Intermediate commit.
Implement status variables to aid the DBA in determining the need
and/or effectiveness of the per-engine mylsq.gtid_slave_pos feature:
transactions_multi_engine
Number of transactions that changed data in multiple (transactional)
storage engines.
rpl_transactions_multi_engine
Number of replicated transactions that involved changes in multiple
(transactional) storage engines, before considering the update of the
mysql.gtid_slave_posXXX table.
transactions_gtid_foreign_engine
Number of replicated transactions where the update of the
mysql.gtid_slave_posXXX table had to choose a storage engine that did not
otherwise participate in the transaction.
Intermediate commit.
This commit implements that record_gtid() selects a gtid_slave_posXXX table
with a storage engine already in use by current transaction, if any.
The default table mysql.gtid_slave_pos is used if no match can be found on
storage engine, or for GTID position updates with no specific storage
engine.
Table discovery of mysql.gtid_slave_pos* happens on initial GTID state load
as well as on every START SLAVE. Some effort is made to make this possible
without additional locking. New tables are added using lock-free atomics.
Removing tables requires stopping all slaves first. A warning is given in
the error log when a table is removed but a non-stopped slave still has a
reference to it.
If multiple mysql.gtid_slave_posXXX tables with same storage engine exist,
one is chosen arbitrarily to be used, with a warning in the error log. GTID
data from all tables is still read, but only one among redundant tables with
same storage engine will be updated.
