While the bug was reported as a regression of
MDEV-11025 Make number of page cleaner threads variable dynamic
in MariaDB Server 10.3, the code that MariaDB Server 10.2
inherited from MySQL 5.7.4 (WL#6642) looks prone to similar errors.

pc_flush_slot(): If there is no work to do, reset the is_requested
signal, to avoid potential busy-waiting in
buf_flush_page_cleaner_worker(). If the coordinator thread has shut
down, avoid resetting the is_requested event, to avoid a potential
hang at shutdown if there are multiple worker threads.

Signed-off-by: Daniel Black <daniel@linux.vnet.ibm.com>

FULLTEXT auxiliary tables

Change the logic to take mdl lock on all tables before tablespaces are
copied, rather than  lock every single tablespace just before it is
copied.

ibuf_merge_or_delete_for_page(): Invoke fil_space_acquire_silent()
instead of fil_space_acquire() in order to avoid displaying
a useless message.

We know perfectly well that a tablespace can be dropped while a
change buffer merge is pending, because change buffer merges skip
any transactional locks.

innobase_commit_by_xid(), innobase_rollback_by_xid(): Decrement
the reference count before freeing the transaction object to the pool.
Failure to do so might corrupt the transaction bookkeeping
if trx_create_low() returns the same object to another thread
before we are done with it.

trx_sys_close(): Detach the recovered XA PREPARE transactions from
trx_sys->rw_trx_list before freeing them.

It was supposed to be called out of mutex, but nevertheless was called
under trx_sys.mutex for normal threads adding one extra condtion in
critical section.

This will allow us to reduce critical section protected by
trx_sys.mutex:
- no need to maintain global m_free list
- eliminate if (trx->read_view == NULL) condition.

On x86_64 sizeof(Readview) is 144 mostly due to padding, sizeof(trx_t)
with ReadView is 1200.

Also don't close ReadView for read-write transactions, just mark it
closed similarly to read-only.

Clean-up: removed n_prepared_recovered_trx and n_prepared_trx, which
accidentally re-appeared after some rebase.

join_tab->distinct=true means "Before doing record read with this
join_tab, call join_tab->remove_duplicates() to eliminate duplicates".
remove_duplicates() assumes that
- there is a temporary table $T with rows that are to be de-duplicated
- there is a previous join_tab (e.g. with join_tab->fields) which was
used to populate the temp.table $T.

When the query has "Impossible WHERE" and window function, then the above
conditions are not met (but we still might need a window function
computation step when the query has implicit grouping).

The fix is to not add remove_duplicates step if the select execution is
degenerate (and we'll have at most one row in the output anyway).

MDEV-14511 tried to avoid some consistency problems related to InnoDB
persistent statistics. The persistent statistics are being written by
an InnoDB internal SQL interpreter that requires the InnoDB data dictionary
cache to be locked.

Before MDEV-14511, the statistics were written during DDL in separate
transactions, which could unnecessarily reduce performance (each commit
would require a redo log flush) and break atomicity, because the statistics
would be updated separately from the dictionary transaction.

However, because it is unacceptable to hold the InnoDB data dictionary
cache locked while suspending the execution for waiting for a
transactional lock (in the mysql.innodb_index_stats or
mysql.innodb_table_stats tables) to be released, any lock conflict
was immediately be reported as "lock wait timeout".

To fix MDEV-14941, an attempt to reduce these lock conflicts by acquiring
transactional locks on the user tables in both the statistics and DDL
operations was made, but it would still not entirely prevent lock conflicts
on the mysql.innodb_index_stats and mysql.innodb_table_stats tables.

Fixing the remaining problems would require a change that is too intrusive
for a GA release series, such as MariaDB 10.2.

Thefefore, we revert the change MDEV-14511. To silence the
MDEV-13201 assertion, we use the pre-existing flag trx_t::internal.

Conflicts:
	sql/sp_rcontext.cc

current_select may point to data from old parser states
when calling a stored procedure with CALL

The failure happens in Item::Item when testing if we are
in having.

Fixed by explicitely reseting current_select in do_execute_sp()
and in sp_rcontext::create(). The later is also needed for
stored functions().

trx->read_view|= 1 was done in a silly attempt to fix race condition
where trx->read_view was closed without trx_sys.mutex lock by read-only
trasnactions.

