buf_LRU_get_free_block(): Initially wait for a single block to be
freed, signaled by buf_pool.done_free. Only if that fails and no
LRU eviction flushing batch is already running, we initiate a
flushing batch that should serve all threads that are currently
waiting in buf_LRU_get_free_block().

Note: In an extreme case, this may introduce a performance regression
at larger numbers of connections. We observed this in sysbench
oltp_update_index with 512MiB buffer pool, 4GiB of data on fast NVMe,
and 1000 concurrent connections, on a 20-thread CPU. The contention point
appears to be buf_pool.mutex, and the improvement would turn into a
regression somewhere beyond 32 concurrent connections.

On slower storage, such regression was not observed; instead, the
throughput was improving and maximum latency was reduced.

The excessive waits were pointed out by Vladislav Vaintroub.

MDEV-23855 changed the way how the page cleaner is signaled by
user threads. If a threshold is exceeded, a mini-transaction commit
would invoke buf_flush_ahead() in order to initiate page flushing
before all writers would eventually grind to halt in
log_free_check(), waiting for the checkpoint age to reduce.

However, buf_flush_ahead() would always initiate 'furious flushing',
making the buf_flush_page_cleaner thread write innodb_io_capacity_max
pages per batch, and sleeping no time between batches, until the
limit LSN is reached. Because this could saturate the I/O subsystem,
system throughput could significantly reduce during these
'furious flushing' spikes.

With this change, we introduce a gentler version of flush-ahead,
which would write innodb_io_capacity_max pages per second until
the 'soft limit' is reached.

buf_flush_ahead(): Add a parameter to specify whether furious flushing
is requested.

buf_flush_async_lsn: Similar to buf_flush_sync_lsn, a limit for
the less intrusive flushing.

buf_flush_page_cleaner(): Keep working until buf_flush_async_lsn
has been reached.

log_close(): Suppress a warning message in the event that a new log
is being created during startup, when old logs did not exist.
Return what type of page cleaning will be needed.

mtr_t::finish_write(): Also when m_log.is_small(), invoke log_close().
Return what type of page cleaning will be needed.

mtr_t::commit(): Invoke buf_flush_ahead() based on the return value of
mtr_t::finish_write().

buf_page_write_complete(): Reduce the buf_pool.mutex hold time,
and do not acquire buf_pool.flush_list_mutex at all.
Instead, mark blocks clean by setting oldest_modification to 1.
Dirty pages of temporary tables will be identified by the special
value 2 instead of the previous special value 1.
(By design of the ib_logfile0 format, actual LSN values smaller
than 2048 are not possible.)

buf_LRU_free_page(), buf_pool_t::get_oldest_modification()
and many other functions will remove the garbage (clean blocks)
from buf_pool.flush_list while holding buf_pool.flush_list_mutex.

buf_pool_t::n_flush_LRU, buf_pool_t::n_flush_list:
Replaced with non-atomic variables, protected by buf_pool.mutex,
to avoid unnecessary synchronization when modifying the counts.

export_vars: Remove unnecessary indirection for
innodb_pages_created, innodb_pages_read, innodb_pages_written.

In commit 7cffb5f6 (MDEV-23399)
the implementation of buf_flush_dirty_pages() was replaced with
a slow one, which would perform excessive scans of the
buf_pool.flush_list and make little progress.

buf_flush_list(), buf_flush_LRU(): Split from buf_flush_lists().
Vladislav Vaintroub noticed that we will not need to invoke
log_flush_task.wait() for the LRU eviction flushing.

buf_flush_list_space(): Replaces buf_flush_dirty_pages().
This is like buf_flush_list(), but operating on a single
tablespace at a time. Writes at most innodb_io_capacity
pages. Returns whether some of the tablespace might remain
in the buffer pool.

Vladislav Vaintroub suggested that invoking log_flush_up_to()
for every page could perform better than invoking a log write
between buf_pool.flush_list batches, like we started doing in
commit 3a9a3be1 (MDEV-23855).
This could depend on the sequence in which pages are being
modified. The buf_pool.flush_list is ordered by
oldest_modification, while the FIL_PAGE_LSN of the pages is
theoretically independent of that. In the pathological case,
we will wait for a log write before writing each individual page.

It turns out that we can defer the call to log_flush_up_to()
until just before submitting the page write. If the doublewrite
buffer is being used, we can submit a write batch of "future" pages
to the doublewrite buffer, and only wait for the log write right
before we are writing an already doublewritten page.
The next doublewrite batch will not be initiated before the last
page write from the current batch has completed.

When a future version introduces asynchronous writes if the log,
we could initiate a write at the start of a flushing batch, to
reduce waiting further.

It turns out that we had some unnecessary waits for no outstanding
write requests to exist. They were basically working around a
bug that was fixed in MDEV-25953.

On write completion callback, blocks will be marked clean.
So, it is sufficient to consult buf_pool.flush_list to determine
which writes have not been completed yet.

