get_stream_topics: 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.

Adds `allow_empty_topic_name` boolean parameter to
`GET /users/me/{stream_id}/topics` endpoint to decide
whether the topic names in the fetched `topics` array
can be empty strings.

If False, the topic names in the fetched response will
have the value of `realm_empty_topic_display_name` field
in `POST /register` response replacing "".

Fixes part of #23291.

api_docs/changelog.md

@@ -124,6 +124,10 @@ deactivated groups.
   Added `allow_empty_topic_name` boolean parameter to decide whether the
   topic names in the fetched message history objects can be empty strings.
 
+* [`GET /users/me/{stream_id}/topics`](/api/get-stream-topics):
+  Added `allow_empty_topic_name` boolean parameter to decide whether the
+  topic names in the fetched `topics` array can be empty strings.
+
 * [`POST /register`](/api/register-queue): For clients that don't support
   the `empty_topic_name` [client capability](/api/register-queue#parameter-client_capabilities),
   the `topic` field in the `unread_msgs` object and `topic_name` field

web/src/stream_topic_history_util.ts

@@ -27,7 +27,7 @@ export function get_server_history(stream_id: number, on_success: () => void): v
 
     void channel.get({
         url,
-        data: {},
+        data: {allow_empty_topic_name: true},
         success(raw_data) {
             const data = stream_topic_history_response_schema.parse(raw_data);
             const server_history = data.topics;

web/tests/stream_topic_history.test.cjs

@@ -331,7 +331,7 @@ test("server_history_end_to_end", () => {
 
     channel.get = (opts) => {
         assert.equal(opts.url, "/json/users/me/99/topics");
-        assert.deepEqual(opts.data, {});
+        assert.deepEqual(opts.data, {allow_empty_topic_name: true});
         assert.ok(stream_topic_history.is_request_pending_for(stream_id));
         get_success_callback = opts.success;
         get_error_callback = opts.error;

zerver/lib/topic.py

@@ -201,7 +201,10 @@ def update_messages_for_topic_edit(
     return messages, propagate
 
 
-def generate_topic_history_from_db_rows(rows: list[tuple[str, int]]) -> list[dict[str, Any]]:
+def generate_topic_history_from_db_rows(
+    rows: list[tuple[str, int]],
+    allow_empty_topic_name: bool,
+) -> list[dict[str, Any]]:
     canonical_topic_names: dict[str, tuple[int, str]] = {}
 
     # Sort rows by max_message_id so that if a topic
@@ -215,13 +218,19 @@ def generate_topic_history_from_db_rows(rows: list[tuple[str, int]]) -> list[dic
 
     history = []
     for max_message_id, topic_name in canonical_topic_names.values():
+        if topic_name == "" and not allow_empty_topic_name:
+            topic_name = Message.EMPTY_TOPIC_FALLBACK_NAME
         history.append(
             dict(name=topic_name, max_id=max_message_id),
         )
     return sorted(history, key=lambda x: -x["max_id"])
 
 
-def get_topic_history_for_public_stream(realm_id: int, recipient_id: int) -> list[dict[str, Any]]:
+def get_topic_history_for_public_stream(
+    realm_id: int,
+    recipient_id: int,
+    allow_empty_topic_name: bool,
+) -> list[dict[str, Any]]:
     cursor = connection.cursor()
     # Uses index: zerver_message_realm_recipient_subject
     # Note that this is *case-sensitive*, so that we can display the
@@ -244,14 +253,21 @@ def get_topic_history_for_public_stream(realm_id: int, recipient_id: int) -> lis
     rows = cursor.fetchall()
     cursor.close()
 
-    return generate_topic_history_from_db_rows(rows)
+    return generate_topic_history_from_db_rows(rows, allow_empty_topic_name)
 
 
 def get_topic_history_for_stream(
-    user_profile: UserProfile, recipient_id: int, public_history: bool
+    user_profile: UserProfile,
+    recipient_id: int,
+    public_history: bool,
+    allow_empty_topic_name: bool,
 ) -> list[dict[str, Any]]:
     if public_history:
-        return get_topic_history_for_public_stream(user_profile.realm_id, recipient_id)
+        return get_topic_history_for_public_stream(
+            user_profile.realm_id,
+            recipient_id,
+            allow_empty_topic_name,
+        )
 
     cursor = connection.cursor()
     # Uses index: zerver_message_realm_recipient_subject
@@ -279,7 +295,7 @@ def get_topic_history_for_stream(
     rows = cursor.fetchall()
     cursor.close()
 
