Since commit 15093639 (MDEV-23484)
the rollback of InnoDB transactions is no longer protected by
dict_operation_lock. Removing that protection revealed a race
condition between transaction rollback and the rollback of an
online table-rebuilding operation (OPTIMIZE TABLE, or any online
ALTER TABLE that is rebuilding the table).

row_undo_mod_clust(): Re-check dict_index_is_online_ddl() after
acquiring index->lock, similar to how row_undo_ins_remove_clust_rec()
is doing it. Because innobase_online_rebuild_log_free() is holding
exclusive index->lock while invoking row_log_free(), this re-check
will ensure that row_log_table_low() will not be invoked when
index->online_log=NULL.

A different race condition is possible between the rollback of a
recovered transaction and the start of online secondary index creation.
Because prepare_inplace_alter_table_dict() is not acquiring an InnoDB
table lock in this case, and because recovered transactions are not
covered by metadata locks (MDL), the dict_table_t::indexes could be
modified by prepare_inplace_alter_table_dict() while the rollback of
a recovered transaction is being executed. Normal transactions would
be covered by MDL, and during prepare_inplace_alter_table_dict() we
do hold MDL_EXCLUSIVE, that is, an online ALTER TABLE operation may
not execute concurrently with other transactions that have accessed
the table.

row_undo(): To prevent a race condition with
prepare_inplace_alter_table_dict(), acquire dict_operation_lock
for all recovered transactions. Before MDEV-23484 we used to acquire
it for all transactions, not only recovered ones.

Note: row_merge_drop_indexes() would not invoke
dict_index_remove_from_cache() while transactional locks
exist on the table, or while any thread is holding an open table handle.
OK, it does that for FULLTEXT INDEX, but ADD FULLTEXT INDEX is not
supported as an online operation, and therefore
prepare_inplace_alter_table_dict() would acquire a table S lock,
which cannot succeed as long as recovered transactions on the table
exist, because they would hold a conflicting IX lock on the table.

In commit fe39d02f (MDEV-20638)
we removed some wake-up signaling of the master thread that should
have been there, to ensure a steady log checkpointing workload.

Common sense suggests that the commit omitted some necessary calls
to srv_inc_activity_count(). But, an attempt to add the call to
trx_flush_log_if_needed_low() as well as to reinstate the function
innobase_active_small() did not restore the performance for the
case where sync_binlog=1 is set.

Therefore, we will revert the entire commit in MariaDB Server 10.2.
In MariaDB Server 10.5, adding a srv_inc_activity_count() call to
trx_flush_log_if_needed_low() did restore the performance, so we
will not revert MDEV-20638 across all versions.

InnoDB transaction rollback includes an unnecessary work-around for
a data corruption bug that was fixed by me in MySQL 5.6.12
mysql/mysql-server@935ba09d52c1908bde273ad1940b5ab919d9763d
and ported to MariaDB 10.0.8 by
commit c291ddfd
in 2013 and 2014, respectively.

By acquiring and releasing dict_operation_lock in shared mode,
row_undo() hopes to prevent the table from being dropped while
the undo log record is being rolled back. But, thanks to mentioned fix,
debug assertions (that we are adding) show that the rollback is
protected by transactional locks (table IX lock, in addition to
implicit or explicit exclusive locks on the records that had been modified).

Because row_drop_table_for_mysql() would invoke
row_add_table_to_background_drop_list() if any locks exist on the table,
the mere existence of locks (which is guaranteed during ROLLBACK) is
enough to protect the table from disappearing. Hence, acquiring and
releasing dict_operation_lock for every row that is being rolled back is
unnecessary.

row_undo(): Remove the unnecessary acquisition and release of
dict_operation_lock.

Note: row_add_table_to_background_drop_list() is mostly working around
bugs outside InnoDB:
MDEV-21175 (insufficient MDL protection of FOREIGN KEY operations)
MDEV-21602 (incorrect error handling of CREATE TABLE...SELECT).

Regretfully, the parameter innodb_log_checksums was introduced
in MySQL 5.7.9 (the first GA release of that series) by
mysql/mysql-server@af0acedd885eb7103e319f79d25fda7386ef1506
which partly replaced a parameter that had been introduced in 5.7.8
mysql/mysql-server@22ba38218e1d76c24f69b5a5595ad3bf5933acb0
as innodb_log_checksum_algorithm.

Given that the CRC-32C operations are accelerated on many processor
implementations (AMD64 with SSE4.2; since MDEV-22669 also on IA-32
with SSE4.2, POWER 8 and later, ARMv8 with some extensions)
and by lookup tables when only generic SISD instructions are available,
there should be no valid reason to disable checksums.

