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Since its introduction, pg_get_expr() has intended to silently return NULL if called with an invalid relation OID, as can happen when scanning the catalogs concurrently with relation drops. However, there is a race condition: we check validity of the OID at the start, but it could get dropped just afterward, leading to failures. This is the cause of some intermittent instability we're seeing in a proposed new test case, and presumably it's a hazard in the field as well. We can fix this by AccessShareLock-ing the target relation for the duration of pg_get_expr(). Since we don't require any permissions on the target relation, this is semantically a bit undesirable. But it turns out that the set_relation_column_names() subroutine already takes a transient AccessShareLock on that relation, and has done since commit 2ffa740be in 2012. Given the lack of complaints about that, it seems like there should be no harm in holding the lock a bit longer. Back-patch to all supported branches. Discussion: https://postgr.es/m/31ddcc01-a71b-4e8c-9948-01d1c47293ca@eisentraut.org
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. Copyright and license information can be found in the file COPYRIGHT. General documentation about this version of PostgreSQL can be found at: https://www.postgresql.org/docs/devel/ In particular, information about building PostgreSQL from the source code can be found at: https://www.postgresql.org/docs/devel/installation.html The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.