This is regression after bc7a1dc1 of
MDEV-15104 - Optimise MVCC snapshot.

Aforementioned revision removes mutex lock around ReadView creation,
which allows views to be created concurrently. Effectively it
invalidates "oldest view" approach: no single view can be considered
oldest anymore. Instead we have to iterate trx_sys.m_views to find
min(m_low_limit_no), min(m_low_limit_id) and all transaction ids below
min(m_low_limit_id), which would form oldest view.

Second regression comes from c0d5d7c0
of MDEV-15104 - Optimise MVCC snapshot.

It removes mutex protection around trx->no assignment, which opens up
a gap between m_max_trx_id increment and transaction serialisation
number becoming visible through rw_trx_hash. While we're in this gap
concurrent thread may come and do MVCC snapshot without seeing allocated
but not yet assigned serialisation number. Then at some point purge
thread may clone this view. As a result it won't see newly allocated
serialisation number and may remove "unnecessary" history data of this
transaction from rollback segments.

MDEV-15215 main.partition_explicit_prune fails in bulidbot with assertion failures and server crashes.

        ha_partition::get_open_file_sample() implemented to be used
        when we need a file sample that is surely opened.

MDEV-14990 mysql_upgrade fails with ERROR 1408 (HY000) at line 566: Event Scheduler: An error occurred when initializing system tables

Don't check mysql.db and mysql.user from event schedule on startup.

Event schedule should only check its own mysql.event table, it has
no business checking other system tables. In particular, it's ridiculous
for event schedule to fail when privilege tables are not the newest,
because sql_acl.cc supports old privilege tables just fine.

Truncate_versioning_priv->Delete_history_priv

because the command and the privilege were renamed

don't expand AS OF in views, and, in particular, don't auto-add
AS OF NOW().

Hide INVISIBLE_SYSTEM columns from writes and from fix_vcol_expr().

enum_mark_columns -> enum_column_usage
mark_used_columns -> column_usage

further commits will replace MARK_COLUMN_NONE with
COLUMN_READ and COLUMN_WRITE that convey the intention
without causing columns to be marked

update all unique keys, not just PK

don't add columns to the drop list, INVISIBLE_SYSTEM columns
cannot be recognized as specified by the user

* don't disclose INVISIBLE_FULL columns in USING and NATURAL JOIN
* other INVISIBLE columns must me mentioned by name in USING,
  they are hidden from NATURAL JOIN

don't duplicate functionality, use TABLE::find_field_by_name()

also, set cached_field_index_ptr for _rowid field

use name hash, just like find_field_in_table() does

replication tests

Standard compatible behavior for UPDATE: all assignments in SET
are executed "simultaneously", not left-to-right. And `SET a=b,b=a`
will swap the values.

cleanup. use "command -v" instead of "which"
simplify some checks.

For placement new, throw() exception specification was removed by
accident in 8fe04a3d , making  debug
tests, that simulate out-of-memory situations, fail (e.g filesort_debug)

Correctly initialize fake IPs.

innobase_add_instant_try(): If the leftmost leaf page does not contain
other records than the 'default row', only empty the table if there
are no successor pages.

When a table or partition which was not empty during a previous
instant ADD COLUMN became empty later, and now with this subsequent
instant ADD COLUMN we have the opportunity to convert the empty table
or partition to 'non-instant' format.

Similarly, if the table or partition is empty to begin with, that is,
it does not even contain a 'default row' record, we can use the
'non-instant' format.

Until now, InnoDB inefficiently compared the aligned fields
FIL_PAGE_PREV, FIL_PAGE_NEXT to the byte-order-agnostic value FIL_NULL.

Timing problem as sometimes table is marked as encrypted but
sometimes we are not sure and table is just marked missing.

Compiler optimizations were switched off due to
MySQL Bug #19424, #36366, #34297, due to an alleged compiler bug.
No  proper analysis of code generation was done back then, thus proof of
a compiler bug is missing.

Even if there was a compiler bug 13 years ago, it could have been fixed.
Will wait and see if there are any complains or crashes

trx_undo_rec_copy(): Use a debug assertion. In a non-debug build,
with len<0 (which this assertion is really testing for)
we should still typically crash due to running out of memory.

Replace all occurrences of the is_clust() method with is_primary(),
because that is what is actually meant. (Also the change buffer
tree would count as a clustered index.)

Rollback attempted to dereference DB_ROLL_PTR=0, which cannot possibly
be a valid undo log pointer. A safer canonical value would be
roll_ptr_t(1) << ROLL_PTR_INSERT_FLAG_POS
which is what was chosen in MDEV-12288, corresponding to reset_trx_id.

No deterministic test case for the bug was found. The simplest test
cases may be related to MDEV-11415, which suppresses undo logging for
ALGORITHM=COPY operations. In those operations, in the spirit of
MDEV-12288, we should actually have written reset_trx_id instead of
using the transaction identifier of the current transaction
(and a bogus value of DB_ROLL_PTR=0). However, thanks to MySQL Bug#28432
which I had fixed in MySQL 5.6.8 as part of WL#6255, access to the
rebuilt table by earlier-started transactions should actually have been
refused with ER_TABLE_DEF_CHANGED.

reset_trx_id: Move the definition to data0type.cc and the declaration
to data0type.h.

btr_cur_ins_lock_and_undo(): When undo logging is disabled, use the
safe value that corresponds to reset_trx_id.

btr_cur_optimistic_insert(): Validate the DB_TRX_ID,DB_ROLL_PTR before
inserting into a clustered index leaf page.

ins_node_t::sys_buf[]: Replaces row_id_buf and trx_id_buf and some
heap usage.

row_ins_alloc_sys_fields(): Init ins_node_t::sys_buf[] to reset_trx_id.

row_ins_buf(): Only if undo logging is enabled, copy trx->id
to node->sys_buf. Otherwise, rely on the initialization in
row_ins_alloc_sys_fields().

row_purge_reset_trx_id(): Invoke mlog_write_string() with reset_trx_id
directly. (No functional change.)

trx_undo_page_report_modify(): Assert that the DB_ROLL_PTR is not 0.

trx_undo_get_undo_rec_low(): Assert that the roll_ptr is valid before
trying to dereference it.

dict_index_t::is_primary(): Check if the index is the primary key.

PageConverter::adjust_cluster_record(): Fix
MDEV-15249 Crash in MVCC read after IMPORT TABLESPACE
by resetting the system fields to reset_trx_id instead of writing
the current transaction ID (which will be committed at the
end of the IMPORT TABLESPACE) and DB_ROLL_PTR=0.
This can partially be viewed as a follow-up fix of MDEV-12288,
because IMPORT should already then have written
DB_TRX_ID=0 and DB_ROLL_PTR=1<<55 to prevent unnecessary
DB_TRX_ID lookups in subsequent accesses to the table.

fix a couple "initialization skipped by goto" and other new errors.

no matching operator delete found; memory will not be freed if initialization throws an exception

Added a no-op delete() for MEM_ROOT based placement-new()

But set _CRT_NONSTDC_NO_WARNINGS to silence silly warnings about
ANSI C function being non-standard

Remove now deprecated GetVersion()/GetVersionEx(),except single case
where where it is really needed, in feedback plugin. Remove checks for
Windows NT.

Avoid old IPv4-only inet_aton, which generated the warning.

The test was used to result in mismatch due to unaccounted specifics
of the master-slave handshake protocol that sets the Slave_IO_Running
status to true while the semisync master status is set to active a bit later.

The test is refined to expect that.