Fix rare assertion failure in standby, if primary is restarted During hot standby, ExpireAllKnownAssignedTransactionIds() and ExpireOldKnownAssignedTransactionIds() functions mark old transactions as no-longer running, but they failed to update xactCompletionCount and latestCompletedXid. AFAICS it would not lead to incorrect query results, because those functions effectively turn in-progress transactions into aborted transactions and an MVCC snapshot considers both as "not visible". But it could surprise GetSnapshotDataReuse() and trigger the "TransactionIdPrecedesOrEquals(TransactionXmin, RecentXmin))" assertion in it, if the apparent xmin in a backend would move backwards. We saw this happen when GetCatalogSnapshot() would reuse an older catalog snapshot, when GetTransactionSnapshot() had already advanced TransactionXmin.
The bug goes back all the way to commit 623a9ba79b in v14 that introduced the snapshot reuse mechanism, but it started to happen more frequently with commit 952365cded6 which removed a GetTransactionSnapshot() call from backend startup. That made it more likely for ExpireOldKnownAssignedTransactionIds() to be called between GetCatalogSnapshot() and the first GetTransactionSnapshot() in a backend. Andres Freund first spotted this assertion failure on buildfarm member 'skink'. Reproduction and analysis by Tomas Vondra. Backpatch-through: 14 Discussion: https://www.postgresql.org/message-id/oey246mcw43cy4qw2hqjmurbd62lfdpcuxyqiu7botx3typpax%40h7o7mfg5zmdj Branch ------ REL_16_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/2f33de3cdbc814bcc4270aafd98880a12d265777 Modified Files -------------- src/backend/storage/ipc/procarray.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+)
