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This approach provides a much tighter binding between a data directory and the associated SysV shared memory block (and SysV or named-POSIX semaphores, if we're using those). Key collisions are still possible, but only between data directories stored on different filesystems, so the situation should be negligible in practice. More importantly, restarting the postmaster with a different port number no longer risks failing to identify a relevant shared memory block, even when postmaster.pid has been removed. A standalone backend is likewise much more certain to detect conflicting leftover backends. (In the longer term, we might now think about deprecating the port as a cluster-wide value, so that one postmaster could support sockets with varying port numbers. But that's for another day.) The hazards fixed here apply only on Unix systems; our Windows code paths already use identifiers derived from the data directory path name rather than the port. src/test/recovery/t/017_shm.pl, which intends to test key-collision cases, has been substantially rewritten since it can no longer use two postmasters with identical port numbers to trigger the case. Instead, use Perl's IPC::SharedMem module to create a conflicting shmem segment directly. The test script will be skipped if that module is not available. (This means that some older buildfarm members won't run it, but I don't think that that results in any meaningful coverage loss.) Patch by me; thanks to Noah Misch and Peter Eisentraut for discussion and review. Discussion: https://postgr.es/m/16908.1557521200@sss.pgh.pa.us
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/.