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An error happening while a slot data is saved on disk in SaveSlotToPath() could cause a state.tmp file (temporary file holding the slot state data, renamed to its permanent name at the end of the function) to remain around after it has been created. This temporary file is created with O_EXCL, meaning that if an existing state.tmp is found, its creation would fail. This would prevent the slot data to be saved, requiring a manual intervention to remove state.tmp before being able to save again a slot. Possible scenarios where this temporary file could remain on disk is for example a ENOSPC case (no disk space) while writing, syncing or renaming it. The bug reports point to a write failure as the principal cause of the problems. Using O_TRUNC has been argued back in 2019 as a potential solution to discard any temporary file that could exist. This solution was rejected as O_EXCL can also act as a safety measure when saving the slot state, crash recovery offering cleanup guarantees post-crash. This commit uses the alternative approach that has been suggested by Andres Freund back in 2019. When the temporary state file cannot be written, synced, closed or renamed (note: not when created!), an unlink() is used to remove the temporary state file while holding the in-progress I/O LWLock, so as any follow-up attempts to save a slot's data would not choke on an existing file that remained around because of a previous failure. This problem has been reported a few times across the years, going back to 2019, but for some reason I have never come back to do something about it and it has been forgotten. A recent report has reminded me that this was still a problem. Reported-by: Kevin K Biju <kevinkbiju@gmail.com> Reported-by: Sergei Kornilov <sk@zsrv.org> Reported-by: Grigory Smolkin <g.smolkin@postgrespro.ru> Discussion: https://postgr.es/m/CAM45KeHa32soKL_G8Vk38CWvTBeOOXcsxAPAs7Jt7yPRf2mbVA@mail.gmail.com Discussion: https://postgr.es/m/3559061693910326@qy4q4a6esb2lebnz.sas.yp-c.yandex.net Discussion: https://postgr.es/m/08bbfab1-a61d-3750-fc18-4ab2c1aa7f09@postgrespro.ru Backpatch-through: 13
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/.