RE: BUG #15460: Error while creating index or constraint

Yes I did an early adoption, but I more or less regret it because the performance (more parallel queries) that I was hoping for didn't work out for me.
But as a pre: I'm more than happy with finding bugs 😊

-----Original Message-----
From: Peter Geoghegan <pg(at)bowt(dot)ie>
Sent: maandag 29 oktober 2018 16:44
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Paul van der Linden <paul(dot)vanderlinden(at)mapcreator(dot)eu>; PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org>; Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Subject: Re: BUG #15460: Error while creating index or constraint

On Mon, Oct 29, 2018 at 3:25 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> So there are a couple of things to complain about here with respect to
> the error message, regardless of the underlying bug:

I agree with you fully.

> Yeah, that works fine on Windows AFAIK. I also note that ENOENT isn't
> an error code that lseek() can deliver, anyway, since it works on an
> already-open FD. The failure here must be coming from opening the
> file.

Good point.

> I'm a little inclined to suspect that the true cause here is workers
> not correctly computing the name of this temp file, which is what led
> me to complain about the error message. Although a weak spot in this
> theory is that it's not clear why they'd not fail later anyway, unless
> maybe this particular file never got touched by workers before.

There just isn't that much to get right there, though. Another weak spot in that theory is that it seems unlikely that the first complaint we'd hear would happen to be from a Windows user. I think that they're very much in the minority, especially among early adopters.

> > I have a strong suspicion that going back to passing the size
> > through shared memory (i.e. partially reverting 445e31bdc74) would
> > make the problem go away, but I won't do that until I actually
> > understand what's going on.
>
> Sounds like papering over the bug ...

I may have been unclear. It would be papering over the bug if I went ahead and did that now.

The advantage of getting the file size from shared memory is that it doesn't leave it up to code like BufFileOpenShared() to find everything through readdir() iteration, an approach that might not be totally portable. We'll reliably fail if all BufFile segments cannot be accounted for with the size-in-shared-memory approach, which seems more robust. I wouldn't be surprised if that actually was the correct fix in the end.

--
Peter Geoghegan