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Fix MVCC bug with prepared xact with subxacts on standby

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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
This commit is contained in:
Heikki Linnakangas
2024-06-27 21:06:32 +03:00
parent ecbf6ac51d
commit cbfbda7841
7 changed files with 85 additions and 17 deletions
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18
src/backend/storage/ipc/procarray.c
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@@ -1106,7 +1106,7 @@ 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
@@ -1258,7 +1258,7 @@ 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;
@@ -1270,6 +1270,18 @@ 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;
}
}
/*
@@ -2833,7 +2845,7 @@ 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(TransamVariables->nextXid);
CurrentRunningXacts->oldestRunningXid = oldestRunningXid;
CurrentRunningXacts->oldestDatabaseRunningXid = oldestDatabaseRunningXid;