user_topic: Add support for empty topic name.

This commit is a part of the work to support empty string
as a topic name.

Previously, empty string was not a valid topic name.

Now, toggling topic visibility policy operation supports
empty topic name.

Adds backward compatibility for:
- `topic_name` field in the `user_topic` event type

api_docs/changelog.md

@@ -41,6 +41,7 @@ format used by the Zulip server that they are interacting with.
     * `subject` field in the `message` event type
     * `topic` field in the `delete_message` event type
     * `orig_subject` and `subject` fields in the `update_message` event type
+    * `topic_name` field in the `user_topic` event type
 
 * [`GET /messages`](/api/get-messages),
   [`GET /messages/{message_id}`](/api/get-message): Added `allow_empty_topic_name`

zerver/actions/user_topics.py

@@ -5,6 +5,7 @@ from django.db import transaction
 from django.utils.timezone import now as timezone_now
 
 from zerver.lib.timestamp import datetime_to_timestamp
+from zerver.lib.topic import maybe_rename_general_chat_to_empty_topic
 from zerver.lib.user_topics import (
     bulk_set_user_topic_visibility_policy_in_database,
     get_topic_mutes,
@@ -26,6 +27,8 @@ def bulk_do_set_user_topic_visibility_policy(
     if last_updated is None:
         last_updated = timezone_now()
 
+    topic_name = maybe_rename_general_chat_to_empty_topic(topic_name)
+
     user_profiles_with_changed_user_topic_rows = bulk_set_user_topic_visibility_policy_in_database(
         user_profiles,
         stream.id,

zerver/openapi/zulip.yaml

@@ -2207,6 +2207,17 @@ paths:
                                   type: string
                                   description: |
                                     The name of the topic.
+
+                                    For clients that don't support the `empty_topic_name` [client capability][client-capabilities],
+                                    if the actual topic name is empty string, this field's value will instead
+                                    be the value of `realm_empty_topic_display_name` found in the
+                                    [`POST /register`](/api/register-queue) response.
+
+                                    **Changes**: Before 10.0 (feature level 334), `empty_topic_name`
+                                    client capability didn't exist and empty string as the topic name for
+                                    channel messages wasn't allowed.
+
+                                    [client-capabilities]: /api/register-queue#parameter-client_capabilities
                                 last_updated:
                                   type: integer
                                   description: |
@@ -10948,6 +10959,13 @@ paths:
                     Clients should use the `max_topic_length` returned by the
                     [`POST /register`](/api/register-queue) endpoint to determine
                     the maximum topic length.
+
+                    Note: When the value of `realm_empty_topic_display_name` found in
+                    the [POST /register](/api/register-queue) response is used for this
+                    parameter, it is interpreted as an empty string.
+
+                    **Changes**: Before Zulip 10.0 (feature level 334), empty string
+                    was not a valid topic name for channel messages.
                   type: string
                   example: dinner
                 visibility_policy:

zerver/tests/test_message_topics.py

@@ -5,6 +5,7 @@ import orjson
 from django.utils.timezone import now as timezone_now
 
 from zerver.actions.streams import do_change_stream_permission
+from zerver.actions.user_topics import do_set_user_topic_visibility_policy
 from zerver.lib.test_classes import ZulipTestCase
 from zerver.lib.user_topics import set_topic_visibility_policy, topic_has_visibility_policy
 from zerver.models import Message, UserMessage, UserTopic
@@ -376,7 +377,7 @@ class EmptyTopicNameTest(ZulipTestCase):
             apply_markdown=True,
             client_type_name="website",
             empty_topic_name=True,
-            event_types=["message", "update_message", "delete_message"],
+            event_types=["message", "update_message", "delete_message", "user_topic"],
             last_connection_time=time.time(),
             queue_timeout=600,
             realm_id=hamlet.realm.id,
@@ -422,6 +423,37 @@ class EmptyTopicNameTest(ZulipTestCase):
         self.assertEqual(events[4]["topic"], "")
         self.assertEqual(events[5]["topic"], "")
 
