MDEV-28771 Assertion `table->in_use&&tdc->flushed' failed after ALTER

don't simply set tdc->flushed, use flush_unused(1) that removes opened
but unused TABLE instances (that would otherwise prevent TABLE_SHARE from
being closed by keeping the ref_count>0).

mysql-test/main/alter_table_online.result

@@ -0,0 +1,56 @@
+#
+# MDEV-28771 Assertion `table->in_use&&tdc->flushed' failed after ALTER
+#
+create table t (a char(1));
+insert into t values ('a'),('b');
+select * from t join t as t2 join t as t3;
+a	a	a
+a	a	a
+b	a	a
+a	b	a
+b	b	a
+a	a	b
+b	a	b
+a	b	b
+b	b	b
+alter table t modify a int;
+ERROR 22007: Truncated incorrect INTEGER value: 'a'
+select * from t;
+a
+a
+b
+drop table t;
+create table t (c double precision key,c2 char,c3 year);
+insert into t values (7,3,1);
+select a from t where a=all (select a from t where b=2 union select a from t where b=2);
+ERROR 42S22: Unknown column 'a' in 'field list'
+insert into t values (3,1,1);
+alter table t change c c date,add key(c);
+ERROR 22007: Incorrect date value: '7' for column `test`.`t`.`c` at row 1
+select * from t;
+c	c2	c3
+7	3	2001
+3	1	2001
+drop table t;
+set sql_mode='';
+create table t (c char unique,c2 int,stamp timestamp);
+insert into t values (1,1,1),(2,2,2),(3,3,3),(4,4,4),(5,5,5);
+Warnings:
+Warning	1265	Data truncated for column 'stamp' at row 1
+Warning	1265	Data truncated for column 'stamp' at row 2
+Warning	1265	Data truncated for column 'stamp' at row 3
+Warning	1265	Data truncated for column 'stamp' at row 4
+Warning	1265	Data truncated for column 'stamp' at row 5
+update t set c=(select * from t) where c in (select * from t);
+ERROR 21000: Operand should contain 1 column(s)
+alter table t modify c date;
+ERROR 23000: Duplicate entry '0000-00-00' for key 'c'
+select * from t;
+c	c2	stamp
+1	1	0000-00-00 00:00:00
+2	2	0000-00-00 00:00:00
+3	3	0000-00-00 00:00:00
+4	4	0000-00-00 00:00:00
+5	5	0000-00-00 00:00:00
+drop table t;
+set sql_mode=default;

mysql-test/main/alter_table_online.test

@@ -0,0 +1,35 @@
+--source include/binlog_combinations.inc
+--source include/have_innodb.inc
+
+--echo #
+--echo # MDEV-28771 Assertion `table->in_use&&tdc->flushed' failed after ALTER
+--echo #
+
+create table t (a char(1));
+insert into t values ('a'),('b');
+select * from t join t as t2 join t as t3;
+--error ER_TRUNCATED_WRONG_VALUE
+alter table t modify a int;
+select * from t;
+drop table t;
+
+create table t (c double precision key,c2 char,c3 year);
+insert into t values (7,3,1);
+--error ER_BAD_FIELD_ERROR
+select a from t where a=all (select a from t where b=2 union select a from t where b=2);
+insert into t values (3,1,1);
+--error ER_TRUNCATED_WRONG_VALUE
+alter table t change c c date,add key(c);
+select * from t;
+drop table t;
+
+set sql_mode='';
+create table t (c char unique,c2 int,stamp timestamp);
+insert into t values (1,1,1),(2,2,2),(3,3,3),(4,4,4),(5,5,5);
+--error ER_OPERAND_COLUMNS
+update t set c=(select * from t) where c in (select * from t);
+--error ER_DUP_ENTRY
+alter table t modify c date;
+select * from t;
+drop table t;
+set sql_mode=default;

sql/sql_table.cc

@@ -12014,7 +12014,7 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
   }
   else if (online) // error was on copy stage
   {
-    from->s->tdc->flushed= 1; // to free the binlog
+    from->s->tdc->flush_unused(1); // to free the binlog
   }
 #endif