MDEV-19490 show tables fails when selecting the information_schema database

The bug was that when using mysql_list_fields, then
table_list->schema_table_name was not filled in.
Fixed by using table_list->schema_table instead, which is always
filled in.

sql/sql_base.cc

@@ -3398,8 +3398,7 @@ open_and_process_table(THD *thd, TABLE_LIST *tables, uint *counter, uint flags,
       Check whether the information schema contains a table
       whose name is tables->schema_table_name
     */
-    ST_SCHEMA_TABLE *schema_table;
-    schema_table= find_schema_table(thd, tables->schema_table_name);
+    ST_SCHEMA_TABLE *schema_table= tables->schema_table;
     if (!schema_table ||
         (schema_table->hidden &&
          ((sql_command_flags[lex->sql_command] & CF_STATUS_COMMAND) == 0 ||
@@ -3410,7 +3409,7 @@ open_and_process_table(THD *thd, TABLE_LIST *tables, uint *counter, uint flags,
           lex->sql_command == SQLCOM_SHOW_KEYS)))
     {
       my_error(ER_UNKNOWN_TABLE, MYF(0),
-               tables->schema_table_name, INFORMATION_SCHEMA_NAME.str);
+               tables->table_name, INFORMATION_SCHEMA_NAME.str);
       DBUG_RETURN(1);
     }
   }

tests/mysql_client_test.c

@@ -8390,6 +8390,26 @@ static void test_list_fields()
 }
 
 
+/* Test mysql_list_fields() with information_schema */
+
+static void test_list_information_schema_fields()
+{
+  MYSQL_RES *result;
+  int rc;
+  myheader("test_list_information_schema_fields");
+
+  rc= mysql_select_db(mysql, "information_schema");
+  myquery(rc);
+  result= mysql_list_fields(mysql, "all_plugins", NULL);
+  mytest(result);
+  rc= my_process_result_set(result);
+  DIE_UNLESS(rc == 0);
+  mysql_free_result(result);
+  rc= mysql_select_db(mysql, current_db);
+  myquery(rc);
+}
+
+
 static void test_bug19671()
 {
   MYSQL_RES *result;
@@ -19903,6 +19923,7 @@ static struct my_tests_st my_tests[]= {
   { "test_fetch_column", test_fetch_column },
   { "test_mem_overun", test_mem_overun },
   { "test_list_fields", test_list_fields },
+  { "test_list_fields", test_list_information_schema_fields },
   { "test_free_result", test_free_result },
   { "test_free_store_result", test_free_store_result },
   { "test_sqlmode", test_sqlmode },