Re: Possibly too stringent Assert() in b-tree code

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Sep 19, 2016 at 7:07 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> I think you have a valid point. It seems we don't need to write WAL
>> for reuse page (aka don't call _bt_log_reuse_page()), if the page is
>> new, as the only purpose of that log is to handle conflict based on
>> transaction id stored in special area which will be anyway zero.

> +1.

This is clearly an oversight in Simon's patch fafa374f2, which introduced
this code without any consideration for the possibility that the page
doesn't have a valid special area. We could prevent the crash by
doing nothing if PageIsNew, a la

if (_bt_page_recyclable(page))
{
/*
* If we are generating WAL for Hot Standby then create a
* WAL record that will allow us to conflict with queries
* running on standby.
*/
- if (XLogStandbyInfoActive() && RelationNeedsWAL(rel))
+ if (XLogStandbyInfoActive() && RelationNeedsWAL(rel) &&
+ !PageIsNew(page))
{
BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);

_bt_log_reuse_page(rel, blkno, opaque->btpo.xact);
}

/* Okay to use page. Re-initialize and return it */

but I'm not very clear on whether this is a safe fix, mainly because
I don't understand what the reuse WAL record really accomplishes.
Maybe we need to instead generate a reuse record with some special
transaction ID indicating worst-case assumptions?

regards, tom lane