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If a FATAL error occurs while holding a lock in a DSM segment (such as a dshash lock) and the process is not in a transaction, a segmentation fault can occur during process exit. The problem sequence is: 1. Process acquires a lock in a DSM segment (e.g., via dshash) 2. FATAL error occurs outside transaction context 3. proc_exit() begins, calling before_shmem_exit callbacks 4. dsm_backend_shutdown() detaches all DSM segments 5. Later, on_shmem_exit callbacks run 6. ProcKill() calls LWLockReleaseAll() 7. Segfault: the lock being released is in unmapped memory This only manifests outside transaction contexts because AbortTransaction() calls LWLockReleaseAll() during transaction abort, releasing locks before DSM cleanup. Background workers and other non-transactional code paths are vulnerable. Fix by calling LWLockReleaseAll() unconditionally at the start of shmem_exit(), before any callbacks run. Releasing locks before callbacks prevents the segfault - locks must be released before dsm_backend_shutdown() detaches their memory. This is safe because after an error, held locks are protecting potentially inconsistent data anyway, and callbacks can acquire fresh locks if needed. Also add a comment noting that LWLockReleaseAll() must be safe to call before LWLock initialization (which it is, since num_held_lwlocks will be 0), plus an Assert for the post-condition. This fix aligns with the original design intent from commit 001a573a2, which noted that backends must clean up shared memory state (including releasing lwlocks) before unmapping dynamic shared memory segments. Reported-by: Rahila Syed <rahilasyed90@gmail.com> Author: Rahila Syed <rahilasyed90@gmail.com> Reviewed-by: Amit Langote <amitlangote09@gmail.com> Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de> Discussion: https://postgr.es/m/CAH2L28uSvyiosL+kaic9249jRVoQiQF6JOnaCitKFq=xiFzX3g@mail.gmail.com 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/.