streams: Extract topics_policy validation.

Fixes: Point 3 of
https://github.com/zulip/zulip/pull/33405#issuecomment-3064452310.
Signed-off-by: apoorvapendse <apoorvavpendse@gmail.com>

zerver/lib/streams.py

@@ -12,6 +12,7 @@ from django.utils.translation import gettext as _
 from zerver.lib.default_streams import get_default_stream_ids_for_realm
 from zerver.lib.exceptions import (
     CannotAdministerChannelError,
+    CannotSetTopicsPolicyError,
     IncompatibleParametersError,
     JsonableError,
     OrganizationOwnerRequiredError,
@@ -24,6 +25,7 @@ from zerver.lib.stream_subscription import (
 from zerver.lib.stream_traffic import get_average_weekly_stream_traffic, get_streams_traffic
 from zerver.lib.string_validation import check_stream_name
 from zerver.lib.timestamp import datetime_to_timestamp
+from zerver.lib.topic import get_topic_display_name
 from zerver.lib.types import APIStreamDict, UserGroupMembersData
 from zerver.lib.user_groups import (
     UserGroupMembershipDetails,
@@ -123,6 +125,33 @@ def get_stream_topics_policy(realm: Realm, stream: Stream) -> int:
     return stream.topics_policy
 
 
+def validate_topics_policy(
+    topics_policy: str | None,
+    user_profile: UserProfile,
+    # Pass None when creating a channel and the channel being edited when editing a channel's settings
+    stream: Stream | None = None,
+) -> StreamTopicsPolicyEnum | None:
+    if topics_policy is not None and isinstance(topics_policy, StreamTopicsPolicyEnum):
+        if (
+            topics_policy != StreamTopicsPolicyEnum.inherit
+            and not user_profile.can_set_topics_policy()
+        ):
+            raise JsonableError(_("Insufficient permission"))
+
+        # Cannot set `topics_policy` to `empty_topic_only` when there are messages
+        # in non-empty topics in the current channel.
+        if (
+            stream is not None
+            and topics_policy == StreamTopicsPolicyEnum.empty_topic_only
+            and channel_has_named_topics(stream)
+        ):
+            raise CannotSetTopicsPolicyError(
+                get_topic_display_name("", user_profile.default_language)
+            )
+        return topics_policy
+    return None
+
+
 def get_default_value_for_history_public_to_subscribers(
     realm: Realm,
     invite_only: bool,

zerver/views/streams.py

@@ -57,7 +57,6 @@ from zerver.lib.default_streams import get_default_stream_ids_for_realm
 from zerver.lib.email_mirror_helpers import encode_email_address, get_channel_email_token
 from zerver.lib.exceptions import (
     CannotManageDefaultChannelError,
-    CannotSetTopicsPolicyError,
     JsonableError,
     OrganizationOwnerRequiredError,
 )
@@ -75,7 +74,6 @@ from zerver.lib.streams import (
     access_stream_for_delete_or_update_requiring_metadata_access,
     access_web_public_stream,
     channel_events_topic_name,
-    channel_has_named_topics,
     check_stream_name_available,
     do_get_streams,
     filter_stream_authorization_for_adding_subscribers,
@@ -85,10 +83,10 @@ from zerver.lib.streams import (
     list_to_streams,
     stream_to_dict,
     user_has_content_access,
+    validate_topics_policy,
 )
 from zerver.lib.subscription_info import gather_subscriptions
 from zerver.lib.topic import (
-    get_topic_display_name,
     get_topic_history_for_public_stream,
     get_topic_history_for_stream,
     maybe_rename_general_chat_to_empty_topic,
@@ -398,20 +396,9 @@ def update_stream_backend(
         if not user_profile.can_create_web_public_streams():
             raise JsonableError(_("Insufficient permission"))
 
-    if topics_policy is not None and isinstance(topics_policy, StreamTopicsPolicyEnum):
-        if not user_profile.can_set_topics_policy():
-            raise JsonableError(_("Insufficient permission"))
-
-        # Cannot set `topics_policy` to `empty_topic_only` when there are messages
-        # in non-empty topics in the current channel.
-        if topics_policy == StreamTopicsPolicyEnum.empty_topic_only and channel_has_named_topics(
-            stream
-        ):
-            raise CannotSetTopicsPolicyError(
-                get_topic_display_name("", user_profile.default_language)
-            )
-
-        do_set_stream_property(stream, "topics_policy", topics_policy.value, user_profile)
+    validated_topics_policy = validate_topics_policy(topics_policy, user_profile, stream)
+    if validated_topics_policy is not None:
+        do_set_stream_property(stream, "topics_policy", validated_topics_policy.value, user_profile)
 
     if (
         is_private is not None
@@ -768,13 +755,9 @@ def add_subscriptions_backend(
         stream_dict_copy["message_retention_days"] = parse_message_retention_days(
             message_retention_days, Stream.MESSAGE_RETENTION_SPECIAL_VALUES_MAP
         )
-        if topics_policy is not None and isinstance(topics_policy, StreamTopicsPolicyEnum):
-            if (
-                topics_policy != StreamTopicsPolicyEnum.inherit
-                and not user_profile.can_set_topics_policy()
-            ):
-                raise JsonableError(_("Insufficient permission"))
-            stream_dict_copy["topics_policy"] = topics_policy.value
+        validated_topics_policy = validate_topics_policy(topics_policy, user_profile)
+        if validated_topics_policy is not None:
+            stream_dict_copy["topics_policy"] = validated_topics_policy.value
         stream_dict_copy["can_add_subscribers_group"] = group_settings_map[
             "can_add_subscribers_group"
         ]