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Both heap_freeze_tuple() and heap_tuple_needs_freeze() neglected to look into a multixact to check the members against cutoff_xid. This means that a very old Xid could survive hidden within a multi, possibly outliving its CLOG storage. In the distant future, this would cause clog lookup failures: ERROR: could not access status of transaction 3883960912 DETAIL: Could not open file "pg_clog/0E78": No such file or directory. This mostly was problematic when the updating transaction aborted, since in that case the row wouldn't get pruned away earlier in vacuum and the multixact could possibly survive for a long time. In many cases, data that is inaccessible for this reason way can be brought back heuristically. As a second bug, heap_freeze_tuple() didn't properly handle multixacts that need to be frozen according to cutoff_multi, but whose updater xid is still alive. Instead of preserving the update Xid, it just set Xmax invalid, which leads to both old and new tuple versions becoming visible. This is pretty rare in practice, but a real threat nonetheless. Existing corrupted rows, unfortunately, cannot be repaired in an automated fashion. Existing physical replicas might have already incorrectly frozen tuples because of different behavior than in master, which might only become apparent in the future once pg_multixact/ is truncated; it is recommended that all clones be rebuilt after upgrading. Following code analysis caused by bug report by J Smith in message CADFUPgc5bmtv-yg9znxV-vcfkb+JPRqs7m2OesQXaM_4Z1JpdQ@mail.gmail.com and privately by F-Secure. Backpatch to 9.3, where freezing of MultiXactIds was introduced. Analysis and patch by Andres Freund, with some tweaks by Álvaro. |
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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. Changes between all PostgreSQL releases are recorded in the file HISTORY. 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/.