Group all the checks in online_alter_check_supported().

There is now two groups of checks:
1. A technical availability of online, that is checked before open_tables,
and affects table_list->lock_type. It's supposed to be safe to make it
TL_READ even if COPY algorithm will fall back to not-online, since MDL is
SHARED_UPGRADEABLE anyway.
2. An 'online' availability for a COPY algorithm. It can be done as late as
just before the copy_data_between_tables call. The lock_type influence is
disclosed above, so the only other place it affects is
Alter_info::supports_lock, where `online` flag is only used to decide
whether to report the error at the inplace preparation stage. We'd want to
make that at the last resort, which is COPY preparation, if no algorithm is
chosen by the user. So it's event better now.

Some changes are required to the autoinc support detection, as the check
now happens after mysql_prepare_alter_table:
* alter_info->drop_list is empty
* instead, dropped columns are in tmp_set
* alter_info->create_list now has every field that's in the new table.
* the column definition's change.str will be nonnull whether the column
  remains in the new table (vs whether it was changed, as before).
  But it also has `field` field set.
* IF EXISTS doesn't have to be dealt anymore

This infers that the changes are now checked in more detail: a field's
definition shouldn't be changed, vs a field shouldn't be mentioned in
the CHANGE list, as it was before. This is reflected by the line 193 test.

Also update abi declarations. The abi itself is unchanged, since const
doesn't affect C-style exported name.

disabie view-protocol for certain places:
1. While a transaction is in progress: a view gets locked for a
transaction duration, so DROP VIEW from a servicing conneciton
deadlocks. We can't just disable servicing connection, as
CREATE VIEW and DROP VIEW commit implicitly.
2. For querying Opened_table_definitions: the actual query executed
is `SELECT * FROM mysqltest_tmp_v`, then the view is dropped

Skip more rpl-related error handling.
Also move the error check inside if (table) -- otherwise the error
should be handled already.

Online alter and replication paths differ quite noticeably. It's time to
fix the mess. Online alter has to unpack the old table's data
(here conv_table), and then convert, while replication requires more
sophisticated track: handle possible conversions, treat extra replica
fields, and carefully deduce master's record length.

* Extract unpack_field.
* handle the unpacking progress through a state machine presented by
  Unpack_record_state struct.
* Stick most of the online alter unpacking together under a single
  conditional branch.

When column is changed to autoinc, ALTER TABLE may update zero/NULL values,
if NO_AUTO_VALUE_ON_ZERO mode is not enabled.

Forbid this for LOCK=NONE for the unreliable cases.
The cases are described in online_alter_check_autoinc.

when committing a big transaction, online_alter_cache_log creates a cache
file. It wasn't properly closed, which was spotted by a memory leak from
my_register_filename. A temporary file also remained open.

Binlog wasn't affected by this, since it features its own file management.

A proper closing is calling close_cached_file. It deinits io_cache and
closes the underlying file. After closing, the file is expected to be
deleted automagically.

Non-transactional engines changes are visible immediately the row operation
succeeds, so we should apply the changes to the online buffer immediately
after the statement ends.

The row events were applied "twice": once for the ha_partition, and one
more time for the underlying storage engine.

There's no such problem in binlog/rpl, because ha_partiton::row_logging
is normally set to false.

The fix makes the events replicate only when the handler is a root handler.
We will try to *guess* this by comparing it to table->file. The same
approach is used in the MDEV-21540 fix, 231feabd. The assumption is made,
that the row methods are only called for table->file (and never for a
cloned handler), hence the assertions are added in ha_innobase and
ha_myisam to make sure that this is true at least for those engines

Also closes MDEV-31040, however the test is not included, since we have no
convenient way to construct a deterministic version.

...in bitmap_intersect

m_cols_ai was accessed during the Delete event, however this field is only
related to Updates.

Moving it to Update_rows_event would require too much effort. So instead:
* Only access m_cols_ai in Update events (conditional branch is added in
  Rows_log_event::do_add_row_data)
* Clean up m_cols_ai operations in Rows_log_event constructor.
  m_cols_ai.bitmap is first set to NULL, indicating invalid event.
  Then it is initialized:
  -> For Update events, a new bitmap is created.
  -> For other events, debug mode, m_cols_ai.bitmap is set to 1, indicating
     that the value is correct, but it shouldn't be accessed. To make sure
     we'll have a failure, n_bits is also set to 1.
  -> In release mode, m_cols_ai mirrors m_cols, providing extra safety
     in production.

