Re: [Tarantool-patches] [PATCH v14 2/6] qsync: update confirmed lsn on initial promote request

[Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

On Sun, Sep 12, 2021 at 05:44:04PM +0200, Vladislav Shpilevoy wrote:
> Thanks for the patch!
> 
> On 10.09.2021 17:29, Cyrill Gorcunov wrote:
> > When promote request is handled we drop last confirmed
> > lsn to zero because its value make sense for sync queue
> > owner only. Still the case where we become queue owner
> > for the first time is special - we need to fetch the
> > obtained lsn from the request and remember it so we
> > will be able to filter any next malformed requests
> > with wrong lsn numbers (see queue filtering procedure
> > in next patch).
> 
> I don't understand anything. Why isn't it needed always? And
> how exactly will it help to filter stuff?

This problem is revealed when run of ./test-run replication/gh-6034-qsync-limbo-ownership.test.lua
with filteration turned on. The confirmed_lsn make sence in bound
with limbo owner as far as I understand. And in test we have
two nodes "default" and "replica". Initially default gets up, filled
with some data into sync space and then we start up a replica node.
The replica get subscribed and then we call box.promote() on it,
since replica itself has not been writting anything its confirmed_lsn = 0,
which we send back to the "default" inside promote request body. And
it get rejected because "default" has non-zero confirmed_lsn. I've
been talking to Serge a lot about this problem and if I'm not missing
somthing obvious updating this filed on first promote arrival is
mostly correct way to handle the issue. I presume we should get
a meeting and talk again.

Or maybe better via email. Serge could you please write the details here?