test: Add test to check that a node does not fail on unknown commit status

error when starting up.

Test that a node is starting successfully if while joining a cluster and becoming a voter, it
receives an unknown commit status error.

Test for scylladb/scylladb#20814

raft/server.cc

@@ -857,6 +857,10 @@ future<add_entry_reply> server_impl::execute_modify_config(server_id from,
 }
 
 future<> server_impl::modify_config(std::vector<config_member> add, std::vector<server_id> del, seastar::abort_source* as) {
+    utils::get_local_injector().inject("raft/throw_commit_status_unknown_in_modify_config", [] {
+        throw raft::commit_status_unknown();
+    });
+
     if (!_config.enable_forwarding) {
         const auto leader = _fsm->current_leader();
         if (leader != _id) {

test/topology_custom/test_error_becoming_voter.py

@@ -0,0 +1,43 @@
+#
+# Copyright (C) 2025-present ScyllaDB
+#
+# SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.0
+#
+import logging
+import pytest
+import asyncio
+import time
+
+from cassandra import ConsistencyLevel  # type: ignore
+from cassandra.query import SimpleStatement  # type: ignore
+from test.pylib.manager_client import ManagerClient
+from test.pylib.util import wait_for_cql_and_get_hosts
+
+
+logger = logging.getLogger(__name__)
+
+
+@pytest.mark.xfail(reason="issue #20814")
+@pytest.mark.asyncio
+async def test_error_while_becoming_voter(request: pytest.FixtureRequest, manager: ManagerClient) -> None:
+    """
+    Test that a node is starting successfully if while joining a cluster and becoming a voter, it
+    receives an unknown commit status error.
+    Issue https://github.com/scylladb/scylladb/issues/20814
+
+    1. Create a new cluster, start 2 nodes normally.
+    2. Run one node with error injection for throwing an exception commit_status_unknown in modify_config,
+       so that after bootstrapping the node would get a commit_status_unknown error.
+    3. Make sure the node was started successfully. In case the error with the handling of commit_status_unknown is
+       not handled properly, the node will fail to start.
+
+    """
+    logger.info("Creating a new cluster")
+    await manager.servers_add(2)
+
+    srv = await manager.server_add(config={
+        "error_injections_at_startup": [
+            {"name": "raft/throw_commit_status_unknown_in_modify_config", "one_shot": True}
+        ]
+    })
+    await manager.server_start(srv.server_id)