Added a test for race condition

dbms/tests/queries/0_stateless/00941_system_columns_race_condition.sh

+#!/usr/bin/env bash
+
+# Test fix for issue #5066
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+. $CURDIR/../shell_config.sh
+
+set -e
+
+$CLICKHOUSE_CLIENT -q "DROP TABLE IF EXISTS alter_table"
+$CLICKHOUSE_CLIENT -q "CREATE TABLE alter_table (a UInt8, b Int16, c Float32, d String, e Array(UInt8), f Nullable(UUID), g Tuple(UInt8, UInt16)) ENGINE = MergeTree ORDER BY a"
+
+function thread1()
+{
+    while true; do $CLICKHOUSE_CLIENT --query "SELECT * FROM system.columns FORMAT Null"; done
+}
+
+function thread2()
+{
+    while true; do $CLICKHOUSE_CLIENT -n --query "ALTER TABLE alter_table ADD COLUMN h String; ALTER TABLE alter_table MODIFY COLUMN h UInt64; ALTER TABLE alter_table DROP COLUMN h;"; done
+}
+
+# https://stackoverflow.com/questions/9954794/execute-a-shell-function-with-timeout
+export -f thread1;
+export -f thread2;
+
+timeout 5 bash -c thread1 &
+timeout 5 bash -c thread2 &
+
+wait
+
+$CLICKHOUSE_CLIENT -q "DROP TABLE alter_table"