+        # reset
+        self.send_stream_message(
+            iago, "Denmark", topic_name="", skip_capture_on_commit_callbacks=True
+        )
+        self.send_stream_message(
+            iago,
+            "Verona",
+            topic_name=Message.EMPTY_TOPIC_FALLBACK_NAME,
+            skip_capture_on_commit_callbacks=True,
+        )
+
+        self.login_user(hamlet)
+        denmark = get_stream("Denmark", hamlet.realm)
+        verona = get_stream("Verona", hamlet.realm)
+        with self.captureOnCommitCallbacks(execute=True):
+            do_set_user_topic_visibility_policy(
+                hamlet,
+                denmark,
+                "",
+                visibility_policy=UserTopic.VisibilityPolicy.FOLLOWED,
+            )
+            do_set_user_topic_visibility_policy(
+                hamlet,
+                verona,
+                Message.EMPTY_TOPIC_FALLBACK_NAME,
+                visibility_policy=UserTopic.VisibilityPolicy.FOLLOWED,
+            )
+        events = client.event_queue.contents()
+        self.assertEqual(events[6]["topic_name"], "")
+        self.assertEqual(events[7]["topic_name"], "")
+
     def test_client_not_supports_empty_topic_name(self) -> None:
         iago = self.example_user("iago")
         hamlet = self.example_user("hamlet")
@@ -430,7 +462,7 @@ class EmptyTopicNameTest(ZulipTestCase):
             apply_markdown=True,
             client_type_name="zulip-mobile",
             empty_topic_name=False,
-            event_types=["message", "update_message", "delete_message"],
+            event_types=["message", "update_message", "delete_message", "user_topic"],
             last_connection_time=time.time(),
             queue_timeout=600,
             realm_id=hamlet.realm.id,
@@ -476,6 +508,37 @@ class EmptyTopicNameTest(ZulipTestCase):
         self.assertEqual(events[4]["topic"], Message.EMPTY_TOPIC_FALLBACK_NAME)
         self.assertEqual(events[5]["topic"], Message.EMPTY_TOPIC_FALLBACK_NAME)
 
+        # reset
+        self.send_stream_message(
+            iago, "Denmark", topic_name="", skip_capture_on_commit_callbacks=True
+        )
+        self.send_stream_message(
+            iago,
+            "Verona",
+            topic_name=Message.EMPTY_TOPIC_FALLBACK_NAME,
+            skip_capture_on_commit_callbacks=True,
+        )
+
+        self.login_user(hamlet)
+        denmark = get_stream("Denmark", hamlet.realm)
+        verona = get_stream("Verona", hamlet.realm)
+        with self.captureOnCommitCallbacks(execute=True):
+            do_set_user_topic_visibility_policy(
+                hamlet,
+                denmark,
+                "",
+                visibility_policy=UserTopic.VisibilityPolicy.FOLLOWED,
+            )
+            do_set_user_topic_visibility_policy(
+                hamlet,
+                verona,
+                Message.EMPTY_TOPIC_FALLBACK_NAME,
+                visibility_policy=UserTopic.VisibilityPolicy.FOLLOWED,
+            )
+        events = client.event_queue.contents()
+        self.assertEqual(events[6]["topic_name"], Message.EMPTY_TOPIC_FALLBACK_NAME)
+        self.assertEqual(events[7]["topic_name"], Message.EMPTY_TOPIC_FALLBACK_NAME)
+
     def test_fetch_messages(self) -> None:
         hamlet = self.example_user("hamlet")
         self.login_user(hamlet)

zerver/tornado/event_queue.py

@@ -1623,6 +1623,25 @@ def process_user_group_name_update_event(event: Mapping[str, Any], users: Iterab
                 client.add_event(user_group_event)
 
 
+def process_user_topic_event(event: Mapping[str, Any], users: Iterable[int]) -> None:
+    empty_topic_name_fallback_event: Mapping[str, Any] | dict[str, Any]
+    if event.get("topic_name") == "":
+        empty_topic_name_fallback_event = dict(event)
+        empty_topic_name_fallback_event["topic_name"] = Message.EMPTY_TOPIC_FALLBACK_NAME
+    else:
+        empty_topic_name_fallback_event = event
+
+    for user_profile_id in users:
+        for client in get_client_descriptors_for_user(user_profile_id):
+            if not client.accepts_event(event):
+                continue
+
+            if client.empty_topic_name:
+                client.add_event(event)
+            else:
+                client.add_event(empty_topic_name_fallback_event)
+
+
 def process_notification(notice: Mapping[str, Any]) -> None:
     event: Mapping[str, Any] = notice["event"]
     users: list[int] | list[Mapping[str, Any]] = notice["users"]
@@ -1651,6 +1670,8 @@ def process_notification(notice: Mapping[str, Any]) -> None:
         # event sent for updating name separately for clients with different
         # capabilities.
         process_user_group_name_update_event(event, cast(list[int], users))
+    elif event["type"] == "user_topic":
+        process_user_topic_event(event, cast(list[int], users))
     elif event["type"] == "cleanup_queue":
         # cleanup_event_queue may generate this event to forward cleanup
         # requests to the right shard.