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The versions of these functions that accept object OIDs are supposed to return NULL, rather than failing, if the target object has been dropped. This makes it safe(r) to use them in queries that scan catalogs, since the functions will be applied to objects that are visible in the query's snapshot but might now be gone according to the catalog snapshot. In most cases we implemented this by doing a SearchSysCacheExists test and assuming that if that succeeds, we can safely invoke the appropriate aclchk.c function, which will immediately re-fetch the same syscache entry. It was argued that if the existence test succeeds then the followup fetch must succeed as well, for lack of any intervening AcceptInvalidationMessages call. Alexander Lakhin demonstrated that this is not so when CATCACHE_FORCE_RELEASE is enabled: the syscache entry will be forcibly dropped at the end of SearchSysCacheExists, and then it is possible for the catalog snapshot to get advanced while re-fetching the entry. Alexander's test case requires the operation to happen inside a parallel worker, but that seems incidental to the fundamental problem. What remains obscure is whether there is a way for this to happen in a non-debug build. Nonetheless, CATCACHE_FORCE_RELEASE is a very useful test methodology, so we'd better make the code safe for it. After some discussion we concluded that the most future-proof fix is to give up the assumption that checking SearchSysCacheExists can guarantee success of a later fetch. At best that assumption leads to fragile code --- for example, has_type_privilege appears broken for array types even if you believe the assumption holds. And it's not even particularly efficient. There had already been some work towards extending the aclchk.c APIs to include "is_missing" output flags, so this patch extends that work to cover all the aclchk.c functions that are used by the has_xxx_privilege() functions. (This allows getting rid of some ad-hoc decisions about not throwing errors in certain places in aclchk.c.) In passing, this fixes the has_sequence_privilege() functions to provide the same guarantees as their cousins: for some reason the SearchSysCacheExists tests never got added to those. There is more work to do to remove the unsafe coding pattern with SearchSysCacheExists in other places, but this is a pretty self-contained patch so I'll commit it separately. Per bug #18014 from Alexander Lakhin. Given the lack of hard evidence that there's a bug in non-debug builds, I'm content to fix this only in HEAD. (Perhaps we should clean up the has_sequence_privilege() oversight in the back branches, but in the absence of field complaints I'm not too excited about that either.) Discussion: https://postgr.es/m/18014-28c81cb79d44295d@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: 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/.