postgres

History

Peter Geoghegan 40aa5ddea1 Make _bt_killitems drop pins it acquired itself. 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		1 year ago
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backend	Make _bt_killitems drop pins it acquired itself.	1 year ago
bin	pg_restore: fix incompatibility with old directory-format dumps.	1 year ago
common	With GB18030, prevent SIGSEGV from reading past end of allocation.	1 year ago
fe_utils	In fmtIdEnc(), handle failure of enlargePQExpBuffer().	1 year ago
include	Make our usage of memset_s() conform strictly to the C11 standard.	1 year ago
interfaces	Don't reduce output request size on non-Unix-socket connections.	1 year ago
makefiles	…
pl	Run pgindent on the previous commit.	1 year ago
port	Make our usage of memset_s() conform strictly to the C11 standard.	1 year ago
template	…
test	Fixed signed/unsigned mismatch in test_dsm_registry.	1 year ago
timezone	Update time zone data files to tzdata release 2025b.	1 year ago
tools	meson: Flush stdout in testwrap	1 year ago
tutorial	…
.gitignore	…
DEVELOPERS	…
Makefile	…
Makefile.global.in	…
Makefile.shlib	…
meson.build	…
nls-global.mk	…