Make _bt_killitems drop pins it acquired itself.HEADmaster

Teach nbtree's _bt_killitems to leave the so->currPos page that it sets
LP_DEAD items on in whatever state it was in when _bt_killitems was
called.  In particular, make sure that so->dropPin scans don't acquire a
pin whose reference is saved in so->currPos.buf.

Allowing _bt_killitems to change so->currPos.buf like this is wrong.
The immediate consequence of allowing it is that code in _bt_steppage
(that copies so->currPos into so->markPos) will behave as if the scan is
a !so->dropPin scan.  so->markPos will therefore retain the buffer pin
indefinitely, even though _bt_killitems only needs to acquire a pin
(along with a lock) for long enough to mark known-dead items LP_DEAD.

This issue came to light following a report of a failure of an assertion
from recent commit e6eed40e.  The test case in question involves the use
of mark and restore.  An initial call to _bt_killitems takes place that
leaves so->currPos.buf in a state that is inconsistent with the scan
being so->dropPin.  A subsequent call to _bt_killitems for the same
position (following so->currPos being saved in so->markPos, and then
restored as so->currPos) resulted in the failure of an assertion that
tests that so->currPos.buf is InvalidBuffer when the scan is so->dropPin
(non-assert builds got a "resource was not closed" WARNING instead).

The same problem exists on earlier releases, though the issue is far
more subtle there.  Recent commit e6eed40e introduced the so->dropPin
field as a partial replacement for testing so->currPos.buf directly.
Earlier releases won't get an assertion failure (or buffer pin leak),
but they will allow the second _bt_killitems call from the test case to
behave as if a buffer pin was consistently held since the original call
to _bt_readpage.  This is wrong; there will have been an initial window
during which no pin was held on the so->currPos page, and yet the second
_bt_killitems call will neglect to check if so->currPos.lsn continues to
match the page's now-current LSN.

As a result of all this, it's just about possible that _bt_killitems
will set the wrong items LP_DEAD (on release branches).  This could only
happen with merge joins (the sole user of nbtree mark/restore support),
when a concurrently inserted index tuple used a recently-recycled TID
(and only when the new tuple was inserted onto the same page as a
distinct concurrently-removed tuple with the same TID).  This is exactly
the scenario that _bt_killitems' check of the page's now-current LSN
against the LSN stashed in currPos was supposed to prevent.

A follow-up commit will make nbtree completely stop conditioning whether
or not a position's pin needs to be dropped on whether the 'buf' field
is set.  All call sites that might need to drop a still-held pin will be
taught to rely on the scan-level so->dropPin field recently introduced
by commit e6eed40e.  That will make bugs of the same general nature as
this one impossible (or make them much easier to detect, at least).

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/545be1e5-3786-439a-9257-a90d30f8b849@gmail.com
Backpatch-through: 13

0 files changed, 0 insertions, 0 deletions