In MariaDB 10.5.2, as a preparation for MDEV-12353, MDEV-19543 deprecated
and ignored the parameter innodb_log_checksums altogether. This should
imply that after a clean shutdown with innodb_log_checksums=OFF one
cannot upgrade to MariaDB Server 10.5 at all.

Due to these problems, let us deprecate the parameter innodb_log_checksums
and honor it only during server startup.
The command SET GLOBAL innodb_log_checksums will always set the
parameter to ON.

Problem:
=======
InnoDB drops the column which has foreign key relations on it. So it
tries to load the foreign key during rename process of copy algorithm
even though the foreign_key_check is disabled.

Solution:
========
During alter copy algorithm, InnoDB ignores the error while loading
the foreign key constraint if foreign key check is disabled. It
should throw the warning about failure of the foreign key constraint
when foreign key check is disabled.

This problem is caused by 6697135c
(MDEV-21572). During recovery, InnoDB prefetches the siblings of
change buffer index leaf page. It does asynchronous page read
and recovery scenario wasn't handled in buf_read_page_background().
It leads to the refusal of startup of the server.

Solution:
=========
  InnoDB shouldn't allow the change buffer index page siblings
to be prefetched.

Remove __bss_start & Co, because systen call "write" check buffer address and return EFAULT if it is wrong.

Adding any unknown option to the "[mysqld_safe]" section makes
mysqld impossible to start with mysqld_multi. For example, after
adding the unknown option "numa_interleave" to the "[mysqld_safe]"
section, mysqld_multi exits with the following diagnostics:

[ERROR] /usr/local/mysql/bin/mysqld: unknown option '--numa_interleave'

To get rid of this behavior, this patch by default adds the "--loose-"
prefix to all unknown (for mysqld_safe) options. This behavior can be
enabled explicitly with the --ignore-unknown option and disabled with
the --no-ignore-unknown option.

Since MDEV-18778, timezone tables get changed to innodb
to allow them to be replicated to other galera nodes.

Even without galera, timezone tables could be declared innodb.
With the standalone innodb tables, the mysql_tzinfo_to_sql takes
approximately 27 seconds.

With the transactions enabled in this patch, 1.2 seconds is
the approximate load time.

While explicit checks for the engine of the time zone tables could be
done, or checks against !opt_skip_write_binlog, non-transactional
storage engines will just ignore the transactional state without
even a warning so its safe to enact globally.

Leap seconds are pretty much ignored as they are a single insert
statement and have gone out of favour as they have caused MariaDB
stalls in the past.

fil_page_decompress(): Remove a rather useless debug check.
We should have test coverage for reading page_compressed pages
from files, either due to buffer pool page eviction or due to
server restarts.

A similar check was removed from fil_space_encrypt() in
commit 0b36c27e (MDEV-20307).

The usage message for the innodb_compression_algorithm system variable
did not list snappy, which was added as an optional compression algorithm
in MariaDB 10.1.3 and might actually work since
commit 90c52e52 (MDEV-12615)
in MariaDB 10.1.24.

Unfortunately, we will include also unavailable compression algorithms
in the list, because ENUM parameters allow numeric values, and we do
not want innodb_compression_algorithm=3 to change meaning depending on
the way how the source code was compiled.

InnoDB only reserves 13 bits for the heap number in the record header,
limiting the heap number to be at most 8191. But, when using
innodb_page_size=64k and secondary index records of 7 bytes each,
it is possible to exceed the maximum heap number.

btr_cur_optimistic_insert(): Let the operation fail if the
maximum number of records would be exceeded.

page_mem_alloc_heap(): Move to the same compilation unit with the
only caller, and let the operation fail if the maximum heap number
has been allocated already.

The debug assertion is bogus, and we had removed it in
commit b1ab211d (MDEV-15053)
in the MariaDB Server 10.5 branch.

For a small data file, fil_space_extend_must_retry() would always
allocate a minimum size of 4*innodb_page_size.

It is possible that random read-ahead will be triggered for
a smaller file than this. In the observed case, the read-ahead
was triggered for a 6-page file that used ROW_FORMAT=COMPRESSED
with 8KiB page size. So, the desired file size was 49152 bytes,
but the actual size was 65536 bytes.

