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DSM handle values can be reused as soon as the underlying shared memory object has been destroyed. That means that for a brief moment we might have two DSM slots with the same handle. While trying to attach, if we encounter a slot with refcnt == 1, meaning that it is currently being destroyed, we should continue our search in case the same handle exists in another slot. The race manifested as a rare "dsa_area could not attach to segment" error, and was more likely in 10 and 11 due to the lack of distinct seed for random() in parallel workers. It was made very unlikely in in master by commit 197e4af9, and older releases don't usually create new DSM segments in background workers so it was also unlikely there. This fixes the root cause of bug report #15585, in which the error could also sometimes result in a self-deadlock in the error path. It's not yet clear if further changes are needed to avoid that failure mode. Back-patch to 9.4, where dsm.c arrived. Author: Thomas Munro Reported-by: Justin Pryzby, Sergei Kornilov Discussion: https://postgr.es/m/20190207014719.GJ29720@telsasoft.com Discussion: https://postgr.es/m/15585-324ff6a93a18da46@postgresql.org
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/.