Re: prevent immature WAL streaming

On Wed, Nov 24, 2021 at 2:10 AM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
>
> On 2021-Nov-23, Tom Lane wrote:
>
> > We're *still* not out of the woods with 026_overwrite_contrecord.pl,
> > as we are continuing to see occasional "mismatching overwritten LSN"
> > failures, further down in the test where it tries to start up the
> > standby:
>
> Augh.
>
> > Looking at adjacent successful runs, it seems that the exact point
> > where the "missing contrecord" starts varies substantially, even after
> > our previous fix to disable autovacuum in this test. How could that be?
>
> Well, there is intentionally some variability. Maybe not as much as one
> would wish, but I expect that that should explain why that point is not
> always the same.
>
> > It's probably for the best though, because I think this is exposing
> > an actual bug that we would not have seen if the start point were
> > completely consistent. I have not dug into the code, but it looks to
> > me like if the "consistent recovery state" is reached exactly at a
> > page boundary (0/1FFE000 in all these cases), then the standby expects
> > that to be what the OVERWRITE_CONTRECORD record will point at. But
> > actually it points to the first WAL record on that page, resulting
> > in a bogus failure.
>
> So what is happening is that we set state->overwrittenRecPtr to the LSN
> of page start, ignoring the page header. Is that the LSN of the first
> record in a page? I'll see if I can reproduce the problem.
>

In XLogReadRecord(), both the variables being compared have
inconsistency in the assignment -- one gets assigned from
state->currRecPtr where other is from RecPtr.

.....
state->overwrittenRecPtr = state->currRecPtr;
.....
state->abortedRecPtr = RecPtr;
.....

Before the place where assembled flag sets, there is a bunch of code
that adjusts RecPtr. I think instead of RecPtr, the latter assignment
should use state->currRecPtr as well.

Regards,
Amul