1ee22d1e87
We have long disallowed all forms of ALTER TABLE if the table is already opened by some outer SQL command in the same session. This has the same purpose as obtaining AccessExclusiveLock, but since a session's own locks don't conflict the lock only blocks use of the table by other sessions, not our own. Without this check, the ALTER might confuse the outer SQL command since any previous inspection of the table would potentially become invalid. However, the RelisBecomingView code path in DefineQueryRewrite never got that memo, and assumed that AccessExclusiveLock is sufficient for performing something morally equivalent to a rather invasive ALTER TABLE. Unsurprisingly, this can confuse an outer command that is trying to do something with the table. This was submitted as a security issue, but the security team has been unable to identify any consequence worse than a null pointer dereference (from trying to access rd_tableam methods that the relation no longer has). Therefore, in accordance with our usual policy, it's not security material and should just be fixed as a routine bug. Fix by disallowing the operation if the table is open locally, exactly as ALTER TABLE does it. Per an anonymous security researcher, via Bundesamt für Sicherheit in der Informationstechnik. Patch v12-v15 only. In v16 and later, we removed this code altogether (cf. commit b23cd185f), so that there's no issue.
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/.