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The async notification queue contains the XID of the sender, and when processing notifications we call TransactionIdDidCommit() on the XID. But we had no safeguards to prevent the CLOG segments containing those XIDs from being truncated away. As a result, if a backend didn't for some reason process its notifications for a long time, or when a new backend issued LISTEN, you could get an error like: test=# listen c21; ERROR: 58P01: could not access status of transaction 14279685 DETAIL: Could not open file "pg_xact/000D": No such file or directory. LOCATION: SlruReportIOError, slru.c:1087 To fix, make VACUUM "freeze" the XIDs in the async notification queue before truncating the CLOG. Old XIDs are replaced with FrozenTransactionId or InvalidTransactionId. Note: This commit is not a full fix. A race condition remains, where a backend is executing asyncQueueReadAllNotifications() and has just made a local copy of an async SLRU page which contains old XIDs, while vacuum concurrently truncates the CLOG covering those XIDs. When the backend then calls TransactionIdDidCommit() on those XIDs from the local copy, you still get the error. The next commit will fix that remaining race condition. This was first reported by Sergey Zhuravlev in 2021, with many other people hitting the same issue later. Thanks to: - Alexandra Wang, Daniil Davydov, Andrei Varashen and Jacques Combrink for investigating and providing reproducable test cases, - Matheus Alcantara and Arseniy Mukhin for review and earlier proposed patches to fix this, - Álvaro Herrera and Masahiko Sawada for reviews, - Yura Sokolov aka funny-falcon for the idea of marking transactions as committed in the notification queue, and - Joel Jacobson for the final patch version. I hope I didn't forget anyone. Backpatch to all supported versions. I believe the bug goes back all the way to commit d1e027221d, which introduced the SLRU-based async notification queue. Discussion: https://www.postgresql.org/message-id/16961-25f29f95b3604a8a@postgresql.org Discussion: https://www.postgresql.org/message-id/18804-bccbbde5e77a68c2@postgresql.org Discussion: https://www.postgresql.org/message-id/CAK98qZ3wZLE-RZJN_Y%2BTFjiTRPPFPBwNBpBi5K5CU8hUHkzDpw@mail.gmail.com 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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