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fixempties() counts the number of in-arcs in the regex NFA and then allocates an array of that many arc pointers. If the NFA contains no arcs, this amounts to malloc(0) for which some platforms return NULL. The code mistakenly treats that as indicating out-of-memory. Thus, we can get a bogus "out of memory" failure for some unsatisfiable regexes. This happens only in v15 and earlier, since bea3d7e38 switched to using palloc() rather than bare malloc(). And at least of the platforms in the buildfarm, only AIX seems to return NULL. So the impact is pretty narrow. But I don't especially want to ship code that is failing its own regression tests, so let's fix this for this week's releases. A quick code survey says that there is only the one place in src/backend/regex/ that is at risk of doing malloc(0), so we'll just band-aid that place. A more future-proof fix could be to install a malloc() wrapper similar to pg_malloc(). But this code seems unlikely to change much more in the affected branches, so that's probably overkill. The only known test case for this involves a complemented character class in a bracket expression, for example [^\s\S], and we didn't support that in v13. So it may be that the problem is unreachable in v13. But I'm not 100% sure of that, so patch v13 too. Discussion: https://postgr.es/m/661fd81b-f069-8f30-1a41-e195c57449b4@gmail.com
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/.