This just made the problem less likely to happen. In fact there was race
condition in const version of trx_get_read_view(): pointer may change to
garbage any moment after MVCC::is_view_active(trx->read_view) check and
before this function returns.

This patch doesn't fix this race condition, but rather makes it's
consequences less destructive.

Simplified away MVCC::get_oldest_view()
Simplified away MVCC::get_view()
Removed unused MVCC::view_release()

trx_erase_lists(): trx->read_view is owned by current thread and thus
doesn't need trx_sys.mutex protection for reading it's value. Move
trx->read_view check out of mutex

trx_start_low(): moved assertion out of mutex.

Call ReadView::creator_trx_id() directly: allows to inline this one-line
method.

These were unused status variables available in debug builds only.
Also removed trx_sys.rw_max_trx_id: not used anymore.

trx_sys.mvcc was allocated dynamically for no good reason.

There is only one transaction system object in InnoDB.
Allocate the storage for it at link time, not at runtime.

lock_rec_fetch_page(): Use the correct fetch mode BUF_GET.
Pages may never be deallocated from a tablespace while
record locks are pointing to them.

Use atomic operations when accessing trx_sys_t::max_trx_id. We can't yet
move trx_sys_t::get_new_trx_id() out of mutex because it must be updated
atomically along with trx_sys_t::rw_trx_ids.

Remove rw_trx_list.

Let lock_print_info_all_transactions() iterate rw_trx_hash instead of
rw_trx_list.

When printing info of locks for transactions, InnoDB monitor doesn't
attempt to read relevant page from disk anymore. The code was prone
to race conditions.

Note that TrxListIterator didn't work as advertised: it iterated
rw_trx_list only.

Let trx_rollback_recovered() iterate rw_trx_hash instead of rw_trx_list.

Let lock_validate_table_locks(), lock_rec_other_trx_holds_expl(),
lock_table_locks_lookup(), trx_recover_for_mysql(), trx_get_trx_by_xid(),
trx_roll_must_shutdown(), fetch_data_into_cache() iterate rw_trx_hash
instead of rw_trx_list.

Removed trx_sys_validate_trx_list(): with rw_trx_hash elements are not
required to be ordered by transaction id. Transaction state is now guarded
by asserts in rw_trx_hash_t.

Removed trx_sys_t::n_prepared_recovered_trx: never used.

Removed trx_sys_t::n_prepared_trx: used only at shutdown, we can perfectly
get this value from rw_trx_hash.

Determine minimum transaction id by iterating rw_trx_hash, not rw_trx_list.

It is more expensive than previous implementation since it does linear
search, especially if there're many concurrent transactions running. But in
such case mutex is much bigger evil. And since it doesn't require
trx_sys->mutex protection it scales better.

For low concurrency performance difference is neglible.

Replaced UT_LIST_GET_LEN(trx_sys->rw_trx_list) with
trx_sys->rw_trx_hash.size().
Moved freeing of trx objects at shutdown to rw_trx_hash destructor.
Small clean-up in trx_rollback_recovered().

Reduce divergence between trx_sys_t::rw_trx_hash and trx_sys_t::rw_trx_list
by not adding recovered COMMITTED transactions to trx_sys_t::rw_trx_list.

Such transactions are discarded immediately without creating trx object.

This also required to split rollback and cleanup phases of recovery. To
reflect these updates the following renames happened:
trx_rollback_or_clean_all_recovered() -> trx_rollback_all_recovered()
trx_rollback_or_clean_is_active -> trx_rollback_is_active
trx_rollback_or_clean_recovered() -> trx_rollback_recovered()
trx_cleanup_at_db_startup() -> trx_cleanup_recovered()

Also removed a hack from lock_trx_release_locks(). Instead let recovery
rollback thread to skip committed XA transactions.

when number of NULLs in IN list reaches in_predicate_conversion_threshold

The bug was fixed by removing an assertion that had been set in order
just to test whether the code could be ever executed.

caused an error

The function subselect_single_select_engine::print() did not print
the WITH clause attached to a subselect with single select engine.
As a result views using suqueries with attached WITH clauses lost
these clauses when saved in frm files.