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synchronous_standby_names cannot be reloaded safely by backends, and the checkpointer is in charge of updating a state in shared memory if the GUC is enabled in WalSndCtl, to let the backends know if they should wait or not for a given LSN. This provides a strict control on the timing of the waiting queues if the GUC is enabled or disabled, then reloaded. The checkpointer is also in charge of waking up the backends that could be waiting for a LSN when the GUC is disabled. This logic had a race condition at startup, where it would be possible for backends to not wait for a LSN even if synchronous_standby_names is enabled. This would cause visibility issues with transactions that we should be waiting for but they were not. The problem lasts until the checkpointer does its initial update of the shared memory state when it loads synchronous_standby_names. In order to take care of this problem, the shared memory state in WalSndCtl is extended to detect if it has been initialized by the checkpointer, and not only check if synchronous_standby_names is defined. In WalSndCtlData, sync_standbys_defined is renamed to sync_standbys_status, a bits8 able to know about two states: - If the shared memory state has been initialized. This flag is set by the checkpointer at startup once, and never removed. - If synchronous_standby_names is known as defined in the shared memory state. This is the same as the previous sync_standbys_defined in WalSndCtl. This method gives a way for backends to decide what they should do until the shared memory area is initialized, and they now ultimately fall back to a check on the GUC value in this case, which is the best thing that can be done. Fortunately, SyncRepUpdateSyncStandbysDefined() is called immediately by the checkpointer when this process starts, so the window is very narrow. It is possible to enlarge the problematic window by making the checkpointer wait at the beginning of SyncRepUpdateSyncStandbysDefined() with a hardcoded sleep for example, and doing so has showed that a 2PC visibility test is indeed failing. On machines slow enough, this bug would cause spurious failures. In 17~, we have looked at the possibility of adding an injection point to have a reproducible test, but as the problematic window happens at early startup, we would need to invent a way to make an injection point optionally persistent across restarts when attached, something that would be fine for this case as it would involve the checkpointer. This issue is quite old, and can be reproduced on all the stable branches. Author: Melnikov Maksim <m.melnikov@postgrespro.ru> Co-authored-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/163fcbec-900b-4b07-beaa-d2ead8634bec@postgrespro.ru Backpatch-through: 13
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/.