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9ee1cf04ab
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Tom Lane 9ee1cf04ab Fix TOAST access failure in RETURNING queries. 
Discussion of commit 3e2f3c2e4 exposed a problem that is of longer
standing: since we don't detoast data while sticking it into a portal's
holdStore for PORTAL_ONE_RETURNING and PORTAL_UTIL_SELECT queries, and we
release the query's snapshot as soon as we're done loading the holdStore,
later readout of the holdStore can do TOAST fetches against data that can
no longer be seen by any of the session's live snapshots.  This means that
a concurrent VACUUM could remove the TOAST data before we can fetch it.
Commit 3e2f3c2e4 exposed the problem by showing that sometimes we had *no*
live snapshots while fetching TOAST data, but we'd be at risk anyway.

I believe this code was all right when written, because our management of a
session's exposed xmin was such that the TOAST references were safe until
end of transaction.  But that's no longer true now that we can advance or
clear our PGXACT.xmin intra-transaction.

To fix, copy the query's snapshot during FillPortalStore() and save it in
the Portal; release it only when the portal is dropped.  This essentially
implements a policy that we must hold a relevant snapshot whenever we
access potentially-toasted data.  We had already come to that conclusion
in other places, cf commits 08e261cbc9 and ec543db77b.

I'd have liked to add a regression test case for this, but I didn't see
a way to make one that's not unreasonably bloated; it seems to require
returning a toasted value to the client, and those will be big.

In passing, improve PortalRunUtility() so that it positively verifies
that its ending PopActiveSnapshot() call will pop the expected snapshot,
removing a rather shaky assumption about which utility commands might
do their own PopActiveSnapshot().  There's no known bug here, but now
that we're actively referencing the snapshot it's almost free to make
this code a bit more bulletproof.

We might want to consider back-patching something like this into older
branches, but it would be prudent to let it prove itself more in HEAD
beforehand.

Discussion: <87vazemeda.fsf@credativ.de>
2016-08-07 17:46:08 -04:00
config Update config.guess and config.sub 2016-05-06 14:02:44 -04:00
contrib Don't propagate a null subtransaction snapshot up to parent transaction. 2016-08-07 13:15:55 -04:00
doc First-draft release notes for 9.5.4. 2016-08-06 22:08:31 -04:00
src Fix TOAST access failure in RETURNING queries. 2016-08-07 17:46:08 -04:00
.dir-locals.el emacs: Set indent-tabs-mode in perl-mode 2015-04-12 23:53:23 -04:00
.gitattributes Fix whitespace and remove obsolete gitattributes entry 2016-03-13 16:03:13 -04:00
.gitignore Add .gitignore entries for AIX-specific intermediate build artifacts. 2015-07-08 20:44:22 -04:00
aclocal.m4 Replace our hacked version of ax_pthread.m4 with latest upstream version. 2015-07-08 20:36:06 +03:00
configure Stamp 9.6beta3. 2016-07-18 16:54:26 -04:00
configure.in Stamp 9.6beta3. 2016-07-18 16:54:26 -04:00
COPYRIGHT Update copyright for 2016 2016-01-02 13:33:40 -05:00
GNUmakefile.in Fix distclean/maintainer-clean targets to remove top-level tmp_install dir. 2015-05-13 18:48:05 -04:00
HISTORY Improve text of stub HISTORY file. 2014-02-12 18:16:17 -05:00
Makefile Allow make check in PL directories 2011-02-15 06:52:12 +02:00
README Don't generate plain-text HISTORY and src/test/regress/README anymore. 2014-02-10 20:48:04 -05:00
README.git Don't generate plain-text HISTORY and src/test/regress/README anymore. 2014-02-10 20:48:04 -05:00

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:

	http://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
http://www.postgresql.org/download/.  For more information look at our
web site located at http://www.postgresql.org/.