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When heap_lock_tuple decides to follow the update chain, it tried to also lock any version of the tuple that was created by an update that was subsequently rolled back. This is pointless, since for all intents and purposes that tuple exists no more; and moreover it causes misbehavior, as reported independently by Marko Tiikkaja and Marti Raudsepp: some SELECT FOR UPDATE/SHARE queries may fail to return the tuples, and assertion-enabled builds crash. Fix by having heap_lock_updated_tuple test the xmin and return success immediately if the tuple was created by an aborted transaction. The condition where tuples become invisible occurs when an updated tuple chain is followed by heap_lock_updated_tuple, which reports the problem as HeapTupleSelfUpdated to its caller heap_lock_tuple, which in turn propagates that code outwards possibly leading the calling code (ExecLockRows) to believe that the tuple exists no longer. Backpatch to 9.3. Only on 9.5 and newer this leads to a visible failure, because of commit 27846f02c176; before that, heap_lock_tuple skips the whole dance when the tuple is already locked by the same transaction, because of the ancient HeapTupleSatisfiesUpdate behavior. Still, the buggy condition may also exist in more convoluted scenarios involving concurrent transactions, so it seems safer to fix the bug in the old branches too. Discussion: https://www.postgresql.org/message-id/CABRT9RC81YUf1=jsmWopcKJEro=VoeG2ou6sPwyOUTx_qteRsg@mail.gmail.com https://www.postgresql.org/message-id/48d3eade-98d3-8b9a-477e-1a8dc32a724d@joh.to |
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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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.