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818119afccd3 has introduced the "generation" concept in pgstats entries, incremented a counter when a pgstats entry is reinitialized, but it did not count on the fact that backends still holding local references to such entries need to be refreshed if the cache age is outdated. The previous logic only updated local references when an entry was dropped, but it needs also to consider entries that are reinitialized. This matters for replication slot stats (as well as custom pgstats kinds in 18~), where concurrent drops and creates of a slot could cause incorrect stats to be locally referenced. This would lead to an assertion failure at shutdown when writing out the stats file, as the backend holding an outdated local reference would not be able to drop during its shutdown sequence the stats entry that should be dropped, as the last process holding a reference to the stats entry. The checkpointer was then complaining about such an entry late in the shutdown sequence, after the shutdown checkpoint is finished with the control file updated, causing the stats file to not be generated. In non-assert builds, the entry would just be skipped with the stats file written. Note that only logical replication slots use statistics. A test case based on TAP is added to test_decoding, where a persistent connection peeking at a slot's data is kept with concurrent drops and creates of the same slot. This is based on the isolation test case that Anton has sent. As it requires a node shutdown with a check to make sure that the stats file is written with this specific sequence of events, TAP is used instead. Reported-by: Anton A. Melnikov Reviewed-by: Bertrand Drouvot Discussion: https://postgr.es/m/56bf8ff9-dd8c-47b2-872a-748ede82af99@postgrespro.ru Backpatch-through: 15
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.