cannot use TABLE:merge_keys for that, because Field::part_of_key
was designed to mark fields for KEY_READ, so a field is not a
"part of key", if only prefix of the field is.

* don't issue an error for ER_KEY_BASED_ON_GENERATED_VIRTUAL_COLUMN
* support keyread on vcols
* callback into the server to compute vcol values from mi_check/mi_repair
* DMLs just work. Automatically.

will be used later by storage engines (e.g. in MyISAM repair or in
InnoDB purge)

Make update_virtual_fields() a method of TABLE, to be consistent
with TABLE::update_default_fields().

TABLE::mark_virtual_col() was polluting table->vcol_set
and that confused the following mark_virtual_col()

and FIELDFLAG_xxx constants

unused freshly initialized record should be trashed
with TRASH_ALLOC, not TRASH_FREE

This fixed several cases where we where using just ulong for sql_mode

This is similar to MysQL Worklog 3253, but with
a different implementation. The disk format and
SQL syntax is identical with MySQL 5.7.

Fetures supported:
- "Any" ammount of any trigger
- Supports FOLLOWS and PRECEDES to be
  able to put triggers in a certain execution order.

Implementation details:
- Class Trigger added to hold information about a trigger.
  Before this trigger information was stored in a set of lists in
  Table_triggers_list and in Table_triggers_list::bodies
- Each Trigger has a next field that poinst to the next Trigger with the
  same action and time.
- When accessing a trigger, we now always access all linked triggers
- The list are now only used to load and save trigger files.
- MySQL trigger test case (trigger_wl3253) added and we execute these
  identically.
- Even more gracefully handling of wrong trigger files than before. This
  is useful if a trigger file uses functions or syntax not provided by
  the server.
- Each trigger now has a "Created" field that shows when the trigger was
  created, with 2 decimals.

Other comments:
- Many of the changes in test files was done because of the new "Created"
  field in the trigger file. This shows up in SHOW ... TRIGGER and when
  using information_schema.trigger.
- Don't check if all memory is released if on uses --gdb;  This is needed
  to be able to get a list from safemalloc of not freed memory while
  debugging.
- Added option to trim_whitespace() to know how many prefix characters
  was skipped.
- Changed a few ulonglong sql_mode to sql_mode_t, to find some wrong usage
  of sql_mode.

This bug in the code of Item_ref::build_clone could
cause corruption of items in where conditions.
Also made sure that equality predicates extracted
from multiple equality items to be pushed into
materialized views were cloned.

Adjusted one result file.

use the new approach with Field->default_value expressions.
But keep the old TIMESTAMP_UN_FIELD for ON UPDATE NOW().

to be calculated at the CREATE TABLE time and stored in
the default row image.

and use the same error message for CREATE and open table time

* revert part of the db7edfed that moved calculations from
  fix_fields to val_str for Item_func_sysconst and descendants
* mark session state dependent functions in check_vcol_func_processor()
* re-run fix_fields for all such functions for every statement
* fix CURRENT_USER/CURRENT_ROLE not to use Name_resolution_context
  (that is allocated on the stack in unpack_vcol_info_from_frm())

Note that NOW(), CURDATE(), etc use lazy initialization and do *not*
force fix_fields to be re-run. The rule is:
* lazy initialization is *not* allowed, if it changes metadata (so,
   e.g. DAYNAME() cannot use it)
* lazy initialization is *preferrable* if it has side effects (e.g.
  NOW() sets thd->time_zone_used=1, so it's better to do it when
  the value of NOW is actually needed, not when NOW is simply prepared)

into "fix" and "check" parts

in a case of a specially crafted invalid frm

Temporary tables created for recursive CTE
were instantiated at the prepare phase. As
a result these temporary tables missed
indexes for look-ups and optimizer could not
use them.

TABLE_SHARE::init_from_binary_frm_image has a rule: if an index
has a partially-covered column (like in "KEY(col(N))" ), then dont
provide "Extended Keys" feature for this index.

The problem was that due to coding error Extended Keys feature was
disabled for *ALL* subsequent indexes. Fixed the error.

this is useless now, flags are recalculated on load anyway.
But storing flags on disk means we cannot easily change (add,
remove, or renumber) them in the new MariaDB version.

because table->map is set much later.
Use check_vcol_func_processor() to detect fields too.

* remove a confusing method name - Field::set_default_expression()
* remove handler::register_columns_for_write()
* rename stuff
* add asserts
* remove unlikely unlikely
* remove redundant if() conditions
* fix mark_unsupported_function() to report the most important violation
* don't scan vfield list for default values (vfields don't have defaults)
* move handling for DROP CONSTRAINT IF EXIST where it belongs
* don't protect engines from Alter_inplace_info::ALTER_ADD_CONSTRAINT
* comments

make it return same errors on CREATE as

CREATE ... ( ... DEFAULT const ... )

and remove all related casts to uchar*
also remove a couple of unused methods

collaterals:
* use %`s, not '%s'
* use correct SQLSTATE codes for these two errors