Add a system variable spider_slave_trx_isolation.
- spider_slave_trx_isolation
  The transaction isolation level when Spider table is used by slave SQL thread.
  -1 : OFF
   0 : READ UNCOMMITTED
   1 : READ COMMITTED
   2 : REPEATABLE READ
   3 : SERIALIZABLE
  The default value is -1

Miscellaneous Spider typos

Change default value of the followings
quick_mode 0 -> 3
quick_page_size 100 -> 1024

Add the following parameter for limiting result page size by byte
- quick_page_byte(qpb)
  Number of bytes in a page when acquisition one by one.
  When quick_mode is 1 or 2, Spider stores at least 1 record even if
  quick_page_byte is smaller than 1 record. When quick_mode is 3,
  quick_page_byte is used for judging using temporary table.
  That is given to priority when server parameter spider_quick_page_byte
  is set.
  The default value is 10485760

Fix "out of sync" issue at using quick_mode = 1 or 2

The fields of the temporary table were not created in create_tmp_table function. Because item->const_item() was true. But the temporary tables that is created by Spider are always used all columns. So Spider should call create_tmp_table function with TMP_TABLE_ALL_COLUMNS flag.

The default value for eq_range_index_dive_limit is set to 10

Replaced wait on COND_thread_count with busy waiting with 1 millisecond
sleep.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.

Signal handler is now respoinsible for setting abort_loop and breaking
poll() in main thread. The rest is handled by main thread itself.

Removed redundant LOCK_error_log init/destroy wrappers.
Removed redundant unireg_end(): it is trivial and it has only one caller.
Removed unused ready_to_exit from PFS.
Removed kill_in_progress: duplicates abort_loop.
Removed shutdown_in_progress: duplicates abort_loop.
Removed ready_to_exit: was used to make sure main thread waits for
cleanups, which are now done by main thread itself.
Removed SIGNALS_DONT_BREAK_READ, MAYBE_BROKEN_SYSCALL,
kill_broken_server: never defined/used.
Make clean_up() static.

fill_status.

Also, remove LOCK_status around calc_sum_of_all_status()

Also, rename LOCK_show_status into LOCK_all_status_vars.
This reflects the variable the lock protects.

modifications (insert/erase) are protected by write lock
iteration over list is protected by read lock.

This way, threads that iterate over the list (as in SHOW PROCESSLIST,
SHOW GLOBAL STATUS) do not block each other.

* fix CRL tests to work
* regenerate certificates to be at least 2048 bit
  (fixes buster and rhel8 in buildbot)
* update generate-ssl-cert.sh to generate crl files
* make all SSL tests to use certificates generated
  in generate-ssl-cert.sh, remove unused certificates

from info to debug

In contrast to thread_count, which is decremented by THD destructor,
this one was most probably intended to be decremented after all THD
destructors are done.

THD_count class was added to achieve similar effect with thread_count.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

Implemented and integrated THD_list as a replacement for the global
thread list. It uses own mutex instead of LOCK_thread_count for THD
list protection.

Removed unused first_global_thread() and next_global_thread().

delayed_insert_threads is now protected by LOCK_delayed_insert. Although
this patch doesn't fix very wrong synchronization of this variable.

After this patch there are only 2 legitimate uses of LOCK_thread_count
left, both in mysqld.cc: thread_count and ready_to_exit.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

LOG_INFO::lock was useless. It could've only protect against concurrent
iterators execution, which was already protected by LOCK_thread_count.

Use LOCK_thd_data instead of LOCK_thread_count as a protection against
THD::current_linfo reset.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

Bootstrap in a separate thread was introduced in 746f0b3b to workaround
OS/2 small stack size. OS/2 support was discontinued in 2006 and modern
operating systems have default stack size a few times larger than
default thread_stack and it is tunable.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

if ssl_cipher is not specified, it must be NULL, not ""

MDEV-18122 Assertion 'table->versioned() == m_prebuilt->table->versioned()' failed in ha_innobase::open

Closes #1134

and gitignore myrocks_hotbackup (as it's now generated)

Closes #1081

Closes #1080

It's too late. Might work or not (and on buster it doesn't).

MDEV-18057 Assertion `(node->state == 5) || (node->state == 6)' failed in row_upd_sec_step upon DELETE after UPDATE failed due to FK violation

The idea of the fix: reset state from previous query.

row_upd_clust_step(): reset cached index before updating a clustered index

Closes #1133

Fix for galera_3nodes galera_ipv6_mysqldump and galera_ipv6_mariabackup

Remove fil_node_t::sync_event.

I had a discussion with kernel fellows and they said it's safe to call
fsync() simultaneously at least on VFS and ext4. So initially I wanted
to disable check for recent Linux but than I realized code is buggy.

Consider a case when one thread is inside fsync() and two others are
waiting inside os_event. First thread after fsync() calls os_event_set()
which is a broadcast! So two waiting threads will awake and may call
fsync() at the same time.

One fix is to add a notify_one() functionality to os_event but I decided
to remove incorrect check completely. Note, it works for one waiting
thread but not for more than one.

IMO it's ok to avoid existing bugs but there is not too much sense in
avoiding possible(!) bugs as this code does.

fil_space_t::is_in_rotation_list(), fil_space_t::is_in_unflushed_spaces():
Replace redundant bool fields with member functions.

fil_node_t::needs_flush: Replaces fil_node_t::modification_counter and
fil_node_t::flush_counter. We need to know whether there _are_ some
unflushed writes and we do not need to know _how many_ writes.

fil_system_t::modification_counter: Remove as not needed.
Even if we needed fil_node_t::modification_counter, every file
could have its own counter that would be incremented on each write.

fil_system_t::modification_counter is a global modification counter
for all files. It was incremented on every write. But whether some
file was flushed or not is an internal fil_node_t deal/state and
this makes fil_system_t::modification_counter useless.

Closes #1061

MDEV-18379: Unification of check for IPv6

Currently, the three-node mtr suite for Galera (galera_3nodes)
uses a separate IPv6 availability check using the "have_ipv6.inc"
file. This check duplicates a more accurate check at suite.pm
level, which can be used by including the file "check_ipv6.inc".

This patch removes this discrepancy between suites. In addition,
one of the files in the galera_3nodes suite does not contain the
option "--bind-address=::" which is needed for the test to work
correctly with IPv6 (at least on some systems), since without
it the server will not wait for connections on the IPv6 interface.

https://jira.mariadb.org/browse/MDEV-18379

Add a simplest regression test. Specifically, I want to be sure that
SYS_COLUMNS.LEN is increased.

Closes #1123

WSREP_LIB_MAINTAINER_MODE was introduced in wsrep-lib cmake options
to enable -Werror. The maintainer mode is disabled by default in order
not to fail compilation on every compiler warning. The wsrep-lib
maintainer mode can be enabled via -DWSREP_LIB_MAINTAINER_MODE:BOOL=ON.