user_topics: Validate 'topic' parameter length <= max_topic_length.

Earlier, 'topic' parameter length for
'/users/me/subscriptions/muted_topics' and '/user_topics' endpoints
were not validated before DB operations which resulted in exception:
'DataError: value too long for type character varying(60)'.

This commit adds validation for the topic name length to be
capped at 'max_topic_length' characters.

The doc is updated to suggest clients that the topic name should
have a maximum length of 'max_topic_length'.

Fixes #27796.

zerver/openapi/zulip.yaml

@@ -9498,6 +9498,10 @@ paths:
           description: |
             The topic to (un)mute. Note that the request will succeed regardless of
             whether any messages have been sent to the specified topic.
+
+            Clients should use the `max_topic_length` returned by the
+            [`POST /register`](/api/register-queue) endpoint to determine
+            the maximum topic length.
           schema:
             type: string
           example: dinner
@@ -9594,6 +9598,10 @@ paths:
             The topic for which the personal preferences needs to be updated.
             Note that the request will succeed regardless of whether
             any messages have been sent to the specified topic.
+
+            Clients should use the `max_topic_length` returned by the
+            [`POST /register`](/api/register-queue) endpoint to determine
+            the maximum topic length.
           schema:
             type: string
           example: dinner

zerver/tests/test_user_topics.py

@@ -13,6 +13,7 @@ from zerver.lib.test_classes import ZulipTestCase
 from zerver.lib.test_helpers import get_subscription
 from zerver.lib.user_topics import get_topic_mutes, topic_has_visibility_policy
 from zerver.models import UserProfile, UserTopic
+from zerver.models.constants import MAX_TOPIC_NAME_LENGTH
 from zerver.models.streams import get_stream
 
 
@@ -199,6 +200,12 @@ class MutedTopicsTestsDeprecated(ZulipTestCase):
         result = self.api_patch(user, url, data)
         self.assert_json_error(result, "Please choose one: 'stream' or 'stream_id'.")
 
+        data = {"stream_id": stream.id, "topic": "a" * (MAX_TOPIC_NAME_LENGTH + 1), "op": "add"}
+        result = self.api_patch(user, url, data)
+        self.assert_json_error(
+            result, f"topic is too long (limit: {MAX_TOPIC_NAME_LENGTH} characters)"
+        )
+
     def test_muted_topic_remove_invalid(self) -> None:
         user = self.example_user("hamlet")
         realm = user.realm
@@ -233,6 +240,12 @@ class MutedTopicsTestsDeprecated(ZulipTestCase):
         result = self.api_patch(user, url, data)
         self.assert_json_error(result, "Please choose one: 'stream' or 'stream_id'.")
 
+        data = {"stream_id": stream.id, "topic": "a" * (MAX_TOPIC_NAME_LENGTH + 1), "op": "remove"}
+        result = self.api_patch(user, url, data)
+        self.assert_json_error(
+            result, f"topic is too long (limit: {MAX_TOPIC_NAME_LENGTH} characters)"
+        )
+
 
 class MutedTopicsTests(ZulipTestCase):
     def test_get_deactivated_muted_topic(self) -> None:
@@ -436,6 +449,17 @@ class MutedTopicsTests(ZulipTestCase):
         result = self.api_post(user, url, data)
         self.assert_json_error(result, "Invalid stream ID")
 
+        stream = get_stream("Verona", user.realm)
+        data = {
+            "stream_id": stream.id,
+            "topic": "a" * (MAX_TOPIC_NAME_LENGTH + 1),
+            "visibility_policy": UserTopic.VisibilityPolicy.MUTED,
+        }
+        result = self.api_post(user, url, data)
+        self.assert_json_error(
+            result, f"topic is too long (limit: {MAX_TOPIC_NAME_LENGTH} characters)"
+        )
+
     def test_muted_topic_remove_invalid(self) -> None:
         user = self.example_user("hamlet")
         self.login_user(user)
@@ -450,6 +474,17 @@ class MutedTopicsTests(ZulipTestCase):
         result = self.api_post(user, url, data)
         self.assert_json_error(result, "Invalid stream ID")
 
+        stream = get_stream("Verona", user.realm)
+        data = {
+            "stream_id": stream.id,
+            "topic": "a" * (MAX_TOPIC_NAME_LENGTH + 1),
+            "visibility_policy": UserTopic.VisibilityPolicy.INHERIT,
+        }
+        result = self.api_post(user, url, data)
+        self.assert_json_error(
+            result, f"topic is too long (limit: {MAX_TOPIC_NAME_LENGTH} characters)"
+        )
+
 
 class UnmutedTopicsTests(ZulipTestCase):
     def test_user_ids_unmuting_topic(self) -> None:

zerver/views/user_topics.py

@@ -15,8 +15,9 @@ from zerver.lib.streams import (
     access_stream_to_remove_visibility_policy_by_name,
     check_for_exactly_one_stream_arg,
 )
-from zerver.lib.validator import check_int, check_int_in, check_string_in
+from zerver.lib.validator import check_capped_string, check_int, check_int_in, check_string_in
 from zerver.models import UserProfile, UserTopic
+from zerver.models.constants import MAX_TOPIC_NAME_LENGTH
 
 
 def mute_topic(
@@ -66,7 +67,7 @@ def update_muted_topic(
     user_profile: UserProfile,
     stream_id: Optional[int] = REQ(json_validator=check_int, default=None),
     stream: Optional[str] = REQ(default=None),
-    topic: str = REQ(),
+    topic: str = REQ(str_validator=check_capped_string(MAX_TOPIC_NAME_LENGTH)),
     op: str = REQ(str_validator=check_string_in(["add", "remove"])),
 ) -> HttpResponse:
     check_for_exactly_one_stream_arg(stream_id=stream_id, stream=stream)
@@ -94,7 +95,7 @@ def update_user_topic(
     request: HttpRequest,
     user_profile: UserProfile,
     stream_id: int = REQ(json_validator=check_int),
-    topic: str = REQ(),
+    topic: str = REQ(str_validator=check_capped_string(MAX_TOPIC_NAME_LENGTH)),
     visibility_policy: int = REQ(json_validator=check_int_in(UserTopic.VisibilityPolicy.values)),
 ) -> HttpResponse:
     if visibility_policy == UserTopic.VisibilityPolicy.INHERIT: