message_fetch: Take a REPEATABLE READ READ ONLY when fetching messages.

zerver/views/message_fetch.py

@@ -1,6 +1,8 @@
 from typing import Dict, Iterable, List, Optional, Tuple, Union
 
+from django.conf import settings
 from django.contrib.auth.models import AnonymousUser
+from django.db import connection, transaction
 from django.http import HttpRequest, HttpResponse
 from django.utils.html import escape as escape_html
 from django.utils.translation import gettext as _
@@ -169,6 +171,31 @@ def get_messages_backend(
         assert log_data is not None
         log_data["extra"] = "[{}]".format(",".join(verbose_operators))
 
+    with transaction.atomic(durable=True):
+        # We're about to perform a search, and then get results from
+        # it; this is done across multiple queries.  To prevent race
+        # conditions, we want the messages returned to be consistent
+        # with the version of the messages that was searched, to
+        # prevent changes which happened between them from leaking to
+        # clients who should not be able to see the new values, and
+        # when messages are deleted in between.  We set up
+        # repeatable-read isolation for this transaction, so that we
+        # prevent both phantom reads and non-repeatable reads.
+        #
+        # In a read-only repeatable-read transaction, it is not
+        # possible to encounter deadlocks or need retries due to
+        # serialization errors.
+        #
+        # You can only set the isolation level before any queries in
+        # the transaction, meaning it must be the top-most
+        # transaction, which durable=True establishes.  Except in
+        # tests, where durable=True is a lie, because there is an
+        # outer transaction for each test.  We thus skip this command
+        # in tests, since it would fail.
+        if not settings.TEST_SUITE:  # nocoverage
+            cursor = connection.cursor()
+            cursor.execute("SET TRANSACTION ISOLATION LEVEL REPEATABLE READ READ ONLY")
+
         query_info = fetch_messages(
             narrow=narrow,
             user_profile=user_profile,
@@ -205,7 +232,9 @@ def get_messages_backend(
             message_ids = [row[0] for row in rows]
 
             # TODO: This could be done with an outer join instead of two queries
-        um_rows = UserMessage.objects.filter(user_profile=user_profile, message_id__in=message_ids)
+            um_rows = UserMessage.objects.filter(
+                user_profile=user_profile, message_id__in=message_ids
+            )
             user_message_flags = {um.message_id: um.flags_list() for um in um_rows}
 
             for message_id in message_ids: