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

On Sun, Apr 10, 2016 at 7:49 AM, Tom Lane <tgl@sss.pgh.pa.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.  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?

Attached patch is intended to fix this.  It zeroes "hole" in both GenericXLogFinish() and generic_redo().