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Re: [Commits] c285dbe: MDEV-5535: Cannot reopen temporary table


Sergei Golubchik <serg@xxxxxxxxxxx> writes:

> On May 26, Nirbhay Choubey wrote:

>> > > +  if (wait_for_prior_commit())

Please call this method tmptable_wait_for_prior_commit() or something like
that. To avoid confusion with THD::wait_for_prior_commit().

>> > you're doing it in almost every method. And when one method calls another,
>> > you're doing it twice. Or thrice. Isn't is too much? (I don't know)
>> I added these additional waits to fix some failing replication tests. But,
>> whit new TMP_TABLE_SHARE approach, this could have changed.
>> I have now reverted them to the original state and will run rpl test to
>> assure if we do not need additional waits.

That won't mean much. Temporary table handling in statement-based
replication is very nasty code, and we surely do not have full test coverage
of possible issues. And bugs only turn up as rare race conditions that are
hard to reproduce.

So you will have to make sure you understand in detail exactly where
wait_for_prior_commit() (and possibly other needed code) is needed to ensure
statement-based parallel replication will work in all cases (or at least
work as well as the non-parallel case).

The issue here is that temporary tables have no locking facilities. And the
same temporary table can be used by multiple different transactions, which
in parallel replication can be run in different threads. There is code that
keeps track of a shared list of temporary tables, and shuffles it around
between threads as needed (this is also needed for non-parallel replication,
since temporary tables might need to be kept across SQL thread destruction
and re-creation). See Relay_log_info::save_temporary_tables (as you probably
already saw). Maybe Monty knows something about this (ISTR that he wrote
that code originally), otherwise careful study of the code is probably the
only way.

Since there is no locking of temporary tables (at least before your patch?),
it would be possible for two worker threads to simultaneously try to use the
same temptable, which fails, obviously. This is handled rather crudely in
the current code by simply serialising all transactions using temporary
tables with all prior transactions. This is what wait_for_prior_commit()
does - it waits for all transactions to commit that come before in the
replication stream (within that replication domain).

Hope this helps,

 - Kristian.

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