The crash is caused by the attempt to refix the constant subquery during
pushdown from HAVING into WHERE optimization.

Every condition that is going to be pushed into WHERE clause is first
cleaned up, then refixed. Constant subqueries are not cleaned or refixed
because they will remain the same after refixing, so this complicated
procedure can be omitted for them (introduced in MDEV-21184).
Constant subqueries are marked with flag IMMUTABLE_FL, that helps to miss
the cleanup stage for them. Also they are marked as fixed, so refixing is
also not done for them.
Because of the multiple equality propagation several references to the same
constant subquery can exist in the condition that is going to be pushed
into WHERE. Before this patch, the problem appeared in the following way.
After the first reference to the constant subquery is processed, the flag
IMMUTABLE_FL for the constant subquery is disabled.
So, when the second reference to this constant subquery is processed, the
flag is already disabled and the subquery goes through the procedure of
cleaning and refixing. That causes a crash.

To solve this problem, IMMUTABLE_FL should be disabled only after all
references to the constant subquery are processed, so after the whole
condition that is going to be pushed is cleaned up and refixed.

Approved by Igor Babaev <igor@maridb.com>

Rollback is not supposed to fail. This prevents false failures in
spider rollback.

Item_exists_subselect::fix_length_and_dec() sets explicit_limit to 1.
In the exists2in transformation it resets select_limit to NULL. For
consistency we should reset explicity_limit too.

This fixes a bug where spider table returns wrong results for queries
that gets through exists2in transformation when semijoin is off.

SPIDER_CONN::loop_check_meraged_first is useless, because all
SPIDER_CONN_LOOP_CHECKs are in SPIDER_CONN::loop_check_queue, which in
spider_db_conn::fin_loop_check() is iterated over.

This fixes the use-after-free issue when there are three spider tables
sharing the same remote, and their corresponding
SPIDER_CONN_LOOP_CHECKs getting merged in
spider_conn_queue_and_merge_loop_check()

This also fixes MDEV-34555

Same cause as MDEV-31996. This prevents reuse of conns freed
previously, e.g. during the commit of previous statement.

It does not occur in 10.4 because of the commit for MDEV-19002 removed
the call to spider_check_trx_and_get_conn().

This allows creation of SPIDER_CONN on demand, if the previous one was
freed.

Also, the first_link_idx's are reset during
spider_check_trx_and_get_conn(), which in the case of remote HANDLER
commands, might not match the link to use correct first_link_idx for
remote HANDLER statement that was later set in
ha_spider::rnd_handler_init() (causing testing regressions in the
spider/handler suite). Therefore we fix the first_link_idx there.

A SPIDER_TRX_HA associated with a SPIDER_TRX could have longer
lifetime than its associated SPIDER_SHARE. And it is identified with
the associated table name. When the SPIDER_SHARE no longer valid, e.g.
when the associated spider table has been dropped and recreated, the
SPIDER_TRX_HA should be reset too.

Since spider could create a new SPIDER_SHARE with the exact same
address of a freed SPIDER_SHARE, we try to mark all SPIDER_TRX_HAs
associated with a SPIDER_SHARE invalid when the SPIDER_SHARE is about
to be freed.

Some documentations come from commit for MDEV-29676, and the rest are
mainly related to SPIDER_TRX_HA.

Removed SPIDER_TRX_HA::trx as it is unused.

This fixes a valgrind failure where the bulk_size is used before
initialised in ha_spider::end_bulk_insert().

To avoid off-by-one in spider_get_share.

And document conn key and conn key length.

MDEV-31788 Factor functions to reduce duplication around spider_check_and_init_casual_read in ha_spider.cc

factored out static functions:
- spider_prep_loop
- spider_start_bg
- spider_send_queries

They are for unnecessary debugging purposes only.

The functions called in blocks protected by this macro remain
undefined as of 11.5 c96b23f9

It's virtually empty now

Show::Column() was added in MDEV-19772
4156b1a2

handler::pre_calculate_checksum was added in MDEV-16249
be5c432a

get_negated() was introduced in MDEV-16707

sql_cache was moved to lex in MDEV-11953 in
de745ecf

{STRING|INT|REAL|DECIMAL|DATE}_ITEM were replaced with CONST_ITEM in
MDEV-14630 c20cd68e

MDEV-16246 was fixed long ago. And this macro was removed in other
versions too

handler::need_info_for_auto_inc() was added in MDEV-7720 / MDEV-7726
in commit dc17ac16

This macro is unused, and not in 11.5 c96b23f9

This macro is unused

The flag argument was added to handler::start_bulk_insert() in
the MDEV-539 commit
ca2cdaad

new_data is const since at least 2017: a05a610d

handler has can_use_for_auto_inc_init() since at latest 2017:
dc17ac16

HA_EXTRA_USE_CMP_REF is undefined, and remains so as of 11.5
c96b23f9

HA_EXTRA_STARTING_ORDERED_INDEX_SCAN was added latest 2018:
921c5e93

net has thd since 2015 in 56aa1998 for MDEV-6152

arg_count was protected since 2015 in commit
afa17734

unifdef -DSPIDER_HAS_HASH_VALUE_TYPE -m storage/spider/spd_* storage/spider/ha_spider.* storage/spider/hs_client/*