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.

GAL-506 breaks galera_defaults MTR test by upping repl.proto_max again. Fix this once and for all by overwriting it with constant string since it makes little sense to check for it in this test.

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.

lock_trx_release_locks(): Relax a debug assertion to allow
recovered TRX_STATE_COMMITTED_IN_MEMORY transactions.

trx_commit_in_memory(): Add DEBUG_SYNC instrumentation.

trx_undo_insert_cleanup(): Skip persistent changes if innodb_read_only
is set. This should only happen when a recovered committed transaction
would be cleaned up at shutdown.

The patch for MDEV-15287 made the code in Item_func_min_max::get_date_native()
dead, as the TIME data type is now handled in get_time_native().
Removing the TIME related code from get_date_native().

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

- Max_index_length is supported by MyISAM and Aria tables.
- Temporary is a placeholder to signal that a table is a
  temporary table. For the moment this is always "N", except
  "Y" for generated information_schema tables and NULL for
  views. Full temporary table support will be done in another task.
  (No reason to have to update a lot of result files twice in a row)

When Mariabackup gets a bad read of the first page of the system
tablespace file, it would inappropriately try to apply the doublewrite
buffer and write changes back to the data file (to the source file)!
This is very wrong and must be prevented.

The correct action would be to retry reading the system tablespace
as well as any other files whose first page was read incorrectly.
Fixing this was not attempted.

xb_load_tablespaces(): Shorten a bogus message to be more relevant.
The message can be displayed by --backup or --prepare.

xtrabackup_backup_func(), os_file_write_func(): Add a missing space
to a message.

Datafile::restore_from_doublewrite(): Do not even attempt the
operation in Mariabackup.

recv_init_crash_recovery_spaces(): Do not attempt to restore the
doublewrite buffer in Mariabackup (--prepare or --export), because
all pages should have been copied correctly in --backup already,
and because --backup should ignore the doublewrite buffer.

SysTablespace::read_lsn_and_check_flags(): Do not attempt to initialize
the doublewrite buffer in Mariabackup.

innodb_make_page_dirty(): Correct the bounds check.

Datafile::read_first_page(): Correct the name of the parameter.

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.

When code from MySQL 5.7.9 was merged to MariaDB 10.2.2
in commit 2e814d47
an assignment validate=true was inadvertently added to the function
dict_check_sys_tables().

This causes InnoDB to open every single .ibd file on startup, even
when no crash recovery was needed.

Simply removing the assignment would make some tests fail. We do the
best to retain almost the same level of inconsistency detection.
In the test innodb.table_flags, access to one of the tables will not
be blocked despite inconsistent flags.

dict_check_sys_tables(): Remove the problematic assignment, and skip
validation in normal startup.

dict_load_table_one(): If the .ibd file cannot be opened, mark the
table as corrupted and unreadable.

fil_node_open_file(): Validate FSP_SPACE_FLAGS with the expected
flags. If reading the tablespace fails, invalidate node->handle
instead of letting it remain stale. This bug was caught by a
fil_validate() assertion failure.

fsp_flags_try_adjust(): If the tablespace file is invalid, do nothing.