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Heap WAL records that clear bits on the visibility map (like inserts and deletes) did not register the visibility map blocks they modified. Because the WAL summarizer only records registered blocks, an incremental backup taken over such operations would omit the changed VM pages. On restore, the VM would retain stale all-visible/all-frozen bits, which can cause wrong results from index-only scans and incorrect relfrozenxid advancement due to vacuum page skipping. Not registering the VM buffer also meant we never emitted FPIs of VM pages when clearing bits. A torn VM page won't raise an error because the VM is read with ZERO_ON_ERROR; with checksums on, it would be detected and zeroed, but with checksums off, it is accepted as-is and can lead to data corruption. Fix this by registering the VM buffer in the WAL record when clearing VM bits. The VM buffer must now be locked throughout the critical section that modifies the VM and heap pages and emits the WAL record. This can slow down operations that clear the VM, since the VM lock is held longer and VM FPIs may be emitted, but it is required for correctness. Note that this fix does not repair existing incremental backups. Author: Melanie Plageman <melanieplageman@gmail.com> Author: Andres Freund <andres@anarazel.de> Reviewed-by: Robert Haas <robertmhaas@gmail.com> Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru> Discussion: https://postgr.es/m/flat/CAAKRu_bn%2Be7F4yPFBgFbnP%2BsyJRKyNK092bjD2LKvZW7O4Svag> Backpatch-through: 17
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.