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By default, wait_for_catchup() waits for the replication connection to reach the primary's write LSN. That's fine, but in an apparent attempt to save one query round-trip, it was coded so that we executed pg_current_wal_lsn() again during each probe query. Thus, we presented the standby with a moving target to be reached. (While the test script itself couldn't be causing the write LSN to advance while it's blocked in wait_for_catchup(), it's plenty plausible that background activity such as autovacuum is emitting more WAL.) That could make the test take longer than necessary, and potentially it could mask bugs by allowing the standby to process more WAL than a strict interpretation of the test scenario allows. So, change wait_for_catchup() to do it "by the book", explicitly collecting the write LSN to wait for at the outset. Also, various call sites were instructing wait_for_catchup() to wait for the standby to reach the primary's insert LSN rather than its write LSN. This also seems like a bad idea. While in most test scenarios those are the same, if they are different then the inserted-but-not-yet-written WAL is not presently available to the standby. The test isn't doing anything to make it become so, so again we have the potential for unwanted test delay, perhaps even a test timeout. (Again, background activity would be needed to make this more than a hypothetical problem.) Hence, change the callers where necessary so that the wait target is always the primary's write LSN. While at it, simplify callers by making use of wait_for_catchup's default arguments wherever possible (the preceding change makes this possible in more places than it was before). And rewrite wait_for_catchup's documentation a bit. Patch by me; thanks to Julien Rouhaud for review. Discussion: https://postgr.es/m/2368336.1641843098@sss.pgh.pa.us |
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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/.