This is a remnant of "MySQL Instance Manager", which was removed in
MySQL-5.5.0 and never existed in MariaDB

Remove callback, simplify and optimize the code accordingly.

MDEV-20818 ER_CRASHED_ON_USAGE or Assertion `length <= column->length' failed in write_block_record on temporary table

The patch for `MDEV-20795 CAST(inet6 AS BINARY) returns wrong result`
unintentionally changed what Item_char_typecast::type_handler()
returns. This broke UNIONs with the BINARY() function, as the Aria
engine started to get columns of unexpected data types.

Restoring previous behaviour, to return
  Type_handler::string_type_handler(max_length).

The prototype for Item_handed_func::return_type_handler() has changed
from:
  const Type_handler *return_type_handler() const
to:
  const Type_handler *return_type_handler(const Item_handled_func *) const

Putting "#ifndef NO_EMBEDDED_ACCESS_CHECKS" around these constant declarations:

const uint max_hostname_length= 60;
const uint max_dbname_length= 64;

Suppress warnings when Field_inet6::store() is called from
read_statistics_for_table() and other optimizer related routines.

This patch does for Field_inet6 the same thing with what Monty's
patch previously did for other Field_xxx:

> commit 1bbe8c5e
> Author: Michael Widenius <monty@mariadb.org>
> Date:   Sun Sep 22 04:08:48 2019 +0300
>
>    Proper fix for disabling warnings in read_statistics_for_table().
>    MDEV-20589: Server still crashes in Field::set_warning_truncated_wrong_value

Alas, some meaningful warnings disappeared.

These tests use only HEAP and MyISAM tables.

Adding the "override" keyword to Item_char_typecast_func_handler_inet6_to_binary::type_handler_for_create_select()

Clang fails to compile without it.

Engine get LIMIT/OFFSET info an can it use/reset.

Kill check moved from send_data() methids in its wrapper:
send_data_with_check().

Now both offset and limit are stored and do not chenged during execution
(offset is decreased during processing in versions before 10.5).

(Big part of this changes made by Monty)

rec_init_offsets(): Relax the assertion that was added in
commit 01f45bec
to catch ROW_FORMAT=REDUNDANT records that have fewer fields
than expected.

This assertion would fail when accessing the records of the
built-in InnoDB table SYS_INDEXES. The column MERGE_THRESHOLD
had been effectively instantly added in MariaDB Server 10.2
(and MySQL 5.7), but is_instant() does not hold for that index.
Relax the assertion, so that it will not fail in this case.

MDEV-20813 Assertion failure in buf_flush_init_for_writing() for innodb_immediate_scrub_data_uncompressed=ON

The assertion that was added in
commit c0c003be
to augment the fix of MDEV-20805 turns out to be invalid when
innodb_immediate_scrub_data_uncompressed is enabled.
In this mode, fsp_init_file_page() will be invoked on data pages
that have been freed, causing writes of almost-all-zero pages.

btr_page_free(): Adjust the comment.

buf_flush_init_for_writing(): Disable the assertion with a note
that it should be re-enabled in MDEV-15528.

When the mysqld_multi script passes the --defaults-group-suffix
option to mysqld, it must remove the initial substring with the
group name ("mysqld") from option value, because otherwise substring
"mysqld" will be added to the group name and then the group name
will contain the word "mysqld" twice, which is wrong, because
mysqld itself adds the suffix received to the group name.

When the mysqld_multi script passes the --defaults-group-suffix
option to mysqld, it must remove the initial substring with the
group name ("mysqld") from option value, because otherwise substring
"mysqld" will be added to the group name and then the group name
will contain the word "mysqld" twice, which is wrong, because
mysqld itself adds the suffix received to the group name.

When the mysqld_multi script passes the --defaults-group-suffix
option to mysqld, it must remove the initial substring with the
group name ("mysqld") from option value, because otherwise substring
"mysqld" will be added to the group name and then the group name
will contain the word "mysqld" twice, which is wrong, because
mysqld itself adds the suffix received to the group name.

When the mysqld_multi script passes the --defaults-group-suffix
option to mysqld, it must remove the initial substring with the
group name ("mysqld") from option value, because otherwise substring
"mysqld" will be added to the group name and then the group name
will contain the word "mysqld" twice, which is wrong, because
mysqld itself adds the suffix received to the group name.

This is another follow-up fix to
commit b393e2cb
which turned out to be still broken.

Replace the C++11 keyword 'constexpr' with #define.

debug_sync_t::str: Remove the zero-length array.
Replace sync->str with reinterpret_cast<char*>(&sync[1]).

During IMPORT TABLESPACE, we do invoke
buf_flush_init_for_writing() with block==NULL and newest_lsn!=0.

Remove unused variables and type mismatch that was introduced
in commit b393e2cb

Also, fix a typo in the documentation of the parameter, and
update the test.

We will remove the InnoDB background operation of merging buffered
changes to secondary index leaf pages. Changes will only be merged as a
result of an operation that accesses a secondary index leaf page,
such as a SQL statement that performs a lookup via that index,
or is modifying the index. Also ROLLBACK and some background operations,
such as purging the history of committed transactions, or computing
index cardinality statistics, can cause change buffer merge.
Encryption key rotation will not perform change buffer merge.

The motivation of this change is to simplify the I/O logic and to
allow crash recovery to happen in the background (MDEV-14481).
We also hope that this will reduce the number of "mystery" crashes
due to corrupted data. Because change buffer merge will typically
take place as a result of executing SQL statements, there should be
a clearer connection between the crash and the SQL statements that
were executed when the server crashed.

