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An update of the GUC stats_fetch_consistency in a transaction would be able to trigger an assertion when doing cache->snapshot. In this case, when retrieving a pgstat entry after the switch, a new snapshot would be rebuilt, confusing pgstat_build_snapshot() because a snapshot is already cached with an unexpected mode ("cache"). In order to fix this problem, this commit adds a flag to force a snapshot clear each time this GUC is changed. Some tests are added to check, while on it. Some optimizations in avoiding the snapshot clear should be possible depending on what is cached and the current GUC value, I guess, but this solution is simple, and ensures that the state of the cache is updated each time a new pgstat entry is fetched, hence being consistent with the level wanted by the client that has set the GUC. Note that cache->none and snapshot->none would not cause issues, as fetching a pgstat entry would be retrieved from shared memory on the second attempt, however a snapshot would still be cached. Similarly, none->snapshot and none->cache would build a new snapshot on the second fetch attempt. Finally, snapshot->cache would cache a new snapshot on the second attempt. Reported-by: Alexander Lakhin Author: Kyotaro Horiguchi Discussion: https://postgr.es/m/17804-2a118cd046f2d0e5@postgresql.org backpatch-through: 15 |
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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/.