Merge branch '10.4-MDEV-19966' of https://github.com/codership/mariadb-server into codership-10.4-MDEV-19966

Added options:
--datadir
--ignore-control-file
--require-control-file

- Improved error messages if open fails
- If control file can't be found/opened, assume that all rows in the
  tables are commited.

Problem was that replace_dynstr_append_mem() assumed strings are
null terminated which is not always the case.

           'index_merge_sort_union=off'

When index_merge_sort_union is set to 'off' and  index_merge_union is set
to 'on' then any evaluated index merge scan must consist only of ROR scans.
The cheapest out of such index merges must be chosen. This index merge
might not be the cheapest index merge.

This was to remove a performance regression between 10.3 and 10.4
In 10.5 we will have a better implementation of records_in_range
that will enable us to get more statistics.
This change was not done in 10.4 because the 10.5 will be part of
a larger change that is not suitable for the GA 10.4 version

Other things:
- Changed default handler block_size to 8192 to fix things statistics
  for engines that doesn't set the block size.
- Fixed a bug in spider when using multiple part const ranges
  (Patch from Kentoku)

It did not work, eventhough the default for Windows was changed to 1
for the corresponding system variable.
This be because  test_flags was zeroed mysql_init_variables.

The patch fixes this glitch.

This should fix the following failure:

+ Warnings:
+ Warning       1639    debug sync point wait timed out

mi_records_in_range(): Because HA_POS_ERROR cannot be accurately
represented in double (it will be off by one), add an explicit
cast to silence the warning.

myrg_open(): Reduce the scope of the variable 'end' and
simplify the code.

For some reason, I got no warning for this code in the 10.2
branch, only 10.3 or later.

The ENGINE=MERGE is covered by the tests main.merge, main.merge_debug,
and main.merge-big.

Problem:
=======
When we upgrade from "mysql" to "mariadb" if slave is using repositories as
tables their data is completely ignored and no warning is issued in error log.

Fix:
===
"mysql_upgrade" test should check for the presence of data in
"mysql.slave_master_info" and "mysql.slave_relay_log_info" tables. When tables
have some data the upgrade script should report a warning which hints users
that the data in repository tables will be ignored.

_ma_fetch_keypage(): Correct an assertion that used to always hold.
Thanks to clang -Wint-in-bool-context for flagging this.

double_to_datetime_with_warn(): Suppress -Wimplicit-int-float-conversion
by adding a cast. LONGLONG_MAX converted to double will actually be
LONGLONG_MAX+1.

All tablespace metadata is buffered in fil_system. There is a LRU
mechanism, but that only controls the opening and closing of
fil_node_t::handle.

It is much more efficient and less error-prone to access data file names
by looking up the fil_space_t object rather than by essentially joining
each row with an access to SYS_DATAFILES via the InnoDB internal SQL parser.

dict_get_first_path(): Declare static. The function may only be needed
when loading or updating the data dictionary. Also, change a condition
in order to avoid a bogus GCC 10 -Wstringop-overflow warning for
mem_strdupl() about len==ULINT_UNDEFINED.

i_s_sys_tablespaces_fill_table(): Do not access other InnoDB internal
dictionary tables than SYS_TABLESPACES.

This fixes GCC 10.0.1 -Wstringop-truncation and some typos.

actually, page_zip_verify_checksum() generally allows all-zeroes
checksums because our CRC32 checksum is something like
crc_1 ^ crc_2 ^ crc_3

Also, all zeroes page is considered correct.

As a side effect fix nasty reinterpret_cast<> UB

Also, since c0f47a4a innodb_checksum_algorithm=full_crc32
exists which computes CRC32 in one go (without bitwise arithmetic)

Also, remove some trailing white space and add missing static
qualifier to free_annotate_event().

It was:
implicit conversion from 'ha_rows' (aka 'unsigned long long') to 'double'
changes value from 18446744073709551615 to 18446744073709551616

Follow what JOIN::get_examined_rows() does for similar code.

Parser: uninitialized Lex->create_last_non_select_table under
mysql_unpack_partition() fix.

Tested with main, parts suites.