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e0ed20d0b6
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Tom Lane e0ed20d0b6 Prevent access to no-longer-pinned buffer in heapam_tuple_lock(). 
heap_fetch() used to have a "keep_buf" parameter that told it to return
ownership of the buffer pin to the caller after finding that the
requested tuple TID exists but is invisible to the specified snapshot.
This was thoughtlessly removed in commit 5db6df0c0, which broke
heapam_tuple_lock() (formerly EvalPlanQualFetch) because that function
needs to do more accesses to the tuple even if it's invisible.  The net
effect is that we would continue to touch the page for a microsecond or
two after releasing pin on the buffer.  Usually no harm would result;
but if a different session decided to defragment the page concurrently,
we could see garbage data and mistakenly conclude that there's no newer
tuple version to chain up to.  (It's hard to say whether this has
happened in the field.  The bug was actually found thanks to a later
change that allowed valgrind to detect accesses to non-pinned buffers.)

The most reasonable way to fix this is to reintroduce keep_buf,
although I made it behave slightly differently: buffer ownership
is passed back only if there is a valid tuple at the requested TID.
In HEAD, we can just add the parameter back to heap_fetch().
To avoid an API break in the back branches, introduce an additional
function heap_fetch_extended() in those branches.

In HEAD there is an additional, less obvious API change: tuple->t_data
will be set to NULL in all cases where buffer ownership is not returned,
in particular when the tuple exists but fails the time qual (and
!keep_buf).  This is to defend against any other callers attempting to
access non-pinned buffers.  We concluded that making that change in back
branches would be more likely to introduce problems than cure any.

In passing, remove a comment about heap_fetch that was obsoleted by
9a8ee1dc6.

Per bug #17462 from Daniil Anisimov.  Back-patch to v12 where the bug
was introduced.

Discussion: https://postgr.es/m/17462-9c98a0f00df9bd36@postgresql.org
2022-04-13 13:35:02 -04:00
config Suppress warning about stack_base_ptr with late-model GCC. 2022-02-17 22:45:34 -05:00
contrib Fix postgres_fdw to check shippability of sort clauses properly. 2022-03-31 14:29:24 -04:00
doc Doc: tweak textsearch.sgml for SEO purposes. 2022-04-12 18:21:26 -04:00
src Prevent access to no-longer-pinned buffer in heapam_tuple_lock(). 2022-04-13 13:35:02 -04:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.gitattributes Add XSL stylesheet to fix up SVG files 2019-06-19 21:26:42 +02:00
.gitignore Support for optimizing and emitting code in LLVM JIT provider. 2018-03-22 11:05:22 -07:00
aclocal.m4 Probe $PROVE not $PERL while checking for modules needed by TAP tests. 2021-11-22 12:54:52 -05:00
configure configure: check for dlsym instead of dlopen. 2022-03-23 12:43:38 -07:00
configure.in configure: check for dlsym instead of dlopen. 2022-03-23 12:43:38 -07:00
COPYRIGHT Update copyright for 2022 2022-01-07 19:04:56 -05:00
GNUmakefile.in Add new make targets world-bin and install-world-bin 2021-07-01 14:51:54 -04:00
HISTORY Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00
Makefile Don't unset MAKEFLAGS in non-GNU Makefile. 2019-06-25 09:36:21 +12:00
README Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00
README.git Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00

README 

PostgreSQL Database Management System
=====================================

This directory contains the source code distribution of the PostgreSQL
database management system.

PostgreSQL is an advanced object-relational database management system
that supports an extended subset of the SQL standard, including
transactions, foreign keys, subqueries, triggers, user-defined types
and functions.  This distribution also contains C language bindings.

PostgreSQL has many language interfaces, many of which are listed here:

	https://www.postgresql.org/download

See the file INSTALL for instructions on how to build and install
PostgreSQL.  That file also lists supported operating systems and
hardware platforms and contains information regarding any other
software packages that are required to build or run the PostgreSQL
system.  Copyright and license information can be found in the
file COPYRIGHT.  A comprehensive documentation set is included in this
distribution; it can be read as described in the installation
instructions.

The latest version of this software may be obtained at
https://www.postgresql.org/download/.  For more information look at our
web site located at https://www.postgresql.org/.