This bug was introduced by the patch for WL#3220.
If the memory allocated for the tree to store unique elements
to be counted is not big enough to include all of them then
an external file is used to store the elements.
The unique elements are guaranteed not to be nulls. So, when 
reading them from the file we don't have to care about the null
flags of the read values. However, we should remove the flag 
at the very beginning of the process. If we don't do it and
if the last value written into the record buffer for the field
whose distinct values needs to be counted happens to be null,
then all values read from the file  are considered to be nulls
and are not counted in.
The fix does not remove a possible null flag for the read values.
Rather it just counts the values in the same way it was done
before WL #3220.

Flip the switch and create Item_cache based on the argument's cmp_type, not argument's result_type().
Fix subselect_engine to calculate cmp_type correctly

sql/item_subselect.h:
  mdev:4284

Incorrect NULL value handling in Item_func_conv_charset fixed.

mysql-test/r/keywords.result:
  Test that option works as table/column/variable
mysql-test/suite/funcs_1/r/storedproc.result:
  OPTION is now a valid identifier
mysql-test/suite/funcs_1/t/storedproc.test:
  OPTION is now a valid identifier
mysql-test/t/keywords.test:
  Test that option works as table/column/variable
sql/sql_yacc.yy:
  OPTION is now a valid identifier

  Adding an include file which checks whether long names are supported

get_datetime_value() should not double-cache its own Item_cache_temporal items,
but it *should* cache other Item_cache items, such as Item_cache_str.

sql/item.h:
  shortcut, to avoid going through the switch in Item::cmp_type()
sql/item_cmpfunc.cc:
  even if the item is Item_cache_str - it still needs to be converted and cached.
sql/item_timefunc.h:
  all descendants of Item_temporal_func always have cmp_type==TIME_RESULT.
  Even Item_date_add_interval, that might have field_type == MYSQL_TYPE_STRING.

MDEV-4267 : do not copy sql_yacc.cc and sql_yacc.h from unpacked source tarball into build directory, if usable bison is installed on the build machine.

This is done in order to avoid possibly stale bison output files in the build directory.

The reason for the problem was  negation of signed longlong value  LONGLON
G_MIN in  Item_func_neg::int_op() - the result of this operation is not defined
(in C/C++ standard).

With this patch, LONGLONG_MIN is handled as special value, and negation is
avoided.

MDEV-4249 : when autodetecting default client charset on Windows, fallback to GetACP() whenever GetConsoleCP() returns 0 (i.e appkication does not have a console , which is the case for GUI apps, Windows services etc)

was not enabled on x86_64 machines. Enabling it. Gives up to 18 times
conversion performance improvement.

modified:
  sql/sql_string.cc

fix the declaration to use the correct type for st_handler_check_param::sort_buffer_length.
remove redundant casts.

This is a bug in the legacy code. It did not manifest itself because
it was masked by other bugs that were fixed by the patches for
mdev-4172 and mdev-4177.

Field_enum incorrectly inherited decimals() from Field_string.
Field_enum should be always integer in numeric context.

add --mysqld option to my_print_defaults
change server-postin script to use that

close (and auto-drop) temporary tables before
rolling back the last transaction in the connection.

        The bug was found by Alyssa Milburn.
        If the number of points of a geometry feature read from
        binary representation is greater than 0x10000000, then
        the (uint32) (num_points * 16) will cut the higher byte,
        which leads to various errors.
        Fixed by additional check if (num_points > max_n_points).

sql/sql_table.cc:
  Don't call allow_access_to_protected_table() if we haven't protected table against usage.
  Table is mainly protected against usage when one disables keys with alter table.

sql/sql_table.cc:
  Remove version protection from share when repair has been done.
  Without this one can't run SHOW commands on the table if it was locked until it's unlocked.
sql/table.h:
  Allow one to remove version protection with allow_access_to_protected_table()

This bug is a regression bug. The regression was introduced by
the patch for mdev-3851, that tried to weaken the condition when
a ref access with an extended key can be converted to an eq_ref
access. The patch incorrectly formed this condition. As a result,
while improving performance for some queries, the patch caused 
worse performance for another queries.

MDEV-4216 : export additional functions  mysql_get_timeout_value(),mysql_get_timeout_value_ms(),  mysql_get_socket() from shared client library. They are documented as part of async API.

Also, remove  functions mysql_close_slow_part_start() and mysql_close_slow_part_cont() from exports - they are not documented anywhere.

Removed the obsolete instructions from the MySQL 5.1 manual. Instead provide a link to https://kb.askmonty.org/en/compiling-mariadb-from-source/

The issue was that there was that SHOW commands could open the table in the store engine, even in cases
where it should not be allowed to do that (ie, the storage engines meta data for that table was under big changes).

The cases where this should not be allowed are:
- ALTER TABLE DISABLE KEYS
- ALTER TABLE ENABLE KEYS
- REPAIR TABLE
- OPTIMIZE TABLE
- DROP TABLE

This patch adds a new mode, protected_against_usage(). If this is used then the SHOW command will wait until the table
is accessable. This is implemented by re-using the already exising 'version' flag for TABLE_SHARE.
It also added functions to be used to change TABLE_SHARE->version instead of changing it directly.
	


mysql-test/r/myisam-metadata.result:
  Added test case
mysql-test/t/myisam-metadata.test:
  Added test case
sql/mysqld.cc:
  Start from refresh_version 2 as 0 and 1 are reserved.
sql/sql_admin.cc:
  Added MYSQL_OPEN_FOR_REPAIR
  Updated call to wait_while_table_is_used()
sql/sql_base.cc:
  Updated call to wait_while_table_is_used()
  - Allow one to specify how the table should be removed (for all commands except show or for all commands).
  - Don't allow one to reopen the table if one has called share->protect_against_usage()
sql/sql_base.h:
  Added TDC_RT_REMOVE_NOT_OWN_AND_MARK_NOT_USABLE, which is used to mark that no one can reopen this table, except with MYSQL_OPEN_FOR_REPAIR .
  - Added MYSQL_OPEN_FOR_REPAIR
  - Updated prototype for wait_while_table_is_used()
sql/sql_table.cc:
  Updated call to wait_while_table_is_used()
  Use MYSQL_OPEN_FOR_REPAIR for open tables that where repaired.
sql/sql_truncate.cc:
  Updated call to wait_while_table_is_used()
sql/table.cc:
  Use set_refresh_version()
sql/table.h:
  Added functions to be used to change TABLE_SHARE->version instead of changing it directly

Do not include BLOB fields into the key to access the temporary
table created for a materialized view/derived table.
BLOB components are not allowed in keys.