MDEV-14581 Server does not clear diagnostics between sessions

Amend previous patch, so it works in all cases (also for "change user"
command, and for RESET CONNECTION in 10.3)

sql/mysqld.cc

@@ -2962,10 +2962,6 @@ static bool cache_thread(THD *thd)
       _db_pop_();
 #endif
 
-    /* Clear warnings. */
-    if (!thd->get_stmt_da()->is_warning_info_empty())
-      thd->get_stmt_da()->clear_warning_info(thd->query_id);
-
     set_timespec(abstime, THREAD_CACHE_TIMEOUT);
     while (!abort_loop && ! wake_thread && ! kill_cached_threads)
     {

sql/sql_class.cc

@@ -1332,6 +1332,11 @@ void THD::change_user(void)
   cleanup_done= 0;
   reset_killed();
   thd_clear_errors(this);
+
+  /* Clear warnings. */
+  if (!get_stmt_da()->is_warning_info_empty())
+    get_stmt_da()->clear_warning_info(0);
+
   init();
   stmt_map.reset();
   my_hash_init(&user_vars, system_charset_info, USER_VARS_HASH_SIZE, 0, 0,

tests/mysql_client_test.c

@@ -16348,6 +16348,7 @@ static void test_change_user()
   const char *db= "mysqltest_user_test_database";
   int rc;
   MYSQL*       conn;
+  MYSQL_RES* res;
   DBUG_ENTER("test_change_user");
   myheader("test_change_user");
 
@@ -16484,6 +16485,20 @@ static void test_change_user()
   rc= mysql_change_user(conn, user_pw, pw, "");
   myquery(rc);
 
+  /* MDEV-14581 : Check that there are no warnings after change user.*/
+  rc = mysql_query(conn,"SIGNAL SQLSTATE '01000'");
+  myquery(rc);
+
+  rc = mysql_change_user(conn, user_pw, pw, "");
+  myquery(rc);
+
+  rc = mysql_query(conn, "SHOW WARNINGS");
+  myquery(rc);
+  res = mysql_store_result(conn);
+  rc = my_process_result_set(res);
+  DIE_UNLESS(rc == 0);
+  mysql_free_result(res);
+
   rc= mysql_change_user(conn, user_no_pw, pw, db);
   DIE_UNLESS(rc);
   if (! opt_silent)