Modernize declaration to C++11.
Add missing const.
Convert TYPE_CODE to constexpr.

Assertion `!table->versioned(VERS_TRX_ID)' failed in
Write_rows_log_event::binlog_row_logging_function during ONLINE ALTER.

trxid-versioned tables can't be replicated.
ONLINE ALTER will also be forbidden for these tables.

The bug is inherent for row-based replication as well.
To reproduce, a virtual (not stored) field of a blob type computed from
another field of a different blob type is required.

The following happens during an update or delete row event:
1. A row is unpacked.
2. Virtual fields are updated. Field b1 stores the pointer in
   Field_blob::value and references it in table->record[0].
3. record[0] is stored to record[1] in Rows_log_event::find_row.
4. A new record is fetched from handler. (e.g. ha_rnd_next)
5. Virtual columns are updated (only non-stored).
6. Field b1 receives new value. Old value is deallocated
   (Field_blob::val_str).
7. record_compare is called. record[0] and record[1] are compared.
8. record[1] contains a reference to a freed value.

record_compare is used in replication to find a matching record for update
or delete. Virtual columns that are not stored should be definitely skipped
both for correctness, and for this bug fix.

STORED virtual columns, on the other hand, may be required and shouldn't be
skipped. Stored columns are not affected, since they are not updated after
handler's fetch.

ASAN showed use-after-free in binlog_online_alter_end_trans, during
running through thd->online_alter_cache_list.

In online_alter_binlog_get_cache_data, new_cache_data was allocated on
thd->mem_root, in case of autocommit=1, but this mem_root could be freed
in sp_head::execute, upon using stored functions.

It appears that thd->transaction->mem_root exists even in single-stmt
transaction mode (i.e autocommit=1), so it can be used in all cases.
This mem_root will remain valid till the end of transaction, including
commit phase.

This is the corner case of ONLINE ALTER vs ha_maria vs App-time Periods.

When a Delete_rows_event (or update) is executed, a lookup handler may be
created, normally to serve long unique index needs, by a call of
handler::prepare_for_insert. This function also creates a lookup handler
if an application-time period exists in a table.

A difference with a usual call of prepare_for_insert is that transactions
are disabled for this table during ALTER TABLE. See
mysql_trans_prepare_alter_copy_data call in copy_data_between_tables.

Then, ha_maria calls _ma_tmp_disable_logging_for_table during
ha_maria::external_lock. It never happened so before, that two handlers
would be created for write to a single ha_maria table under transactions
disabled.

Hence, the fix handles this scenario.
It could be done otherwise, by not creating this lookup handler (since it's
not used anyway during ONLINE ALTER), but architecturally, two handlers
should be supported.

Avoiding the creation of lookup handler could be done here additionally,
but with a cost of slowing down other more generic cases, with an
additional check of online alter table active.

ONLINE ALTER TABLE uses binlog events like the replication does.

Before it was never used outside of replication, so significant
change was required. For example, a single event had a statement-like
befavior: it locked the tables, opened it, and closed them in the end. But
for ONLINE ALTER we use preopened table.

A crash scenario is following: lex->query_tables was set to NULL in
restore_empty_query_table_list when alter event is applied.
Then lex->query_tables->prev_global was write-accessed in
LEX::first_lists_tables_same, leading to a segfault.

In replication restore_empty_query_table_list would mean resetting lex
before next query or event.

In ONLINE ALTER TABLE we reuse a locked table between the events, so
we should avoid it. Here the need to reset lex state (or close the tables)
can be determined by nonzero rgi->tables_to_lock_count.
If no table is locked, then event doesn't own the tables.

The same was already done before for rgi->slave_close_thread_tables call.

previously, fields with DEFAULTs were allowed just when expression is
deterministic. In case of online alter, we should recursively check that
underlying fields of expression also either have explicit values, or
have DEFAULT following this validity rule.

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.