message: Add an O(1)-query variant of bulk_access_messages.

We'll use this in the push-notifications code, in a context where
there should definitely already be UserMessage rows if everything's
gone normally... but explicitly checking at the top seems like the
right pattern from a secure-coding perspective.

zerver/lib/message.py

@@ -555,6 +555,26 @@ def bulk_access_messages(user_profile: UserProfile, messages: Sequence[Message])
             filtered_messages.append(message)
     return filtered_messages
 
+def bulk_access_messages_expect_usermessage(
+        user_profile_id: int, message_ids: Sequence[int]) -> List[int]:
+    '''
+    Like bulk_access_messages, but faster and potentially stricter.
+
+    Returns a subset of `message_ids` containing only messages the
+    user can access.  Makes O(1) database queries.
+
+    Use this function only when the user is expected to have a
+    UserMessage row for every message in `message_ids`.  If a
+    UserMessage row is missing, the message will be omitted even if
+    the user has access (e.g. because it went to a public stream.)
+
+    See also: `access_message`, `bulk_access_messages`.
+    '''
+    return UserMessage.objects.filter(
+        user_profile_id=user_profile_id,
+        message_id__in=message_ids,
+    ).values_list('message_id', flat=True)
+
 def render_markdown(message: Message,
                     content: str,
                     realm: Optional[Realm]=None,