MDEV-21469: Implement crash-safe logging of the user XA #2364
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of the user XA participants list. This patch makes sure that at handling of the user XA the binlog hton heads the list of the transaction branches which was not the case for the multi-engine XA. Also a recovered XA's completion (XA COMMIT or ROLLBACK through a "foreign" connection) is made to invoke first binlog hton's "complete"_by_xid before the general hton loop (and naturally skip the binlog hton action in the loop). The work is a prerequisite for MDEV-21469.
Make XA PREPARE, XA COMMIT and XA ROLLBACK statements crash-safe when --log-bin is specified. At execution of XA PREPARE, XA COMMIT and XA ROLLBACK their replication events are made written into the binary log prior to execution of the commands in storage engines. In case the server crashes, after writing to binary log but before all involved engines have processed the command, the following recovery will execute the command's replication events to equalize the states of involved engines with that of binlog. That applies to all XA PREPARE *group* and XA COMMIT or ROLLBACK. On the implementation level the recovery time binary log parsing is augmented to pay attention to the user XA xids to identify the XA transactions' state:s in binary log, and eventually match them against their states in engines, see MYSQL_BIN_LOG::recover_explicit_xa_prepare(). In discrepancy cases the outdated state in the engines is corrected with resubmitting the transaction prepare group of events, or completion ones. The multi-engine partly prepared XA PREPARE case the XA is rolled back first. The fact of multiple-engine involved is registered into Gtid_log_event::flags2 as one bit. The boolean value is sufficient and precise to deal with two engines in XA transaction. With more than 2 recoverable engines the flag method is still correct though may be pessimistic, as it treats all recoverable engines as XA participants. So when the number of such Engines exceeds the number of prepared engines of the XA that XA is treated as partially completed, with all that ensued. As an optimization no new bit is allocated in flags2, instead a pre-existing ones (of MDEV-742) are reused, observing that A. XA "COMPLETE" does not require multi-engine hint for its recovery and that B. the MDEV-742 XA-completion bit is not anyhow used by XA-PREPARE and its GTID log event. Notice the multi-engine flagging is superceded by MDEV-21117 extension in Gtid log event so this part should be taken from there.
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Created a PR from bb-10.5-MDEV_21469 to make it easier to review. The first two commits were slightly adjusted to rebase on top of the current 10.5 branch.