[PATCH] Fix minor race in commit_ts SLRU truncation vs lookups

Hi all

There's a minor race between commit_ts SLRU truncation and concurrent
commit_ts lookups, where a lookup can check the lower valid bound xid
without knowing it's already been truncated away. This would result in
a SLRU lookup error.

It's pretty low-harm since it's hard to trigger and the only problem
is an error being emitted when we should otherwise return null/zero.
Most notably you have to pass an xid that used to be within the
datrozenxid but due to a concurrent vacuum has just moved outside it.
This can't happen if you're passing the xmin of a tuple that still
exists so it only matters for callers passing arbitrary XIDs in via
pg_xact_commit_timestamp(...).

The race window is bigger on standby because there we don't find out
about the advance of the lower commit ts bound until the next
checkpoint. But you still have to be looking at very old xids that
don't exist on the heap anymore.

We might as well fix it in HEAD, but it's totally pointless to
back-patch, and the only part of the race that can be realistically
hit is on standby, where we can't backpatch a fix w/o changing the
xlog format. Nope. We could narrow the scope by limiting commit_ts
slru truncation to just before a checkpoint, but given how hard this
is to hit... I don't care.

(This came up as part of the investigation I've been doing on the
txid_status thread, where Robert pointed out a similar problem that
can arise where txid_status races with clog truncation. I noticed the
issue with standby while looking into that.)

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services