818119afcc 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such entries need to be refreshed if the cache age is outdated.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backend holding an outdated local reference would not be able to drop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
dc5f905418