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If a multixid with zero offset is left behind after a crash, and that multixid later becomes the oldest multixid, truncation might try to look up its offset and read the zero value. In the worst case, we might incorrectly use the zero offset to truncate valid SLRU segments that are still needed. I'm not sure if that can happen in practice, or if there are some other lower-level safeguards or incidental reasons that prevent the caller from passing an unwritten multixid as the oldest multi. But better safe than sorry, so let's add an explicit check for it. In stable branches, we should perhaps do the same check for 'oldestOffset', i.e. the offset of the old oldest multixid (in master, 'oldestOffset' is gone). But if the old oldest multixid has an invalid offset, the damage has been done already, and we would never advance past that point. It's not clear what we should do in that case. The check that this commit adds will prevent such an multixid with invalid offset from becoming the oldest multixid in the first place, which seems enough for now. Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru> Discussion: Discussion: https://www.postgresql.org/message-id/000301b2-5b81-4938-bdac-90f6eb660843@iki.fi Backpatch-through: 14
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/.