MDEV-31646 Online alter applies binlog cache limit to cache writes
1. Make online disk writes unlimited, same as filesort does.
2. Make proper error handling -- in 32-bit build IO_CACHE capacity limit is
4GB, so it is quite possible to overfill there.
3. Event_log::write_cache complicated with event reparsing, and as it was
proven by QA, contains some mistakes. Rewrite introbuce a simpler and much
faster version, not featuring reparsing and therefore copying a whole
buffer at once. This also disables checksums and crypto.
As a result, online alter is untied of several binlog variables, which was
a second aim of this patch.
Also another segfault was caused by accessing rpl_write_set on slave during
the row update/delete.
The reason was a default_column_bitmaps() call, which also sets
rpl_write_set to NULL.
Previously, the related behavior was changed in commit afd3ee97ad, where
one such call was removed from Update_rows_log_event::do_exec_row, but the
same one was mistakenly left in Delete_rows_log_event. Now it's also
removed.
MDEV-30984 Online ALTER table is denied with non-informative error messages
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.
