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xl_tot_len comes first in a WAL record. Usually we don't trust it to be the true length until we've validated the record header. If the record header was split across two pages, previously we wouldn't do the validation until after we'd already tried to allocate enough memory to hold the record, which was bad because it might actually be garbage bytes from a recycled WAL file, so we could try to allocate a lot of memory. Release 15 made it worse. Since 70b4f82a4b5, we'd at least generate an end-of-WAL condition if the garbage 4 byte value happened to be > 1GB, but we'd still try to allocate up to 1GB of memory bogusly otherwise. That was an improvement, but unfortunately release 15 tries to allocate another object before that, so you could get a FATAL error and recovery could fail. We can fix both variants of the problem more fundamentally using pre-existing page-level validation, if we just re-order some logic. The new order of operations in the split-header case defers all memory allocation based on xl_tot_len until we've read the following page. At that point we know that its first few bytes are not recycled data, by checking its xlp_pageaddr, and that its xlp_rem_len agrees with xl_tot_len on the preceding page. That is strong evidence that xl_tot_len was truly the start of a record that was logged. This problem was most likely to occur on a standby, because walreceiver.c recycles WAL files without zeroing out trailing regions of each page. We could fix that too, but that wouldn't protect us from rare crash scenarios where the trailing zeroes don't make it to disk. With reliable xl_tot_len validation in place, the ancient policy of considering malloc failure to indicate corruption at end-of-WAL seems quite surprising, but changing that is left for later work. Also included is a new TAP test to exercise various cases of end-of-WAL detection by writing contrived data into the WAL from Perl. Back-patch to 12. We decided not to put this change into the final release of 11. Author: Thomas Munro <thomas.munro@gmail.com> Author: Michael Paquier <michael@paquier.xyz> Reported-by: Alexander Lakhin <exclusion@gmail.com> Reviewed-by: Noah Misch <noah@leadboat.com> (the idea, not the code) Reviewed-by: Michael Paquier <michael@paquier.xyz> Reviewed-by: Sergei Kornilov <sk@zsrv.org> Reviewed-by: Alexander Lakhin <exclusion@gmail.com> Discussion: https://postgr.es/m/17928-aa92416a70ff44a2%40postgresql.org
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/.