Re: TransactionIdIsInProgress() cache

Alvaro Herrera wrote:
> I didn't check whether your transformation is correct, but if so then it
> can be changed like this and save the extra XidDidCommit call:
>
> xvac_committed = TransactionIdDidCommit(xvac);
> if (xvac_committed)
> {
> /* committed */
> }
> else if (!TransactionIdIsInProgress(xvac))
> {
> if (xvac_committed)
> {
> /* committed */
> }
> else
> {
> /* aborted */
> }
> }
> else
> {
> /* in-progress */
> }

Nope, that's not good. Per comments in tqual.c, you have to call
TransactionIdIsInProgress *before* TransactionIdDidCommit, to avoid this
race condition:

1. Xact A inserts a record
2. Xact B does TransactionIdDidCommit, which retuns false because it's
still in progress
3. Xact B commits
4. Xact B does TransactionIdIsInProgress to see if A is still in
progress. It returns false. We conclude that A aborted, while it
actually committed.

My proposal was basically to add an extra TransactionIdDidCommit call
before the TransactionIdIsInProgress call, in the hope that it returns
true and we can skip the rest.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com