Re: Some other things about contrib/bloom and generic_xlog.c

On Sun, Apr 10, 2016 at 1:49 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> 1. It doesn't seem like generic_xlog.c has thought very carefully about
> the semantics of the "hole" between pd_lower and pd_upper. The mainline
> XLOG code goes to some lengths to ensure that the hole stays all-zeroes;
> for example RestoreBlockImage() explicitly zeroes the hole when restoring
> from a full-page image that has a hole.

Yes.

> But generic_xlog.c's redo routine
> does not do anything comparable, nor does GenericXLogFinish make any
> effort to ensure that the "hole" is all-zeroes after normal application of
> a generic update. The reason this is of interest is that it means the
> contents of the "hole" could diverge between master and slave, or differ
> between the original state of a database and what it is after a crash and
> recovery. That would at least complicate forensic comparisons of pages,
> and I think it might also break checksumming. We thought that this was
> important enough to take the trouble of explicitly zeroing holes during
> mainline XLOG replay. Shouldn't generic_xlog.c take the same trouble?

One look at checksum_impl.h shows up that the page hole is taken into
account in the checksum calculation, so we definitely has better zero
the page. And looking at generic_xlog.c, this code either logs in a
full page, or a delta.

Actually, as I look at this code for the first time, I find that
GenericXLogFinish() is a very confusing interface. It makes no
distinction between a page and the meta data associated to this page,
this is controlled by a flag isNew and buffer data is saved as some
delta. Actually, fullmage is not exactly true, this may refer to a
page that has a hole. It seems to me that we should not have one but
two routines: GenericXLogRegisterBuffer and
GenericXLogRegisterBufData, similar to what the normal XLOG routines
offer.
--
Michael