-    return generate_topic_history_from_db_rows(rows)
+    return generate_topic_history_from_db_rows(rows, allow_empty_topic_name)
 
 
 def get_topic_resolution_and_bare_name(stored_name: str) -> tuple[bool, str]:

zerver/openapi/zulip.yaml

@@ -10220,6 +10220,22 @@ paths:
         user subscribed.
       parameters:
         - $ref: "#/components/parameters/ChannelIdInPath"
+        - name: allow_empty_topic_name
+          in: query
+          description: |
+            Whether the client supports processing the empty string as
+            a topic name in the returned data.
+
+            If `false`, the value of `realm_empty_topic_display_name`
+            found in the [`POST /register`](/api/register-queue) response is
+            returned replacing the empty string as the topic name.
+
+            **Changes**: New in Zulip 10.0 (feature level 334). Previously,
+            the empty string was not a valid topic.
+          schema:
+            type: boolean
+            default: false
+          example: true
       responses:
         "200":
           description: Success.

zerver/tests/test_message_topics.py

@@ -843,3 +843,28 @@ class EmptyTopicNameTest(ZulipTestCase):
         self.assertEqual(
             state_data["user_topics"][1]["topic_name"], Message.EMPTY_TOPIC_FALLBACK_NAME
         )
+
+    def test_get_channel_topics(self) -> None:
+        hamlet = self.example_user("hamlet")
+        self.login_user(hamlet)
+        channel_one = self.make_stream("channel_one")
+        channel_two = self.make_stream("channel_two")
+        self.subscribe(hamlet, channel_one.name)
+        self.subscribe(hamlet, channel_two.name)
+
+        self.send_stream_message(hamlet, channel_one.name, topic_name="")
+        self.send_stream_message(
+            hamlet, channel_two.name, topic_name=Message.EMPTY_TOPIC_FALLBACK_NAME
+        )
+
+        params = {"allow_empty_topic_name": "false"}
+        for channel_id in [channel_one.id, channel_two.id]:
+            result = self.client_get(f"/json/users/me/{channel_id}/topics", params)
+            data = self.assert_json_success(result)
+            self.assertEqual(data["topics"][0]["name"], Message.EMPTY_TOPIC_FALLBACK_NAME)
+
+        params = {"allow_empty_topic_name": "true"}
+        for channel_id in [channel_one.id, channel_two.id]:
+            result = self.client_get(f"/json/users/me/{channel_id}/topics", params)
+            data = self.assert_json_success(result)
+            self.assertEqual(data["topics"][0]["name"], "")

zerver/views/streams.py

@@ -929,6 +929,7 @@ def get_topics_backend(
     maybe_user_profile: UserProfile | AnonymousUser,
     *,
     stream_id: PathOnly[NonNegativeInt],
+    allow_empty_topic_name: Json[bool] = False,
 ) -> HttpResponse:
     if not maybe_user_profile.is_authenticated:
         is_web_public_query = True
@@ -943,7 +944,9 @@ def get_topics_backend(
         realm = get_valid_realm_from_request(request)
         stream = access_web_public_stream(stream_id, realm)
         result = get_topic_history_for_public_stream(
-            realm_id=realm.id, recipient_id=assert_is_not_none(stream.recipient_id)
+            realm_id=realm.id,
+            recipient_id=assert_is_not_none(stream.recipient_id),
+            allow_empty_topic_name=allow_empty_topic_name,
         )
 
     else:
@@ -956,6 +959,7 @@ def get_topics_backend(
             user_profile=user_profile,
             recipient_id=stream.recipient_id,
             public_history=stream.is_history_public_to_subscribers(),
+            allow_empty_topic_name=allow_empty_topic_name,
         )
 
     return json_success(request, data=dict(topics=result))