On FLUSH TABLES...FOR EXPORT we must still wait for all pending
asynchronous writes to complete, because buf_flush_file_space()
would merely guarantee that writes will have been initiated.

redo_rseg_mutex, noredo_rseg_mutex: Remove the PERFORMANCE_SCHEMA keys.
The rollback segment mutex will be uninstrumented.

trx_sys_t: Remove pointer indirection for rseg_array, temp_rseg.
Align each element to the cache line.

trx_sys_t::rseg_id(): Replaces trx_rseg_t::id.

trx_rseg_t::ref: Replaces needs_purge, trx_ref_count, skip_allocation
in a single std::atomic<uint32_t>.

trx_rseg_t::latch: Replaces trx_rseg_t::mutex.

trx_rseg_t::history_size: Replaces trx_sys_t::rseg_history_len

trx_sys_t::history_size_approx(): Replaces trx_sys.rseg_history_len
in those places where the exact count does not matter. We must not
acquire any trx_rseg_t::latch while holding index page latches, because
normally the trx_rseg_t::latch is acquired before any page latches.

trx_sys_t::history_exists(): Replaces trx_sys.rseg_history_len!=0
with an approximation.

We remove some unnecessary trx_rseg_t::latch acquisition around
trx_undo_set_state_at_prepare() and trx_undo_set_state_at_finish().
Those operations will only access fields that remain constant
after trx_rseg_t::init().

recv_sys_t::recover_deferred(): Set the file size to match the number
of pages. Mariabackup might copy the file while it was being extended.

dict_check_sys_tables(): Correctly advance the cursor position.
This fixes a regression that was caused by
commit 49e2c8f0 (MDEV-25743).

Cherry-picking only test case.

commit_try_rebuild(): Invoke trx_t::drop_table_statistics()
with the correct (original) name of the table.
This fixes a regression that was introduced in
commit 1bd681c8 (MDEV-25506 part 3).

If an ALTER TABLE that affected a table with FULLTEXT INDEX was
in progress while a backup was made, InnoDB would crash or hang
during the first startup after restoring the backup,
while trying to drop the #sql-alter- table for the DDL operation.

drop_garbage_tables_after_restore(): Invoke dict_sys.unlock()
before invoking the FTS functions. Also, invoke purge_sys.stop_FTS()
in debug builds to silence debug assertions. (Purge is not yet
running at this point.)

trx_undo_mem_create_at_db_start(): Relax too strict upgrade checks
that were introduced in
commit e46f76c9 (MDEV-15912).
On commit, pages will typically be set to TRX_UNDO_CACHED state.
Having the type TRX_UNDO_INSERT in such pages is common and
unproblematic; the type would be reset in trx_undo_reuse_cached().

trx_rseg_array_init(): On failure, clean up the rollback segments
that were initialized so far, to avoid an assertion failure later
during shutdown.

init_crypt_key(): On failure, set info->key_version to
ENCRYPTION_KEY_VERSION_INVALID.

log_crypt_101_read_block(): Refuse to attempt decryption if
info->key_version is ENCRYPTION_KEY_VERSION_INVALID.

This bug affected queries with views / derived_tables / CTEs whose
specifications were of the form
  (SELECT ... LIMIT <n>) ORDER BY ...
Units representing such specifications contains one SELECT_LEX structure
for (SELECT ... LIMIT <n>) and additionally SELECT_LEX structure for
fake_select_lex. This fact should have been taken into account in the
function mysql_derived_fill().

This patch has to be applied to 10.2 and 10.3 only.

This bug affected queries with views / derived_tables / CTEs whose
specifications were of the form
  (SELECT ... LIMIT <n>) ORDER BY ...
Units representing such specifications contains one SELECT_LEX structure
for (SELECT ... LIMIT <n>) and additionally SELECT_LEX structure for
fake_select_lex. This fact should have been taken into account in the
function mysql_derived_fill().

This patch has to be applied to 10.2 and 10.3 only.

trx_undo_report_row_operation(): Fix a race condition that was introduced
in commit f74023b9 (MDEV-15090).
We must not access undo_block after the page latch has been released
in mtr_t::commit(), because the block could be evicted or replaced.

fil_crypt_rotate_page(): If a page is already marked as dirty,
do not move it in buf_pool.flush_list. Actual re-encryption
will happen in buf_flush_page(), and moving the page in
buf_pool.flush_list may only postpone the operation.

In the merge commit d3e4fae7
a message about innodb_force_recovery was accidentally duplicated.

- Adding socket check for MariaDB/Mysql driver

Reviewed by: serg@mariadb.com

mytop fall-back to DBD::mysql if DBD::MariaDB is not available

Apply #1546

Let us simply refuse an upgrade from earlier versions if the
upgrade procedure was not followed. This simplifies the purge,
commit, and rollback of transactions.

Before upgrading to MariaDB 10.3 or later, a clean shutdown
of the server (with innodb_fast_shutdown=1 or 0) is necessary,
to ensure that any incomplete transactions are rolled back.
The undo log format was changed in MDEV-12288. There is only
one persistent undo log for each transaction.

Builds on travis-ci.org ceased on 2021-06-15.