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

On Mon, Dec 14, 2020 at 8:27 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> 2.
> + /*
> + * Flags are determined from the state of the transaction. We know we
> + * always get PREPARE first and then [COMMIT|ROLLBACK] PREPARED, so if
> + * it's already marked as committed then it has to be COMMIT PREPARED (and
> + * likewise for abort / ROLLBACK PREPARED).
> + */
> + if (rbtxn_commit_prepared(txn))
> + flags = LOGICALREP_IS_COMMIT_PREPARED;
> + else if (rbtxn_rollback_prepared(txn))
> + flags = LOGICALREP_IS_ROLLBACK_PREPARED;
> + else
> + flags = LOGICALREP_IS_PREPARE;
>
> I don't like clubbing three different operations under one message
> LOGICAL_REP_MSG_PREPARE. It looks awkward to use new flags
> RBTXN_COMMIT_PREPARED and RBTXN_ROLLBACK_PREPARED in ReordeBuffer so
> that we can recognize these operations in corresponding callbacks. I
> think setting any flag in ReorderBuffer should not dictate the
> behavior in callbacks. Then also there are few things that are not
> common to those APIs like the patch has an Assert to say that the txn
> is marked with prepare flag for all three operations which I think is
> not true for Rollback Prepared after the restart. We don't ensure to
> set the Prepare flag if the Rollback Prepare happens after the
> restart. Then, we have to introduce separate flags to distinguish
> prepare/commit prepared/rollback prepared to distinguish multiple
> operations sent as protocol messages. Also, all these operations are
> mutually exclusive so it will be better to send separate messages for
> each of these and I have changed it accordingly in the attached patch.
>

While looking at the two-phase protocol messages (with a view to
documenting them) I noticed that the messages for
LOGICAL_REP_MSG_PREPARE, LOGICAL_REP_MSG_COMMIT_PREPARED,
LOGICAL_REP_MSG_ROLLBACK_PREPARED are all sending and receiving flag
bytes which *always* has a value 0.

----------
e.g.
uint8 flags = 0;
pq_sendbyte(out, flags);

and
/* read flags */
uint8 flags = pq_getmsgbyte(in);
if (flags != 0)
elog(ERROR, "unrecognized flags %u in commit prepare message", flags);
----------

I think this patch version v31 is where the flags became redundant.

Is there some reason why these unused flags still remain in the protocol code?

Do you have any objection to me removing them?
Otherwise, it might seem strange to document a flag that has no function.

------
KInd Regards,
Peter Smith.
Fujitsu Australia