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Accidentally the code in LWLockWakeup() checked the list of to-be-woken up processes to see if LW_FLAG_HAS_WAITERS should be unset. That means that HAS_WAITERS would not get unset immediately, but only during the next, unnecessary, call to LWLockWakeup(). Luckily, as the code stands, this is just a small efficiency issue. However, if there were (as in a patch of mine) a case in which LWLockWakeup() would not find any backend to wake, despite the wait list not being empty, we'd wrongly unset LW_FLAG_HAS_WAITERS, leading to potentially hanging. While the consequences in the backbranches are limited, the code as-is confusing, and it is possible that there are workloads where the additional wait list lock acquisitions hurt, therefore backpatch. Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff 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. 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/.