertEqual(message.content, "TestStreamEmailMessages body")
         self.assert_message_stream_name(message, stream.name)
-        self.assertEqual(message.topic_name(), "(no topic)")
+        self.assertEqual(message.topic_name(), "Email with no subject")
 
     def test_receive_stream_email_messages_subject_with_nonprintable_chars(
         self,
@@ -358,7 +358,7 @@ class TestStreamEmailMessagesSuccess(ZulipTestCase):
         process_message(incoming_valid_message)
         message = most_recent_message(user_profile)
 
-        self.assertEqual(message.topic_name(), "(no topic)")
+        self.assertEqual(message.topic_name(), "Email with no subject")
 
     def test_receive_private_stream_email_messages_success(self) -> None:
         user_profile = self.example_user("hamlet")
@@ -1617,12 +1617,12 @@ class TestStreamEmailMessagesSubjectStripping(ZulipTestCase):
         message = most_recent_message(user_profile)
         self.assertEqual("Test", message.topic_name())
 
-        # If after stripping we get an empty subject, it should get set to (no topic)
+        # If after stripping we get an empty subject, it should get set to Email with no subject
         del incoming_valid_message["Subject"]
         incoming_valid_message["Subject"] = "Re: Fwd: Re: "
         process_message(incoming_valid_message)
         message = most_recent_message(user_profile)
-        self.assertEqual("(no topic)", message.topic_name())
+        self.assertEqual("Email with no subject", message.topic_name())
 
     def test_strip_from_subject(self) -> None:
         subject_list = orjson.loads(self.fixture_data("subjects.json", type="email"))