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MDEV-8302: Duplicate key with parallel replication

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This bug is essentially another variant of MDEV-7458.

If a transaction conflict caused a deadlock kill of T2 in record_gtid()
during commit, the code would do a rollback _before_ running
rgi->unmark_start_commit(). This creates a race where following transactions
could start too early (before T2 has completed its transaction retry). This
in turn could lead to replication failure, if there was a conflict that
caused eg. duplicate key error or similar.

The fix is to remove these rollbacks (in Query_log_event::do_apply_event()
and Xid_log_event::do_apply_event(). They seem out-of-place; code in
log_event.cc generally does not roll back on error, this is handled higher
up.

In addition, because of the extreme difficulty of reproducing bugs like
MDEV-7458 and MDEV-8302, this patch adds some extra precations to try to
detect (in debug builds) or prevent (in release builds) similar bugs.
ha_rollback_trans() will now call unmark_start_commit() if needed (and
assert in debug build when a caller does rollback without unmark first).

We also add an extra check for thd->killed() so that we avoid doing
mark_start_commit() if we already have a pending deadlock kill.

And we add a missing unmark_start_commit() call in the error case, found by
the above assertion.
This commit is contained in:
Kristian Nielsen
2015-08-04 11:20:03 +02:00
parent a6087e7dc1
commit 9b9c5e890c
5 changed files with 147 additions and 6 deletions
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18
sql/handler.cc
View File

@ -1582,6 +1582,24 @@ int ha_rollback_trans(THD *thd, bool all)
DBUG_ASSERT(thd->transaction.stmt.ha_list == NULL ||
trans == &thd->transaction.stmt);
if (is_real_trans)
{
/*
In parallel replication, if we need to rollback during commit, we must
first inform following transactions that we are going to abort our commit
attempt. Otherwise those following transactions can run too early, and
possibly cause replication to fail. See comments in retry_event_group().
There were several bugs with this in the past that were very hard to
track down (MDEV-7458, MDEV-8302). So we add here an assertion for
rollback without signalling following transactions. And in release
builds, we explicitly do the signalling before rolling back.
*/
DBUG_ASSERT(!(thd->rgi_slave && thd->rgi_slave->did_mark_start_commit));
if (thd->rgi_slave && thd->rgi_slave->did_mark_start_commit)
thd->rgi_slave->unmark_start_commit();
}
if (thd->in_sub_stmt)
{
DBUG_ASSERT(0);