In a Sysbench oltp_update_index workload that involves 1 table,
a serious contention between the workload and the purge of history
was observed. This was the worst when the table contained only 1 record.

This turned out to be fixed by setting innodb_purge_batch_size=128,
which corresponds to the number of usable persistent rollback segments.
When we go above that, there would be contention between row_purge_poss_sec()
and the workload, typically on the clustered index page latch, sometimes
also on a secondary index page latch. It might be that with smaller
batches, trx_sys.history_size() will end up pausing all concurrent
transaction start/commit frequently enough so that purge will be able
to make some progress, so that there would be less contention on the
index page latches between purge and SQL execution.

In commit aa719b50 (part of MDEV-32050)
the interpretation of the parameter innodb_purge_batch_size was slightly
changed. It would correspond to the maximum desired size of the
purge_sys.pages cache. Before that change, the parameter was referring to
a number of undo log pages, but the accounting might have been inaccurate.

To avoid a regression, we will reduce the default value to
innodb_purge_batch_size=127, which will also be compatible with
innodb_undo_tablespaces>1 (which will disable rollback segment 0).

Additionally, some logic in the purge and MVCC checks is simplified.
The purge tasks will make use of purge_sys.pages when accessing undo
log pages to find out if a secondary index record can be removed.
If an undo page needs to be looked up in buf_pool.page_hash, we will
merely buffer-fix it. This is correct, because the undo pages are
append-only in nature. Holding purge_sys.latch or purge_sys.end_latch
or the fact that the current thread is executing as a part of an
in-progress purge batch will prevent the contents of the undo page from
being freed and subsequently reused. The buffer-fix will prevent the
page from being evicted form the buffer pool. Thanks to this logic,
we can refer to the undo log record directly in the buffer pool page
and avoid copying the record.

buf_pool_t::page_fix(): Look up and buffer-fix a page. This is useful
for accessing undo log pages, which are append-only by nature.
There will be no need to deal with change buffer or ROW_FORMAT=COMPRESSED
in that case.

purge_sys_t::view_guard::view_guard(): Allow the type of guard to be
acquired: end_latch, latch, or no latch (in case we are a purge thread).

purge_sys_t::view_guard::get(): Read-only accessor to purge_sys.pages.

purge_sys_t::get_page(): Invoke buf_pool_t::page_fix().

row_vers_old_has_index_entry(): Replaced with row_purge_is_unsafe()
and row_undo_mod_sec_unsafe().

trx_undo_get_undo_rec(): Merged to trx_undo_prev_version_build().

row_purge_poss_sec(): Add the parameter mtr and remove redundant
or unused parameters sec_pcur, sec_mtr, is_tree. We will use the
caller's mtr object but release any acquired page latches before
returning.

btr_cur_get_page(), page_cur_get_page(): Do not invoke page_align().

row_purge_remove_sec_if_poss_leaf(): Return the value of PAGE_MAX_TRX_ID
to be checked against the page in row_purge_remove_sec_if_poss_tree().
If the secondary index page was not changed meanwhile, it will be
unnecessary to invoke row_purge_poss_sec() again.

trx_undo_prev_version_build(): Access any undo log pages using
the caller's mini-transaction object.

row_purge_vc_matches_cluster(): Moved to the only compilation unit that
needs it.

Reviewed by: Debarun Banerjee