Bug #25385590 DROP TABLE CRASHES IF INNODB_FORCE_RECOVERY > 4

Problem:
========
 - Drop table assert if innodb_force_recovery is set to 5 or 6.
For innodb_force_recovery 5 and 6, InnoDB doesn't scan the undo log
and it makes the redo rollback segment as NULL. There is no way for
transaction to write any undo log.

 - If innodb_force_recovery is set to 6 then InnoDB does not do the
redo log roll-forward in connection with recovery. In this case,
log_sys will be initalized only and it will not have latest
checkpoint information. Checkpoint is done during shutdown even
innodb_force_recovery is set to 6. So it leads to incorrect
information update in checkpoint header.

Solution:
========
1) Allow drop table only if innodb_force_recovery < 5.
2) Make innodb as read-only if innodb_force_recovery is set to 6.
3) During shutdown, remove the checkpoint if innodb_force_recovery
is set to 6.
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>
RB: 15075

storage/innobase/handler/ha_innodb.cc

@@ -13797,7 +13797,8 @@ ha_innobase::delete_table(
 	extension, in contrast to ::create */
 	normalize_table_name(norm_name, name);
 
-	if (srv_read_only_mode) {
+	if (srv_read_only_mode
+	    || srv_force_recovery >= SRV_FORCE_NO_UNDO_LOG_SCAN) {
 		DBUG_RETURN(HA_ERR_TABLE_READONLY);
 	}
 

storage/innobase/srv/srv0start.cc

@@ -1442,6 +1442,10 @@ innobase_start_or_create_for_mysql(void)
 	size_t		dirnamelen;
 	unsigned	i = 0;
 
+	if (srv_force_recovery == SRV_FORCE_NO_LOG_REDO) {
+		srv_read_only_mode = true;
+	}
+
 	high_level_read_only = srv_read_only_mode
 		|| srv_force_recovery > SRV_FORCE_NO_TRX_UNDO;