In many cases, a slight performance improvement was observed.

This is joint work with Thirunarayanan Balathandayuthapani
and was tested by Axel Schwenke and Matthias Leich.

The InnoDB monitor counter innodb_ibuf_merge_usec will be removed.

On slow shutdown (innodb_fast_shutdown=0), we will continue to
merge all buffered changes (and purge all undo log history).

Two InnoDB configuration parameters will be changed as follows:

innodb_disable_background_merge: Removed.
This parameter existed only in debug builds.
All change buffer merges will use synchronous reads.

innodb_force_recovery will be changed as follows:
* innodb_force_recovery=4 will be the same as innodb_force_recovery=3
(the change buffer merge cannot be disabled; it can only happen as
a result of an operation that accesses a secondary index leaf page).
The option used to be capable of corrupting secondary index leaf pages.
Now that capability is removed, and innodb_force_recovery=4 becomes 'safe'.
* innodb_force_recovery=5 (which essentially hard-wires
SET GLOBAL TRANSACTION ISOLATION LEVEL READ UNCOMMITTED)
becomes safe to use. Bogus data can be returned to SQL, but
persistent InnoDB data files will not be corrupted further.
* innodb_force_recovery=6 (ignore the redo log files)
will be the only option that can potentially cause
persistent corruption of InnoDB data files.

Code changes:

buf_page_t::ibuf_exist: New flag, to indicate whether buffered
changes exist for a buffer pool page. Pages with pending changes
can be returned by buf_page_get_gen(). Previously, the changes
were always merged inside buf_page_get_gen() if needed.

ibuf_page_exists(const buf_page_t&): Check if a buffered changes
exist for an X-latched or read-fixed page.

buf_page_get_gen(): Add the parameter allow_ibuf_merge=false.
All callers that know that they may be accessing a secondary index
leaf page must pass this parameter as allow_ibuf_merge=true,
unless it does not matter for that caller whether all buffered
changes have been applied. Assert that whenever allow_ibuf_merge
holds, the page actually is a leaf page. Attempt change buffer
merge only to secondary B-tree index leaf pages.

btr_block_get(): Add parameter 'bool merge'.
All callers of btr_block_get() should know whether the page could be
a secondary index leaf page. If it is not, we should avoid consulting
the change buffer bitmap to even consider a merge. This is the main
interface to requesting index pages from the buffer pool.

ibuf_merge_or_delete_for_page(), recv_recover_page(): Replace
buf_page_get_known_nowait() with much simpler logic, because
it is now guaranteed that that the block is x-latched or read-fixed.

mlog_init_t::mark_ibuf_exist(): Renamed from mlog_init_t::ibuf_merge().
On crash recovery, we will no longer merge any buffered changes
for the pages that we read into the buffer pool during the last batch
of applying log records.

buf_page_get_gen_known_nowait(), BUF_MAKE_YOUNG, BUF_KEEP_OLD: Remove.

btr_search_guess_on_hash(): Merge buf_page_get_gen_known_nowait()
to its only remaining caller.

buf_page_make_young_if_needed(): Define as an inline function.
Add the parameter buf_pool.

buf_page_peek_if_young(), buf_page_peek_if_too_old(): Add the
parameter buf_pool.

fil_space_validate_for_mtr_commit(): Remove a bogus comment
about background merge of the change buffer.

btr_cur_open_at_rnd_pos_func(), btr_cur_search_to_nth_level_func(),
btr_cur_open_at_index_side_func(): Use narrower data types and scopes.

ibuf_read_merge_pages(): Replaces buf_read_ibuf_merge_pages().
Merge the change buffer by invoking buf_page_get_gen().

buf_flush_init_for_writing(): Assert that FIL_PAGE_TYPE is set
except when creating a new data file with a dummy first page.

buf_dblwr_create(): Ensure that FIL_PAGE_TYPE on all pages
will be initialized. Reset buf_dblwr_being_created at the end.

In the function recv_parse_or_apply_log_rec_body() there are debug checks
for validating the state of the page when redo log records are being
applied. Most notably, FIL_PAGE_TYPE should be set before anything else
is being written to the page.

ibuf_add_free_page(): Set FIL_PAGE_TYPE before performing any other changes.

Apply the correct pattern for debug instrumentation:

SET @save_dbug=@@debug_dbug;
SET debug_dbug='+d,...';
...
SET debug_dbug=@save_dbug;

Numerous tests use statements of the form

SET debug_dbug='-d,...';

which will inadvertently enable all DBUG tracing output,
causing unnecessary waste of resources.

The test main.index_merge_innodb is taking very much time,
especially on later versions (10.2 and 10.3).

Some of this could be attributed to the use of INSERT...SELECT,
which is time-consumingly creating explicit record locks in InnoDB
for the locking read in the SELECT part.

In 10.3 and later, some slowness can be attributed to MDEV-12288,
which makes the InnoDB purge thread spend time to reset transaction
identifiers in the inserted records. If we prevent purge from
running before all tables are dropped, the test seems to be
10% faster on an unoptimized debug build on 10.5. (A proper fix
would be to implement MDEV-515 and stop writing row-level undo log
records for inserts into an empty table or partition.)

At the same time, it should not hurt to make main.index_merge_myisam
to use the sequence engine. Not only could it be a little faster,
but the test would be slightly more readable.