MDEV-29068 Cascade foreign key updates do not apply in online alter

mysql-test/main/alter_table_online.result

@@ -80,3 +80,43 @@ XA END 'xid';
 XA COMMIT 'xid' ONE PHASE;
 DROP TABLE t;
 disconnect con1;
+connection default;
+#
+# MDEV-29068 Cascade foreign key updates do not apply in online alter
+#
+create table t1 (a int primary key) engine=InnoDB;
+insert into t1 values (1),(2),(3);
+create table t2 (b int, foreign key (b)
+references t1 (a)
+on update cascade) engine=InnoDB;
+insert into t2 values (1),(2),(3);
+alter table t2 add c int, algorithm=copy, lock=none;
+ERROR 0A000: LOCK=NONE is not supported. Reason: COPY algorithm requires a lock. Try LOCK=SHARED
+alter table t2 add c int, algorithm=inplace, lock=none;
+create or replace table t2 (b int, foreign key (b)
+references t1 (a)
+on delete set null) engine=InnoDB;
+alter table t2 add c int, algorithm=copy, lock=none;
+ERROR 0A000: LOCK=NONE is not supported. Reason: COPY algorithm requires a lock. Try LOCK=SHARED
+alter table t2 add c int, algorithm=inplace, lock=none;
+create or replace table t2 (b int, foreign key (b)
+references t1 (a)
+on delete no action) engine=InnoDB;
+insert into t2 values (1),(2),(3);
+alter table t2 add c int, algorithm=copy, lock=none;
+create or replace table t2 (b int, foreign key (b)
+references t1 (a)
+on update restrict) engine=InnoDB;
+insert into t2 values (1),(2),(3);
+alter table t2 add c int, algorithm=copy, lock=none;
+drop table t2, t1;
+create table t1 (a int primary key, b int unique) engine=InnoDB;
+insert into t1 values (1, 1),(2, 2),(3, 3);
+create table t2 (a int references t1 (a),
+b int references t1 (b) on update cascade) engine=InnoDB;
+insert into t2 values (1, 1),(2, 2);
+alter table t2 add c int, algorithm=copy, lock=none;
+ERROR 0A000: LOCK=NONE is not supported. Reason: COPY algorithm requires a lock. Try LOCK=SHARED
+alter table t2 add c int, algorithm=copy;
+alter table t2 add d int, algorithm=inplace;
+drop table t2, t1;

mysql-test/main/alter_table_online.test

@@ -64,3 +64,54 @@ XA COMMIT 'xid' ONE PHASE;
 
 DROP TABLE t;
 --disconnect con1
+--connection default
+
+--echo #
+--echo # MDEV-29068 Cascade foreign key updates do not apply in online alter
+--echo #
+create table t1 (a int primary key) engine=InnoDB;
+insert into t1 values (1),(2),(3);
+create table t2 (b int, foreign key (b)
+                        references t1 (a)
+                        on update cascade) engine=InnoDB;
+insert into t2 values (1),(2),(3);
+
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table t2 add c int, algorithm=copy, lock=none;
+alter table t2 add c int, algorithm=inplace, lock=none;
+
+create or replace table t2 (b int, foreign key (b)
+                            references t1 (a)
+                            on delete set null) engine=InnoDB;
+
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table t2 add c int, algorithm=copy, lock=none;
+alter table t2 add c int, algorithm=inplace, lock=none;
+
+create or replace table t2 (b int, foreign key (b)
+                            references t1 (a)
+                            on delete no action) engine=InnoDB;
+
+insert into t2 values (1),(2),(3);
+alter table t2 add c int, algorithm=copy, lock=none;
+
+create or replace table t2 (b int, foreign key (b)
+                            references t1 (a)
+                            on update restrict) engine=InnoDB;
+
+insert into t2 values (1),(2),(3);
+alter table t2 add c int, algorithm=copy, lock=none;
+drop table t2, t1;
+
+create table t1 (a int primary key, b int unique) engine=InnoDB;
+insert into t1 values (1, 1),(2, 2),(3, 3);
+create table t2 (a int references t1 (a),
+                 b int references t1 (b) on update cascade) engine=InnoDB;
+insert into t2 values (1, 1),(2, 2);
+
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table t2 add c int, algorithm=copy, lock=none;
+alter table t2 add c int, algorithm=copy;
+alter table t2 add d int, algorithm=inplace;
+# Cleanup
+drop table t2, t1;

sql/sql_table.cc

@@ -10071,6 +10071,18 @@ bool mysql_alter_table(THD *thd, const LEX_CSTRING *new_db,
   
   online= online && !table->s->tmp_table;
 
+  List<FOREIGN_KEY_INFO> fk_list;
+  table->file->get_foreign_key_list(thd, &fk_list);
+  for (auto &fk: fk_list)
+  {
+    if (fk_modifies_child(fk.delete_method)
+        || fk_modifies_child(fk.update_method))
+    {
+      online= false;
+      break;
+    }
+  }
+
 #ifdef WITH_WSREP
   if (WSREP(thd) &&
       (thd->lex->sql_command == SQLCOM_ALTER_TABLE ||