This was caused by two different bugs:
1) Information_schema tables where not locked by lock_tables, but
   get_lock_data() was not filtering these out. This caused a crash when
   mysql_unlock_some_tables() tried to unlock tables early, including
   not locked information schema tables.

Fixed by not locking SYSTEM_TMP_TABLES

2) In some cases the optimizer will notice that we do not need to read
   the information_schema tables at all. In this case
   join_tab->read_record is not set, which caused a crash in
   get_schema_tables_result()

Fixed by ignoring const tables in get_schema_tables_result()

1. wait for the binlog thread to reach the certain state, don't use
   a debug_sync that's incorrectly placed to detect the state
2. no need to do a (non-deterministic) `show binlog events` to verify
   what is guaranteed by the directly preceding line

if it's the whole content of a test anyway.

add a new "debugger" to mtr, that runs the executable
under valgrind in gdb. valgrind pid is auto-detected,
but the delay (sleep) and vgdb path are hard-coded for now

"debugger" is anything that wraps execution of a target
binary (mysqld or mysqltest). Currently the list includes:
gdb, ddd, dbx, lldb, valgrind, strace, ktrace, rr,
devenv, windbg, vsjitdebugger.

for every debugger xxx, mtr will recognize four options:
--xxx, --boot-xxx, --manual-xxx, --client-xxx.
They all support an optional "=string" argument. String
being a semicolon-separated list of commands (e.g. for gdb)
or one (not semicolon-separated) command line of options
(e.g. for valgrind). Or both (e.g. --gdb='-quiet -nh;info files'

In embedded both --xxx and --client-xxx work.

Functionality changed/removed:
* --rr-args is gone
* --rr-dir is gone
* --manual-debug is gone
* --debugger={devenv|vc|windbg|vc_express|vsjitdebugger} is gone
* --strace-option is gone
* --stracer={strace|ktrace} is gone
* --valgrind only enables it for the server, not for everything
* --valgrind-all is gone
* --valgrind-mysqltest is gone
* --valgrind-mysqld is gone
* --valgrind-options is gone
* --valgrind-option is gone
* --valgrind-path is gone
* --callgrind is gone
* one cannot combine --valgrind --gdb anymore
* valgrind report doesn't add a fake test line to the output
* vc and vcexpress on windows are no longer supported

don't allocate Column_statistics_collected objects that won't
be used.

minor style fixes (StringBuffer<>, etc)

only collect persistent stats for columns explicitly listed
by the user in the  ANALYZE TABLE PERSISTENT FOR COLUMNS (...)
clause. The engine can extend table->read_set as much as
it wants, it should not affect the collected statistics.

Test case from the 3b94309a applies - it used to crash,
because ha_partition extended table->read_set after the loop that
initialized some objects based on bits in the read_set but before the
loop that used these objects based on bits in the read_set.

This reverts the commit 3b94309a but keeps the test

Because the fix is a hack that isn't supposed to do anything,
and relies on a side-effect of rnd_init inside ha_partition.

A different fix is coming up.

           on used subqueries

If a query was based on a table value constructor that contained subqueries
then EXPLAIN for such query did not contain any lines explaining the
execution plans of the subqueries.
This happened because
- no optimize() method was called for any subquery used by the table value
  constructor when EXPLAIN command for the query was processed;
- EXPLAIN node created for the table value constructor itself did not
  assume that some child nodes could be attached to it.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

- This is caused by commit ad6171b9
(MDEV-22456). InnoDB reloads the evicted table again from dictionary.
In that case, AHI entries and current index object mismatches
happens. When index object mismatches then InnoDB should drop
the page hash AHI entries for the block. In
btr_search_drop_page_hash_index(), InnoDB should take exclusive
lock on the AHI latch if index is already freed to avoid the
freed memory access during buf_pool_resize()

This can cause crashes when accessing already released memory

The issue was the Item_default created a internal field, pointing to
share->default_values, to be used with the DEFAULT() function.
This does not work for BLOB fields as these are freed at end of query.
Fixed by storing BLOB field data inside and area allocated by
Item_default_value,  like we do for nondeterministic default values.

