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This is a step toward storing stats in dynamic shared memory. As dynamic shared memory segments are detached from just after before_shmem_exit() callbacks are processed, but before on_shmem_exit() callbacks are, no stats can be collected after before_shmem_exit() callbacks have been processed. Parallel worker shutdown can cause stats to be emitted during DSM detach callbacks, e.g. for SharedFileSet (which closes its files, which can causes fd.c to emit stats about temporary files). Therefore parallel worker shutdown needs to complete during the processing of before_shmem_exit callbacks. One might think this problem could instead be solved by carefully ordering the attaching to DSM segments, so that the pgstats segments get detached from later than the parallel query ones. That turns out to not work because the stats hash might need to grow which can cause new segments to be allocated, which then will be detached from earlier. There are two code changes: First, call ParallelWorkerShutdown() via before_shmem_exit. That's a good idea on its own, because other shutdown callbacks like ShutdownPostgres and ShutdownAuxiliaryProcess are called via before_*. Second, explicitly detach from the parallel query DSM segment, thereby ensuring all stats are emitted during ParallelWorkerShutdown(). There are nicer solutions to these problems, but it's not obvious which of those solutions is the correct one. As the shared memory stats work already is a huge amount of work... Author: Andres Freund <andres@anarazel.de> Discussion: https://postgr.es/m/20210405092914.mmxqe7j56lsjfsej@alap3.anarazel.de Discussion: https://postgr.es/m/20210803023612.iziacxk5syn2r4ut@alap3.anarazel.de
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/.