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During hot standby, ExpireAllKnownAssignedTransactionIds() and ExpireOldKnownAssignedTransactionIds() functions mark old transactions as no-longer running, but they failed to update xactCompletionCount and latestCompletedXid. AFAICS it would not lead to incorrect query results, because those functions effectively turn in-progress transactions into aborted transactions and an MVCC snapshot considers both as "not visible". But it could surprise GetSnapshotDataReuse() and trigger the "TransactionIdPrecedesOrEquals(TransactionXmin, RecentXmin))" assertion in it, if the apparent xmin in a backend would move backwards. We saw this happen when GetCatalogSnapshot() would reuse an older catalog snapshot, when GetTransactionSnapshot() had already advanced TransactionXmin. The bug goes back all the way to commit623a9ba79bin v14 that introduced the snapshot reuse mechanism, but it started to happen more frequently with commit952365cdedwhich removed a GetTransactionSnapshot() call from backend startup. That made it more likely for ExpireOldKnownAssignedTransactionIds() to be called between GetCatalogSnapshot() and the first GetTransactionSnapshot() in a backend. Andres Freund first spotted this assertion failure on buildfarm member 'skink'. Reproduction and analysis by Tomas Vondra. Backpatch-through: 14 Discussion: https://www.postgresql.org/message-id/oey246mcw43cy4qw2hqjmurbd62lfdpcuxyqiu7botx3typpax%40h7o7mfg5zmdj
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/.