this should simplify run-time cluster management

reset thd->lex->query_tables_own_last,
because open_table() uses it and will try to dereference
whatever garbage it might have

MDEV-24873 : galera.galera_as_slave_ctas MTR failed: Assertion `(&(&LOCK_thd_data)->m_mutex)->count > 0 && pthread_equal(pthread_self(), (&(&LOCK_thd_data)->m_mutex)->thread)' failed in sql_class.cc on THD::awake(killed_state)

Problem was that thd::awake assumes now that you hold THD::LOCK_thd_data
so we need to keep it when we call wsrep_thd_awake from
wsrep_abort_transaction.

Problem was that we tried to lock THD::LOCK_thd_data after we have
acquired lock_sys mutex. This is against mutex ordering rules.

Stabilize test case.

Let us avoid the excessive allocation of explicit record locks
(a work-around of MDEV-24813) so that the test will execute
much faster under AddressSanitizer, MemorySanitizer, Valgrind.

For truncate we try to find out possible foreign key tables
using open_tables. However, table_list was not cleaned up
properly and there was no error handling. Fixed by cleaning
table_list and adding proper error handling.

The issue here was the read_set bitmap was not set for a field which
was used as a reference in an inner select.
We need to make sure that if we are in an inner select and we have
references from outer select then we update the table bitmaps for
such references.

Introduced a function in the class Item_subselect that would
update bitmaps of table for the references within a
subquery that are defined in outer selects.

Follow-up fix to commit 26f50335(MDEV-23449)
The GROUP BY clause inside IN/ALL/ANY subquery is removed
when there is no aggregate function or HAVING clause in the subquery.

When the GROUP BY clause is removed, a subquery can also be removed
if it part of the GROUP BY clause. This is done inside the function
remove_redundant_subquery_clauses. Here we walk over the GROUP BY list
and remove a subselect from its unit via the callback function
eliminate_subselect_processor.

The issue here was that when the query was being re-executed it was trying
to reinitialize the select that was removed as stated above.
This is not required, so the fix would be to remove select_lex
both from tree lex structure and the global list of nodes so that
we don't do the reinitialization again.

The issue here was histogram statistics were being used even when
the level of optimizer_use_condition_selectivity doesn't allow
usage of statistics from histogram.

The histogram statistics are read for a table only when
optimizer_use_condition_selectivity > 3. But the TABLE structure can be
stored in the internal table cache and be reused for the next query.
So in this case the histogram statistics will be available for the next query.

The fix would be to make sure to use the histogram statistics only when
optimizer_use_condition_selectivity > 3.

The test innodb.innodb_bug60049 used to check that the record
(ID,NAME)=(12,'SYS_FOREIGN_COLS') is the last record in the
secondary index of the system table SYS_TABLES.
But, ever since commit 23365584
or mysql/mysql-server@082d59670f2616f68af37666fac0f23dbeb43099
that record no longer is the last one in the table!

The more recent test innodb.purge_secondary covers the purge
functionality much better.

wsrep_cluster_address_update() causes LOCK_wsrep_slave_threads
to be locked under LOCK_wsrep_cluster_config, while normally
the order should be the opposite.

Fix: don't protect @@wsrep_cluster_address value with the
LOCK_wsrep_cluster_config, LOCK_global_system_variables is enough.

Only protect wsrep reinitialization with the LOCK_wsrep_cluster_config.
And make it use a local copy of the global @@wsrep_cluster_address.

Also, introduce a helper function that checks whether
wsrep_cluster_address is set and also asserts that it can be safely
read by the caller.

2 different problems:
- MYSQL_BIN_LOG::write() did not check if mdl_context.acquire_lock() failed
- Sql_cmd_optimize_table::execute() and Sql_cmd_repair_table::execute()
  called write_bin_log(), which could fail if sql_admin() had already
  called my_eof()

Fixed by adding check for aquire_lock() return status and protect
write_bin_log() in the above two functions with set_overwrite_status().