MDEV-11639 Server crashes in update_virtual_field, gcol.innodb_virtual_basic fails in buildbot

don't use thd->query_id check in background purge threads
(it doesn't work, because thd->query_id is never incremented there)
instead use thd->open_tables directly, there can be only one table
there anyway, and this is the table opened by this purge thread.

sql/sql_class.cc

@@ -4448,6 +4448,14 @@ TABLE *open_purge_table(THD *thd, const char *db, size_t dblen,
   DBUG_RETURN(error ? NULL : tl->table);
 }
 
+TABLE *get_purge_table(THD *thd)
+{
+  /* see above, at most one table can be opened */
+  DBUG_ASSERT(thd->open_tables == NULL || thd->open_tables->next == NULL);
+  return thd->open_tables;
+}
+
+
 /** Find an open table in the list of prelocked tabled
 
   Used for foreign key actions, for example, in UPDATE t1 SET a=1;

storage/innobase/handler/ha_innodb.cc

@@ -132,6 +132,7 @@ void destroy_thd(MYSQL_THD thd);
 void reset_thd(MYSQL_THD thd);
 TABLE *open_purge_table(THD *thd, const char *db, size_t dblen,
 			const char *tb, size_t tblen);
+TABLE *get_purge_table(THD *thd);
 
 /** Check if user has used xtradb extended system variable that
 is not currently supported by innodb or marked as deprecated. */
@@ -22119,9 +22120,18 @@ innobase_find_mysql_table_for_vc(
 	THD*		thd,
 	dict_table_t*	table)
 {
+	TABLE *mysql_table;
+	bool	bg_thread = THDVAR(thd, background_thread);
+
+	if (bg_thread) {
+		if ((mysql_table = get_purge_table(thd))) {
+			return mysql_table;
+		}
+	} else {
 		if (table->vc_templ->mysql_table_query_id == thd_get_query_id(thd)) {
 			return table->vc_templ->mysql_table;
 		}
+	}
 
 	char	dbname[MAX_DATABASE_NAME_LEN + 1];
 	char	tbname[MAX_TABLE_NAME_LEN + 1];
@@ -22150,14 +22160,13 @@ innobase_find_mysql_table_for_vc(
 	tbnamelen = filename_to_tablename(tbname, t_tbname,
 					  MAX_TABLE_NAME_LEN + 1);
 
-	TABLE *mysql_table = find_fk_open_table(thd, t_dbname, dbnamelen,
+	if (bg_thread) {
+		return open_purge_table(thd, t_dbname, dbnamelen,
 					t_tbname, tbnamelen);
+	}
 
-	if (!mysql_table && THDVAR(thd, background_thread)) {
-		/* only open the table in background purge threads */
-		mysql_table = open_purge_table(thd, t_dbname, dbnamelen,
+	mysql_table = find_fk_open_table(thd, t_dbname, dbnamelen,
 					t_tbname, tbnamelen);
-	}
 
 	table->vc_templ->mysql_table = mysql_table;
 	table->vc_templ->mysql_table_query_id = thd_get_query_id(thd);