Re: [HACKERS] logical decoding of two-phase transactions

Hi Tomas,

>>
>
> Uh? Simply rechecking if MyProc->decodeGroupLeader is NULL obviously
> does not fix the race condition - it might get NULL right after the
> check. So we need to either lookup the PROC again (and then get the
> associated lwlock), or hold some other type of lock.
>

I realized my approach was short-sighted while coding it up. So now we
lookup the leader pgproc, recheck if the XID is the same that we are
interested in and go ahead.

>
>>>
>>> 3) I don't quite understand why BecomeDecodeGroupMember does the
>>> cross-check using PID. In which case would it help?
>>>
>>
>> When I wrote this support, I had written it with the intention of
>> supporting both 2PC (in which case pid is 0) and in-progress regular
>> transactions. That's why the presence of PID in these functions. The
>> current use case is just for 2PC, so we could remove it.
>>
>
> Sure, but why do we need to cross-check the PID at all? I may be missing
> something here, but I don't see what does this protect against?
>

The fact that PID is 0 in case of prepared transactions was making me
nervous. So, I had added the assert that pid should only be 0 when
it's a prepared transaction and not otherwise. Anyways, since we are
dealing with only 2PC, I have removed the PID argument now. Also
removed is_prepared argument for the same reason.

>>
>> Ok, I am looking at your provided patch and incorporating relevant
>> changes from it. WIll submit a patch set soon.
>>
>
> OK.
>
PFA, latest patch set.

Regards,
Nikhils
--
Nikhil Sontakke http://www.2ndQuadrant.com/
PostgreSQL/Postgres-XL Development, 24x7 Support, Training & Services