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

1 files changed, 15 insertions, 3 deletions

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 4cfb22222b1..7eb431ed231 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1099,7 +1099,7 @@ 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
@@ -1251,7 +1251,7 @@ 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;
 
@@ -1263,6 +1263,18 @@ 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;
+		}
 	}
 
 	/*
@@ -2897,7 +2909,7 @@ 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->nextXid);
 	CurrentRunningXacts->oldestRunningXid = oldestRunningXid;
 	CurrentRunningXacts->latestCompletedXid = latestCompletedXid;