102bdaa9be
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.
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