Re: Strange coding in _mdfd_openseg()

Hi,

On 2019-04-04 09:24:49 +1300, Thomas Munro wrote:
> On Wed, Apr 3, 2019 at 5:34 PM Kyotaro HORIGUCHI
> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> > I may be missing something, but it seems possible that
> > _mdfd_getseg calls it with segno > opensegs.
> >
> > | for (nextsegno = reln->md_num_open_segs[forknum];
>
> Here nextsegno starts out equal to opensegs.
>
> > | nextsegno <= targetseg; nextsegno++)
>
> Here we add one to nextsegno...
>
> > | ...
> > | v = _mdfd_openseg(reln, forknum, nextsegno, flags);
>
> ... after adding one to opensegs. So they're always equal. This fits
> a general programming pattern when appending to an array, the next
> element's index is the same as the number of elements. But I claim
> the coding is weird, because _mdfd_openseg's *looks* like it can
> handle opening segments in any order, except that the author
> accidentally wrote "<=" instead of ">=". In fact it can't open them
> in any order, because we don't support "holes" in the array. So I
> think it should really be "==", and it should be an assertion, not a
> condition.

Yea, I totally agree it's weird. I'm not sure if I'd go for an assertion
of equality, or just invert the >= (which I agree I probably just
screwed up and didn't notice when reviewing the patch because it looked
close enough to correct and it didn't have a measurable effect).

Greetings,

Andres Freund