Re: [Tarantool-patches] [PATCH v5 3/5] box/applier: fix nil dereference in applier rollback

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Wed, Feb 05, 2020 at 12:55:24PM +0300, Konstantin Osipov wrote:
> * Cyrill Gorcunov <gorcunov@gmail.com> [20/02/05 11:31]:
> > > I don't understand this comment. How can it be lost exactly?
> > 
> > Hmmm, I think you're right. Actually unweaving the all possible
> > call traces by hands (which I had to do) is quite exhausting task
> > so I might be wrong here.
> 
> It was added by parallel applier patch, so the most likely cause
> of the bug is that this way of cancelling parallel appliers on
> conflict was not tested well enough. Previously we only had a
> single applier per peer so did not need to coordinate.

I see, thanks!

> > > Let's begin by explaining why we need to cancel the reader fiber here.
> > 
> > This fiber_cancel has been already here, I only added diag_set(FiberIsCancelled)
> > to throw an exception thus the caller would zap this applier fiber.
> > Actually I think we could retry instead of reaping off the fiber
> > completely but it requies more deep understanding of how applier
> > works. So I left it in comment.
> 
> No, we can't and shouldn't retry. Retry handling is done elsewhere already -
> see replication_skip_conflict.

Sure, will do, thanks for pointing.

> If replication is stopped by an apply error, it's most likely a
> transaction conflict, indicating that active-active setup is
> broken, so it has to be resolved by a DBA (which can set
> replication_skip_conflict). This is why it's critical to preserve
> the original error.
> > 
> > Not exactly, if I understand the initial logic of this applier
> > try/cath branch  we need to setup replicaset.applier.diag and
> > then on FiberIsCancelled we should move it from replicaset.applier.diag
> > back to current fiber->diag.
> 
> Please dig into what is "current" here. Which fiber is current if
> there are many fibers handling a single peer?

Will do, tahnks!