On Wed, Nov 16, 2022 at 11:56 PM Andres Freund <and...@anarazel.de> wrote: > > On 2022-11-16 14:22:01 +0530, Amit Kapila wrote: > > On Wed, Nov 16, 2022 at 7:30 AM Andres Freund <and...@anarazel.de> wrote: > > > > > > On 2022-11-15 16:20:00 +0530, Amit Kapila wrote: > > > > On Tue, Nov 15, 2022 at 8:08 AM Andres Freund <and...@anarazel.de> > > > > wrote: > > > > > nor do we enforce in an obvious place that we > > > > > don't already hold a snapshot. > > > > > > > > > > > > > We have a check for (FirstXactSnapshot == NULL) in > > > > RestoreTransactionSnapshot->SetTransactionSnapshot. Won't that be > > > > sufficient? > > > > > > I don't think that'd e.g. catch a catalog snapshot being held, yet that'd > > > still be bad. And I think checking in SetTransactionSnapshot() is too > > > late, > > > we've already overwritten MyProc->xmin by that point. > > > > > > > So, shall we add the below Asserts in SnapBuildInitialSnapshot() after > > we have the Assert for Assert(!FirstSnapshotSet)? > > Assert(FirstXactSnapshot == NULL); > > Assert(!HistoricSnapshotActive()); > > I don't think that'd catch a catalog snapshot. But perhaps the better answer > for the catalog snapshot is to just invalidate it explicitly. The user doesn't > have control over the catalog snapshot being taken, and it's not too hard to > imagine the walsender code triggering one somewhere. > > So maybe we should add something like: > > InvalidateCatalogSnapshot(); /* about to overwrite MyProc->xmin */ >
The comment "/* about to overwrite MyProc->xmin */" is unclear to me. We already have a check (/* so we don't overwrite the existing value */ if (TransactionIdIsValid(MyProc->xmin))) in SnapBuildInitialSnapshot() which ensures that we don't overwrite MyProc->xmin, so the above comment seems contradictory to me. -- With Regards, Amit Kapila.
