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This fixes a set of race conditions with cumulative statistics where a shared stats entry could be dropped while it should still be valid in the event when it is reused: an entry may refer to a different object but requires the same hash key. This can happen with various stats kinds, like: - Replication slots that compute internally an index number, for different slot names. - Stats kinds that use an OID in the object key, where a wraparound causes the same key to be used if an OID is used for the same object. - As of PostgreSQL 18, custom pgstats kinds could also be an issue, depending on their implementation. This issue is fixed by introducing a counter called "generation" in the shared entries via PgStatShared_HashEntry, initialized at 0 when an entry is created and incremented when the same entry is reused, to avoid concurrent issues on drop because of other backends still holding a reference to it. This "generation" is copied to the local copy that a backend holds when looking at an object, then cross-checked with the shared entry to make sure that the entry is not dropped even if its "refcount" justifies that if it has been reused. This problem could show up when a backend shuts down and needs to discard any entries it still holds, causing statistics to be removed when they should not, or even an assertion failure. Another report involved a failure in a standby after an OID wraparound, where the startup process would FATAL on a "can only drop stats once", stopping recovery abruptly. The buildfarm has been sporadically complaining about the problem, as well, but the window is hard to reach with the in-core tests. Note that the issue can be reproduced easily by adding a sleep before dshash_find() in pgstat_release_entry_ref() to enlarge the problematic window while repeating test_decoding's isolation test oldest_xmin a couple of times, for example, as pointed out by Alexander Lakhin. Reported-by: Alexander Lakhin, Peter Smith Author: Kyotaro Horiguchi, Michael Paquier Reviewed-by: Bertrand Drouvot Discussion: https://postgr.es/m/CAA4eK1KxuMVyAryz_Vk5yq3ejgKYcL6F45Hj9ZnMNBS-g+PuZg@mail.gmail.com Discussion: https://postgr.es/m/17947-b9554521ad963c9c@postgresql.org Backpatch-through: 15 |
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