From e9c8747ee934db9b5bad0b0f5aa3709a41ad0a9c Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Thu, 27 Jun 2024 21:06:32 +0300 Subject: Fix MVCC bug with prepared xact with subxacts on standby We did not recover the subtransaction IDs of prepared transactions when starting a hot standby from a shutdown checkpoint. As a result, such subtransactions were considered as aborted, rather than in-progress. That would lead to hint bits being set incorrectly, and the subtransactions suddenly becoming visible to old snapshots when the prepared transaction was committed. To fix, update pg_subtrans with prepared transactions's subxids when starting hot standby from a shutdown checkpoint. The snapshots taken from that state need to be marked as "suboverflowed", so that we also check the pg_subtrans. Backport to all supported versions. Discussion: https://www.postgresql.org/message-id/6b852e98-2d49-4ca1-9e95-db419a2696e0@iki.fi --- src/backend/storage/ipc/procarray.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) (limited to 'src/backend/storage/ipc/procarray.c') diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c index 7583e30e034..9dbf92360ec 100644 --- a/src/backend/storage/ipc/procarray.c +++ b/src/backend/storage/ipc/procarray.c @@ -734,7 +734,7 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running) * If the snapshot isn't overflowed or if its empty we can reset our * pending state and use this snapshot instead. */ - if (!running->subxid_overflow || running->xcnt == 0) + if (running->subxid_status != SUBXIDS_MISSING || running->xcnt == 0) { /* * If we have already collected known assigned xids, we need to @@ -886,7 +886,7 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running) * missing, so conservatively assume the last one is latestObservedXid. * ---------- */ - if (running->subxid_overflow) + if (running->subxid_status == SUBXIDS_MISSING) { standbyState = STANDBY_SNAPSHOT_PENDING; @@ -898,6 +898,18 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running) standbyState = STANDBY_SNAPSHOT_READY; standbySnapshotPendingXmin = InvalidTransactionId; + + /* + * If the 'xids' array didn't include all subtransactions, we have to + * mark any snapshots taken as overflowed. + */ + if (running->subxid_status == SUBXIDS_IN_SUBTRANS) + procArray->lastOverflowedXid = latestObservedXid; + else + { + Assert(running->subxid_status == SUBXIDS_IN_ARRAY); + procArray->lastOverflowedXid = InvalidTransactionId; + } } /* @@ -2129,7 +2141,7 @@ GetRunningTransactionData(void) CurrentRunningXacts->xcnt = count - subcount; CurrentRunningXacts->subxcnt = subcount; - CurrentRunningXacts->subxid_overflow = suboverflowed; + CurrentRunningXacts->subxid_status = suboverflowed ? SUBXIDS_IN_SUBTRANS : SUBXIDS_IN_ARRAY; CurrentRunningXacts->nextXid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid); CurrentRunningXacts->oldestRunningXid = oldestRunningXid; CurrentRunningXacts->latestCompletedXid = latestCompletedXid; -- cgit v1.2.3