topic_mentions: Fetch users to be notified of @topic mentions.

This commit adds the 'topic_wildcard_mention_user_ids' and
'topic_wildcard_mention_in_followed_topic_user_ids'
attributes to the 'RecipientInfoResult' dataclass.

Only topic participants are notified of @topic mentions.

Topic participants are anyone who sent a message to a topic
or reacted to a message on the topic.

'topic_wildcard_mention_in_followed_topic_user_ids' stores the
ids of the topic participants who follow the topic and have
enabled the wildcard mention notifications for followed topics.

'topic_wildcard_mention_user_ids' stores the ids of the topic
participants for whom 'user_allows_notifications_in_StreamTopic'
with setting 'wildcard_mentions_notify' returns True.

zerver/lib/topic.py

@@ -1,15 +1,15 @@
 from datetime import datetime
-from typing import Any, Dict, List, Optional, Tuple
+from typing import Any, Dict, List, Optional, Set, Tuple
 
 import orjson
 from django.db import connection
-from django.db.models import Q, QuerySet
+from django.db.models import Q, QuerySet, Subquery
 from sqlalchemy.sql import ColumnElement, column, func, literal
 from sqlalchemy.types import Boolean, Text
 
 from zerver.lib.request import REQ
 from zerver.lib.types import EditHistoryEvent
-from zerver.models import Message, Stream, UserMessage, UserProfile
+from zerver.models import Message, Reaction, Stream, UserMessage, UserProfile
 
 # Only use these constants for events.
 ORIG_TOPIC = "orig_subject"
@@ -284,3 +284,22 @@ def get_topic_resolution_and_bare_name(stored_name: str) -> Tuple[bool, str]:
         return (True, stored_name[len(RESOLVED_TOPIC_PREFIX) :])
 
     return (False, stored_name)
+
+
+def participants_for_topic(recipient_id: int, topic_name: str) -> Set[int]:
+    """
+    Users who either sent or reacted to the messages in the topic.
+    The function is expensive for large numbers of messages in the topic.
+    """
+    messages = Message.objects.filter(recipient_id=recipient_id, subject__iexact=topic_name)
+    participants = set(
+        UserProfile.objects.filter(
+            Q(id__in=Subquery(messages.values("sender_id")))
+            | Q(
+                id__in=Subquery(
+                    Reaction.objects.filter(message__in=messages).values("user_profile_id")
+                )
+            )
+        ).values_list("id", flat=True)
+    )
+    return participants