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Buildfarm members hornet and sungazer have shown multiple instances of "Failed test 'xmin of non-cascaded slot with hs feedback has changed'". The reason seems to be that the test is checking the current xmin of the master server's replication slot against a past xmin of the first slave server's replication slot. Even though the latter slot is downstream of the former, it's possible for its reported xmin to be ahead of the former's reported xmin, because those numbers are updated whenever the respective downstream walreceiver feels like it (see logic in WalReceiverMain). Instrumenting this test shows that indeed the slave slot's xmin does often advance before the master's does, especially if an autovacuum transaction manages to occur during the relevant window. If we happen to capture such an advanced xmin as $xmin, then the subsequent wait_slot_xmins call can fall through before the master's xmin has advanced at all, and then if it advances before the get_slot_xmins call, we can get the observed failure. Yeah, that's a bit of a long chain of deduction, but it's hard to explain any other way how the test can get past an "xmin <> '$xmin'" check only to have the next query find that xmin does equal $xmin. Fix by keeping separate images of the master and slave slots' xmins and testing their has-xmin-advanced conditions independently. |
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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/.