MDEV-9121: innodb_force_recovery = 6 cannot recover ANY DATA when

change buffer not empty

Fix: Allow not empty change buffer when innodb_force_recovery >= 5
and output only a warning to error log.

Note: Before using force recovery you should always take backup
of your database.

storage/innobase/dict/dict0boot.cc

@@ -458,12 +458,22 @@ dict_boot(void)
 	if (err == DB_SUCCESS) {
 		if (srv_read_only_mode && !ibuf_is_empty()) {
 
+			if (srv_force_recovery < SRV_FORCE_NO_IBUF_MERGE) {
 				ib_logf(IB_LOG_LEVEL_ERROR,
 					"Change buffer must be empty when --innodb-read-only "
-				"is set!");
+					"is set! "
+					"You can try to recover the database with innodb_force_recovery=5");
 
 				err = DB_ERROR;
 			} else {
+				ib_logf(IB_LOG_LEVEL_WARN,
+					"Change buffer not empty when --innodb-read-only "
+					"is set! but srv_force_recovery = %d, ignoring.",
+					srv_force_recovery);
+			}
+		}
+
+		if (err == DB_SUCCESS) {
 			/* Load definitions of other indexes on system tables */
 
 			dict_load_sys_table(dict_sys->sys_tables);

storage/xtradb/dict/dict0boot.cc

@@ -464,12 +464,22 @@ dict_boot(void)
 	if (err == DB_SUCCESS) {
 		if (srv_read_only_mode && !ibuf_is_empty()) {
 
+			if (srv_force_recovery < SRV_FORCE_NO_IBUF_MERGE) {
 				ib_logf(IB_LOG_LEVEL_ERROR,
 					"Change buffer must be empty when --innodb-read-only "
-				"is set!");
+					"is set!"
+					"You can try to recover the database with innodb_force_recovery=5");
 
 				err = DB_ERROR;
 			} else {
+				ib_logf(IB_LOG_LEVEL_WARN,
+					"Change buffer not empty when --innodb-read-only "
+					"is set! but srv_force_recovery = %d, ignoring.",
+					srv_force_recovery);
+			}
+		}
+
+		if (err == DB_SUCCESS) {
 			/* Load definitions of other indexes on system tables */
 
 			dict_load_sys_table(dict_sys->sys_tables);