Обсуждение: Subtraction carry bug in xlog.c in 7.3 and 7.4

The attached patches against 7.3 and 7.4 fix a subtraction carry bug in
xlog.c.

--
John Nield
jrnield@usol.com

"J.R. Nield" <jrnield@usol.com> writes:
> The attached patches against 7.3 and 7.4 fix a subtraction carry bug in
> xlog.c.

This is simply a waste of cycles, because xrecoff can never be zero
(if it were, it'd be pointing at a page header, which is not a valid
record location).  If you look around in the code, you'll note that
xrecoff==0 is actually used as an invalid-value flag in a couple of
contexts.

Can you point to anyplace where the behavior would change?  If so
I think there's a deeper bug to fix.

            regards, tom lane

Should we add an Assert() to make it clear the current code is OK?

---------------------------------------------------------------------------

Tom Lane wrote:
> "J.R. Nield" <jrnield@usol.com> writes:
> > The attached patches against 7.3 and 7.4 fix a subtraction carry bug in
> > xlog.c.
>
> This is simply a waste of cycles, because xrecoff can never be zero
> (if it were, it'd be pointing at a page header, which is not a valid
> record location).  If you look around in the code, you'll note that
> xrecoff==0 is actually used as an invalid-value flag in a couple of
> contexts.
>
> Can you point to anyplace where the behavior would change?  If so
> I think there's a deeper bug to fix.
>
>             regards, tom lane
>
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Should we add an Assert() to make it clear the current code is OK?

A comment maybe, but not an assert.

            regards, tom lane