sjaakola 81ef68784e MDEV-38869 sequence conflicts with streaming replication ...

Sequence access conflicts with streaming replication could cause the
server to hang, as shown in MDEV-38869.

A sequence table is written from SEQUENCE::next_value() while
SEQUENCE::mutex is held. For a streaming transaction the row write in
handler::ha_write_row() would then replicate a fragment and block waiting
for certification and commit order, while an applier may be waiting for
the same mutex in SEQUENCE::set_value(). Neither side can proceed, the
node deadlocks and the BF abort of the local transaction can never be
delivered.

This commit avoids the deadlock by skipping the streaming replication
step for sequence table rows. The row is already in the write set and is
replicated with the following fragment, or at commit.

Only that one step is skipped. The skip is passed down as a parameter to
wsrep_after_row() and wsrep_after_row_internal() rather than by not
calling them at all, so the row is still counted against
wsrep_max_ws_rows and wsrep_check_pk() still runs. A transaction using
sequences heavily therefore cannot silently exceed the configured write
set row limit.

The commit has also a new mtr test for three sequence/SR conflict
scenarios: galera.galera_sequences_bf_kill_sr

 - a streaming transaction and an applier competing for SEQUENCE::mutex,
   where both are expected to commit

 - the same, but with the applier also BF aborting the local transaction
   over a gap lock. A streaming transaction cannot be replayed, so it is
   rolled back and the client gets ER_LOCK_DEADLOCK. The applier is held
   at the abort_trx_end sync point until the abort has been issued, so
   that the local transaction cannot finish its fragment first

 - twelve row inserts on both nodes with wsrep_trx_fragment_unit=rows, so
   that each node reserves several sequence cache ranges and the sequence
   table writes land inside fragments carrying several rows. The values
   the two nodes hand out must not overlap

2026-08-27 15:24:51 +03:00

269 KiB

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