[Tarantool-patches] [PATCH 3/7] txn_limbo: fix promote term filtering

[Vladislav Shpilevoy]

Hi! Thanks for the fixes!

> 15.06.2021 23:57, Vladislav Shpilevoy пишет:
>> Thanks for the patch!
>>
>> On 10.06.2021 15:32, Serge Petrenko via Tarantool-patches wrote:
>>> txn_limbo_process() used to filter out promote requests whose term was
>>> equal to the greatest term seen. This wasn't correct for PROMOTE entries
>>> with term 1.
>>>
>>> Such entries appear after box.ctl.promote() is issued on an instance
>>> with disabled elections. Every PROMOTE entry from such an instance has
>>> term 1, but should still be applied. Fix this in the patch.
>> Didn't we agree that PROMOTE should bump the term always? I see no purpose
>> for the PROMOTE which never bumps the term except for the tests, but for
>> such occasion it would be better to have internal.make_leader() or something
>> which bypasses everything. Although the best option would to bump the term
>> always.
>>
>> I see you did something in the last commit about that, but the part
>> about `limbo->promote_greatest_term > 1` still remains. It looks very
>> illegal to point out some terms as "special".
> 
> What about old instances?
> They may issue multiple promotes for the same term when elections are off.

What if they issue multiple promotes for the same term > 1? They could have
elections enabled, then the term was bumped a few times, then elections were
turned off again. Next promotes will use the current term which is > 1,
won't they?

> Previous behaviour (wrong, because we forgot to filter out too old promotes at all)
> made the instance apply all such promotes.
> 
> I thought we should allow multiple promotes, at least for the default term (1), which
> happen only when elections are turned off and have never been turned on.
> 
> Otherwise, having a new instance together with the old one in the same cluster,
> you'll have one of them apply all the promotes for one term, and have one dataset,
> and the other ignore all but the first promote, and have possibly different data.> I did fix the issue in the last commit. All the new instances will issue promotes with
> increasing term each time, and the term will be >= 2.
> 
> So all the promotes with term == 1 surely will be from the old instances.