don't crash if ALTER TABLE fails and a long transaction blocks MDL upgrade

mysql-test/main/alter_table_online.test

@@ -34,6 +34,38 @@ set debug_sync= 'now SIGNAL end';
 --reap
 select * from t1;
 
+--echo # Insert, error
+create or replace table t1 (a int);
+insert t1 values (5), (5);
+
+--connection con2
+--send
+set debug_sync= 'now WAIT_FOR ended';
+
+--connection default
+set session lock_wait_timeout=1;
+set debug_sync= 'alter_table_copy_end SIGNAL ended WAIT_FOR end';
+
+--send
+alter table t1 add unique (a), algorithm= copy, lock= none;
+
+--connection con2
+--reap
+start transaction;
+insert into t1 values (123), (456), (789);
+set debug_sync= 'now SIGNAL end';
+
+--connection default
+--error ER_DUP_ENTRY
+--reap
+--connection con2
+commit;
+--connection default
+select variable_value into @otd from information_schema.session_status where variable_name='Opened_table_definitions';
+select * from t1;
+select variable_value-@otd from information_schema.session_status where variable_name='Opened_table_definitions';
+set session lock_wait_timeout=default;
+
 --echo # long transaction and add column
 create or replace table t1 (a int);
 insert t1 values (5);