Re: Missing extension locks in the nbtree code

On 2015-07-06 23:21:12 +0200, Andres Freund wrote:
> There've recently been more and more reports of "unexpected data beyond
> EOF in block %u of relation %s" for me to think that it's likely to be
> caused by a kernel bug. It's now been reproduced at least on somewhat
> recent linux and freebsd versions.
>
> So I started looking around for causes. Not for the first time.
>
> One, probably harmless thing is that _bt_getroot() creates the initial
> root page without an extension lock. That's not pretty, but it should
> happen on the first write and be safe due to the content lock on the
> metapage. ISTM we should still not do that, but it's probably not the
> explanation.

Uh, this was a mixup, I didn't want to send this email yet. The below
obviously is from a different thread.

> The fix is just to change
> if (fd == -1 || XLByteInSeg(change->lsn, curOpenSegNo))
> into
> if (fd == -1 || !XLByteInSeg(change->lsn, curOpenSegNo))
>
> the bug doesn't have any correctness implications afaics, just
> performance. We read all the spilled files till the end, so even change
> spilled to the wrong segment gets picked up.