We can't rely on keys formed with columns that were added during this
ALTER. These columns can be set with non-deterministic values, which can
end up with broken or incorrect search.

The same applies to the keys that contain reliable columns, but also have
bogus ones. Using them can narrow the search, but they're also ignored.

Also, added columns shouldn't be considered during the record match. To
determine them, table->has_value_set bitmap is used.

To fill has_value_set bitmap in the find_key call, extra unpack_row call
has been added.

For replication case, extra replica columns can be considered for this
case. We try to ignore them, too.

use table->pos_in_table_list instead.

Also, table->in_use is always set

ONLINE ALTER TABLE adds binlog handlerton into ha_list, so any
rollback command can end up calling binlog_rollback having no cache_mngr,
if binlog is not enabled.

The assertion should be fixed in the same manner as DBUG_ASSERT(WSREP(thd))

1. ER_KEY_NOT_FOUND
general replcation problem, already fixed earlier.
test added.

2. ER_LOCK_WAIT_TIMEOUT
This is a long unique specific problem.

Sometimes, lookup_handler is created for to->file. To properly free it,
ha_reset should be called. It is usually done by calling
close_thread_table, but ALTER TABLE makes it differently. Hence, a single
ha_reset call is added to mysql_alter_table.

Also, event_mem_root is removed. Normally, no per-event data should be
allocated on thd->mem_root, that would mean a leak. And otherwise,
lookup_handler is lazily allocated, but its lifetime matches statement,
not event.

* in online ALTER it must include the complete new row,
  note that an UPDATE should set all extra columns to their
  default values, as if UPDATE was completely done before the ALTER.
* in rpl WRITE_ROWS_EVENT it must include all extra slave columns,
  but not existing columns unmarked in the m_cols (sequences do that)
* in rpl UPDATE/DELETE events it should follow m_cols_ai

also: default values must be updated for WRITE_ROWS_EVENT and
for UPDATE/DELETE in the online ALTER mode, see above.
Update the result file accordingly.

Extend bitmap_copy() to support arguments of different lengths

MDEV-28816 Assertion `wsrep_thd_is_applying(thd)' failed in int wsrep_ignored_error_code(Log_event*, int)

wsrep expected that any Rows_log_event::do_apply_event() failures
can only happen in the applier thread, because no other thread ever
applies row events. With online alter it's no longer true.

because online means we'll apply events from the binlog, and
ignore means that bad rows will be skipped. So a bad Write_row_log_event
will be skipped and a following Update_row_log_event will fail to
apply.

This test case was also fixed by adding update_virtual_columns.

Division by zero is a good example. sql_mode is basically ignored by
replication, see Bug#56662.

The behavior for ONLINE should remain the same as for non-ONLINE ALTER.

mark fields that have explicit values,
so that table->update_default_fields() would know what to update

We shouldn't rely on `fill_extra_persistent_columns`, as it only updates
fields which have an index > cols->n_bits (replication bitmap width).
Actually, it should never be used, as its approach is error-prone.

Normal update_virtual_fields+update_default_fields should be done.

if ALTER TABLE ... LOCK=xxx is executed under LOCK TABLES,
ignore the LOCK clause, because ALTER should not downgrade
already taken EXCLUSIVE table lock to SHARED or NONE.

This commit preserves the existing behavior (LOCK was de facto ignored),
but makes it explicit.

ALTER ONLINE TABLE acquires table with TL_READ. Myisam normally acquires
TL_WRITE for DML, which makes it hang until table is freed.

We deadlock once ALTER upgrades its MDL lock.

Solution:
Unlock table earlier. We don't need to hold TL_READ once we finished
copying. Relay log replication requires no data locks on `from` table.

MDEV-28967 Assertion `marked_for_write_or_computed()' failed in Field_new_decimal::store_value / online_alter_read_from_binlog`

in the catch-up phase of the online alter we apply row events,
they're unpacked into `from->record[0]` and then converted
to `to->record[0]`.

This needs all fields of `from` to be in the `write_set`.

Although practically `Field::unpack()` does not assert the `write_set`,
and `Field::reset()` - used when a field value is not present in the
after-image - also doesn't assert the `write_set` for many types,
`Field_new_decimal::reset()` does.

in RBR - only show warnings for values that are to be written into a
table, that is, only for the after-image. Don't show data conversion
warnings for the before-image.

* don't disable warnings when catching up
* do propagate warnings up like copy_data_between_tables() does

If online alter fails, TABLE_SHARE can be freed while concurrent
transactions still have row events in their online_alter_cache_data.
On commit they try'll to flush them, writing to TABLE_SHARE's
Cache_flip_event_log, which is already freed.
This causes a crash in main.alter_table_online_debug test

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).

even if called from binlog_rollback()

because some engines (e.g. rocksdb) only do it readonly.
instead, downgrade the lock after reading the first row.

instead use only one (trx) IO_CACHE and truncate it if the
statement is rolled back.

don't use binlog_cache_mngr to accumulate the data,
use binlog_cache_data instead.

(binlog_cache_data owns one IO_CACHE, binlog_cache_mngr owns
two binlog_cache_data's, trx and stmt).