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GiST's getNextNearest() function attempts to pfree the previously-returned tuple if any (that is, scan->xs_hitup in HEAD, or scan->xs_itup in older branches). However, if we are rescanning a plan node after ending a previous scan early, those tuple pointers could be pointing to garbage, because they would be pointing into the scan's pageDataCxt or queueCxt which has been reset. In a debug build this reliably results in a crash, although I think it might sometimes accidentally fail to fail in production builds. To fix, clear the pointer field anyplace we reset a context it might be pointing into. This may be overkill --- I think probably only the queueCxt case is involved in this bug, so that resetting in gistrescan() would be sufficient --- but dangling pointers are generally bad news, so let's avoid them. Another plausible answer might be to just not bother with the pfree in getNextNearest(). The reconstructed tuples would go away anyway in the context resets, and I'm far from convinced that freeing them a bit earlier really saves anything meaningful. I'll stick with the original logic in this patch, but if we find more problems in the same area we should consider that approach. Per bug #14641 from Denis Smirnov. Back-patch to 9.5 where this logic was introduced. Discussion: https://postgr.es/m/20170504072034.24366.57688@wrigleys.postgresql.org |
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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/.