Re: Consider pipeline implicit transaction as a transaction block

On Thu, Oct 31, 2024 at 03:32:39PM +0900, Michael Paquier wrote:
> @Tom added in CC: Is there a specific reason why CheckTransactionBlock()
> did not include a check based on XACT_FLAGS_PIPELINING when it got
> introduced in 20432f873140, while IsInTransactionBlock() considers it?

This also makes LOCK able to work flawlessly within pipelines as far
as I can see, as an error would be reported on HEAD. That's nice.

The behavior of the first command is interesting, still this saves
from the checks after any follow-up commands, and this is intended
from what I can see in postgres.c, because the xact flag is only set
once the first command completes.

Now, here is a fancy case: SAVEPOINT and its two brothers. An error
would be reported on HEAD if attempting a SAVEPOINT, complaining that
we are not in a transaction block. The patch causes a different, more
confusing, failure:
FATAL: DefineSavepoint: unexpected state STARTED

This is a bit user-unfriendly. I am not sure to see the point of
supporting savepoints in this context, so perhaps we should just issue
a cleaner error when we are under a XACT_FLAGS_PIPELINING? Reporting
that we are not in a transaction block, while, well, we are in an
implicit transaction block because of the use of pipelines is
confusing. The new error is actually worse.
--
Michael