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LockRelationOid and sibling routines supposed that, if our session already holds the lock they were asked to acquire, they could skip calling AcceptInvalidationMessages on the grounds that we must have already read any remote sinval messages issued against the relation being locked. This is normally true, but there's a critical special case where it's not: processing inside AcceptInvalidationMessages might attempt to access system relations, resulting in a recursive call to acquire a relation lock. Hence, if the outer call had acquired that same system catalog lock, we'd fall through, despite the possibility that there's an as-yet-unread sinval message for that system catalog. This could, for example, result in failure to access a system catalog or index that had just been processed by VACUUM FULL. This is the explanation for buildfarm failures we've been seeing intermittently for the past three months. The bug is far older than that, but commits a54e1f158 et al added a new recursion case within AcceptInvalidationMessages that is apparently easier to hit than any previous case. To fix this, we must not skip calling AcceptInvalidationMessages until we have *finished* a call to it since acquiring a relation lock, not merely acquired the lock. (There's already adequate logic inside AcceptInvalidationMessages to deal with being called recursively.) Fortunately, we can implement that at trivial cost, by adding a flag to LOCALLOCK hashtable entries that tracks whether we know we have completed such a call. There is an API hazard added by this patch for external callers of LockAcquire: if anything is testing for LOCKACQUIRE_ALREADY_HELD, it might be fooled by the new return code LOCKACQUIRE_ALREADY_CLEAR into thinking the lock wasn't already held. This should be a fail-soft condition, though, unless something very bizarre is being done in response to the test. Also, I added an additional output argument to LockAcquireExtended, assuming that that probably isn't called by any outside code given the very limited usefulness of its additional functionality. Back-patch to all supported branches. Discussion: https://postgr.es/m/12259.1532117714@sss.pgh.pa.us
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/.