MDEV-33450 Assertion fails in main.alter_table_online_debug

Assertion "from->s->online_alter_binlog == NULL" fails in
copy_data_between_tables, signalizing that a table share is being reused
(in another alter) after a lock upgrade to EXCLUSIVE fails.

Commit 3059f274 relaxed the lock to be upgraded to MDL_SHARED_NO_WRITE, leaving
it to happen later by a common path wait_while_table_is_used() call.

However the error handling there is not enough for online alter case, where we
require (for now) the table to be flushed, in order to clean up the memory
properly.

* Add another lock upgrade (to MDL_EXCLUSIVE) after the second replication stage
 in copy_data_between_tables.

The error from this upgrade will be handled by the branch presented further in
the function.

MDEV-33450 Assertion fails in main.alter_table_online_debug

`TABLE_SHARE` that is being online-altered has a shared `s->online_alter_binlog`
member that all concurrent DMLs are writing to. Online alter thread deletes it
under the MDL_EXCLUSIVE. If upgrading the lock to MDL_EXCLUSIVE fails, table as
marked as `flushed` and it's freed automatically when its usage drops to zero.

In commit 3059f274 the lock upgrade was relaxed to MDL_SHARED_NO_WRITE to allow
concurrent SELECT threads during the final `online_alter_read_from_binlog()`
pass. An attempt to upgrade the lock to MDL_EXCLUSIVE was still happening, but
much later — after the code that marked the table `flushed`.

That is, if the upgrade failed, the table was left with a stale
`s->online_alter_binlog` triggering an assert in a future online alter.

To fix this, upgrade the lock to MDL_EXCLUSIVE earlier, after the final
`online_alter_read_from_binlog()`.

sql/sql_table.cc

@@ -12423,6 +12423,17 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
       thd_progress_next_stage(thd);
       error= online_alter_read_from_binlog(thd, &rgi, binlog, &found_count);
     }
+
+    /*
+      We'll do it earlier than usually in mysql_alter_table to handle the
+      online-related errors more comfortably.
+    */
+    lock_error= thd->mdl_context.upgrade_shared_lock(from->mdl_ticket,
+                                                     MDL_EXCLUSIVE,
+                                      (double)thd->variables.lock_wait_timeout);
+    if (!error)
+      error= lock_error;
+
     to->pos_in_table_list= NULL; // Safety
     DBUG_ASSERT(thd->lex->sql_command == saved_sql_command);
     thd->lex->sql_command= saved_sql_command; // Just in case