In 10.3, DBUG_ASSERT() may expand to something that includes
__builtin_expect(), which expects integer arguments, not pointers.
To avoid any compiler warnings, let us use an explicit rather than
implicit comparison to the null pointer.

innobase_pk_order_preserved(): Treat an added AUTO_INCREMENT
column in the same way as an added existing column.
In either case, the column values are not guaranteed to
be constant, and thus the ordering may change if such a column
is added before any existing PRIMARY KEY columns.

prepare_inplace_alter_table_dict(): Initialize
dict_table_t::persistent_autoinc before invoking
innobase_pk_order_preserved().

fil_system_t::keyrotate_next(): If space && space->is_in_rotation_list
does not hold, iterate from the start of the list.

In debug builds, we would typically have hit SIGSEGV because the
iterator would have wrapped a null pointer. It might also be that
we are dereferencing a stale pointer.

There is no test case, because the encryption is very nondeterministic
in nature, due to the use of background threads.

This scenario can be hit by setting the following:

SET GLOBAL innodb_encryption_threads=5;
SET GLOBAL innodb_encryption_rotate_key_age=0;

For some reason, GCC emits more -Wmaybe-uninitialized warnings
when using the flag -Og than when using -O2. Many of the warnings
look genuine.

The problem is caused by the fact that adding the
--defaults-group-suffix option to fix MDEV-18863 causes
mysqld to read all options from the appropriate sections
of the config file, including options specific to mysqld_multi.
Reading unknown options (which are not supported by mysqld)
causes mysqld to terminate with an error.

However, the MDEV-18863 problem has been completely fixed
by passing options on the command line, and now there is no
need to specify the --defaults-group-suffix option (we just
need to give priority to options passed through the command
line, so as not to break MDEV-18863).

During SST we need to let FTWRL to use normal timeout method
even when client is disconnected.

Some tests relied on the fact that DATETIME->DATE conversion
always produce a truncation (with a warning). This is not the case
when the SQL statement is executed at current time '00:00:00' sharp.

Adding a new SET TIMESTAMP statements to make sure time is not '00:00:00'.

Implement a workaround to shut the "memory not freed" message.

The test encryption.create_or_replace would occasionally fail,
because some fil_space_t::n_pending_ops would never be decremented.

fil_crypt_find_space_to_rotate(): If rotate_thread_t::should_shutdown()
holds due to innodb_encryption_threads having been reduced, do
release the reference.

fil_space_remove_from_keyrotation(), fil_space_next(): Declare the
functions static, simplify a little, and define in the same compilation
unit with the only caller, fil_crypt_find_space_to_rotate().

fil_crypt_key_mutex: Remove (unused).

Example of the failure:
http://buildbot.askmonty.org/buildbot/builders/bld-p9-rhel7/builds/4417/steps/mtr/logs/stdio
```
main.mysqld--help 'unix'                 w17 [ fail ]
        Test ended at 2020-06-20 18:51:45

CURRENT_TEST: main.mysqld--help
--- /opt/buildbot-slave/bld-p9-rhel7/build/mysql-test/main/mysqld--help.result	2020-06-20 16:06:49.903604179 +0300
+++ /opt/buildbot-slave/bld-p9-rhel7/build/mysql-test/main/mysqld--help.reject	2020-06-20 18:51:44.886766820 +0300
@@ -1797,10 +1797,10 @@
 sync-relay-log-info 10000
 sysdate-is-now FALSE
 system-versioning-alter-history ERROR
-table-cache 421
+table-cache 2000
 table-definition-cache 400
-table-open-cache 421
-table-open-cache-instances 1
+table-open-cache 2000
+table-open-cache-instances 8
 tc-heuristic-recover OFF
 tcp-keepalive-interval 0
 tcp-keepalive-probes 0

mysqltest: Result length mismatch
```
mtr: table_open_cache_basic autosized:

Lets assume that >400 are available and that
we can set the result back to the start value.

All of these system variables are autosized and can
generate MTR output differences.

Closes #1527

hang.

MDEV-22626: mysql_tzinfo_to_sql not replicates timezone to galeranodes if only 1 timezone will be loaded.

Move alter to InnoDB earlier to more correct place to handle
also if only a one timezone file is loaded.

fix_fields for the arguments of the NTH_VALUE function was updating the same reference,
so for the second argument (or after the first argument) the items were not resolved
to their corresponding field from the view as they were updating the reference to the
first argument.

depending on build config the error might be hidded,
in particular liblz4.so and libjemalloc.so make it to disappear,
but with -DWITH_INNODB_LZ4=NO -DWITH_JEMALLOC=NO it reappears.

Removing the ORDER BY clause from the UNION when UNION is inside an IN/ALL/ANY/EXISTS subquery.
The rewrites are done for subqueries but this rewrite is not done for the fake_select of
the UNION.