Export subxip[] for snapshots taken during recovery. A snapshot taken during recovery stores all of its in-progress XIDs in subxip, every running top-level XID included, leaving xip empty. Unlike with other snapshots, its suboverflowed flag does not mean that subxip is redundant. We nevertheless treated it that way during snapshot export, so an importing session could see in-progress transactions as aborted. This misbehavior could also lead to hint bits being incorrectly set on the standby; affected tuples then wrongly appeared visible or invisible to sessions that never imported the snapshot.
To fix, teach snapshot export to include the subxip[] array regardless of the overflow flag when the snapshot is taken during recovery. This is in line with how CopySnapshot() and SerializeSnapshot() already handle the same issue. Claude Code diagnosed this problem. The committed TAP test is a simplified version of the one that it wrote to demonstrate this bug. Oversight in commit 6c2003f8a, which enabled snapshot export and import during recovery. Author: Peter Geoghegan <[email protected]> Author: Bertrand Drouvot <[email protected]> Bug: #17846 Discussion: https://postgr.es/m/CAH2-WzmHVeYY%3Dpjz9x8DhhxVjXHX0pvoQ-MdiB1Tt6%3Do2GTiKg%40mail.gmail.com Discussion: https://postgr.es/m/[email protected] Backpatch-through: 14 Branch ------ REL_18_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/74c9832ca61037eec25b8f214fc71883494900d8 Modified Files -------------- src/backend/utils/time/snapmgr.c | 58 ++++++++---- src/test/recovery/meson.build | 1 + src/test/recovery/t/056_standby_snapshot_export.pl | 104 +++++++++++++++++++++ 3 files changed, 147 insertions(+), 16 deletions(-)
