[Tarantool-patches] [PATCH 1/1] applier: process synchro rows after WAL write

[Cyrill Gorcunov]

On Thu, Apr 08, 2021 at 01:32:06PM +0300, Serge Petrenko wrote:
> > Serge, you mean the scenario when some instances in replicaset
> > have the patch applied and some are not?
> 
> No. Let's suppose everyone has this patch applied.
> Now look at one particular instance. It may happen that while one of
> its appliers is writing this synchro row (either CONFIRM or ROLLBACK,
> doesn't matter), some other applier may still apply requests coming
> from other replicaset members.

Ah, indeed. I must confess I forgot that there are a number of applier
fibers and blocking write does block only the fiber which has initiated
the write procedure, not other fibers.

> I was wondering what would happen if someone else sent this instance
> another synchro row. Looks like nothing bad but I just wanted to
> double-check.
> 
> And looks like there's a bug, which I'm speaking of below. It's about
> someone sending us normal rows (either synchronous transactions or
> asynchronous, but not CONFIRM/ROLLBACK entries) while we're waiting for
> syncro row's write to end.
> 
> Say, limbo was owned by instance 1, and instance 2 has written CONFIRM
> for everything there was. While we wait for 2's CONFIRM to be written to
> WAL, we may receive some rows from instance 3, who has already applied 2's
> CONFIRM. Since we haven't written the CONFIRM yet, we haven't applied it,
> and the limbo on our instance still isn't empty. All the rows coming from
> 3 will get rejected and replication between 3 and us will be broken.

Hmm, looks so, need to think about.