MDEV-11943 I_S.TABLES inconsistencies with tables with unknown storage engine

Try harder to show the table's engine.

If the table's engine is not loaded, the table won't open.
But we can still read the engine name from frm as a string.

mysql-test/r/drop_bad_db_type.result

@@ -10,7 +10,7 @@ table_name	t1
 select table_schema, table_name, engine, version from information_schema.tables where table_name like 't1';
 table_schema	test
 table_name	t1
-engine	NULL
+engine	ARCHIVE
 version	NULL
 Warnings:
 Level	Warning
@@ -19,7 +19,7 @@ Message	Unknown storage engine 'ARCHIVE'
 select table_schema, table_name, engine, row_format from information_schema.tables where table_name like 't1';
 table_schema	test
 table_name	t1
-engine	NULL
+engine	ARCHIVE
 row_format	NULL
 Warnings:
 Level	Warning

sql/sql_show.cc

@@ -4056,6 +4056,22 @@ make_table_name_list(THD *thd, Dynamic_array<LEX_STRING*> *table_names,
 }
 
 
+static void get_table_engine_for_i_s(THD *thd, char *buf, TABLE_LIST *tl,
+                                     LEX_STRING *db, LEX_STRING *table)
+{
+  LEX_STRING engine_name= { buf, 0 };
+
+  if (thd->get_stmt_da()->sql_errno() == ER_UNKNOWN_STORAGE_ENGINE)
+  {
+    char path[FN_REFLEN];
+    build_table_filename(path, sizeof(path) - 1,
+                         db->str, table->str, reg_ext, 0);
+    if (dd_frm_type(thd, path, &engine_name) == FRMTYPE_TABLE)
+      tl->option= engine_name.str;
+  }
+}
+
+
 /**
   Fill I_S table with data obtained by performing full-blown table open.
 
@@ -4126,9 +4142,9 @@ fill_schema_table_by_open(THD *thd, bool is_show_fields_or_keys,
 
   /*
     Since make_table_list() might change database and table name passed
-    to it we create copies of orig_db_name and orig_table_name here.
-    These copies are used for make_table_list() while unaltered values
-    are passed to process_table() functions.
+    to it (if lower_case_table_names) we create copies of orig_db_name and
+    orig_table_name here.  These copies are used for make_table_list()
+    while unaltered values are passed to process_table() functions.
   */
   if (!thd->make_lex_string(&db_name,
                             orig_db_name->str, orig_db_name->length) ||
@@ -4215,6 +4231,10 @@ fill_schema_table_by_open(THD *thd, bool is_show_fields_or_keys,
   }
   else
   {
+    char buf[NAME_CHAR_LEN + 1];
+    if (thd->is_error())
+      get_table_engine_for_i_s(thd, buf, table_list, &db_name, &table_name);
+
     result= schema_table->process_table(thd, table_list,
                                         table, result,
                                         orig_db_name,
@@ -4525,7 +4545,9 @@ static int fill_schema_table_from_frm(THD *thd, TABLE *table,
     }
     else
     {
-      table_list.table= &tbl;
+      char buf[NAME_CHAR_LEN + 1];
+      get_table_engine_for_i_s(thd, buf, &table_list, db_name, table_name);
+
       res= schema_table->process_table(thd, &table_list, table,
                                        true, db_name, table_name);
     }
@@ -4939,6 +4961,11 @@ static int get_schema_tables_record(THD *thd, TABLE_LIST *tables,
     else
       table->field[3]->store(STRING_WITH_LEN("BASE TABLE"), cs);
 
+    if (tables->option)
+    {
+      table->field[4]->store(tables->option, strlen(tables->option), cs);
+      table->field[4]->set_notnull();
+    }
     goto err;
   }