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Avoid consuming an XID during vac_truncate_clog().

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vac_truncate_clog() uses its own transaction ID as the comparison point in
a sanity check that no database's datfrozenxid has already wrapped around
"into the future".  That was probably fine when written, but in a lazy
vacuum we won't have assigned an XID, so calling GetCurrentTransactionId()
causes an XID to be assigned when otherwise one would not be.  Most of the
time that's not a big problem ... but if we are hard up against the
wraparound limit, consuming XIDs during antiwraparound vacuums is a very
bad thing.

Instead, use ReadNewTransactionId(), which not only avoids this problem
but is in itself a better comparison point to test whether wraparound
has already occurred.

Report and patch by Alexander Korotkov.  Back-patch to all versions.

Report: <CAPpHfdspOkmiQsxh-UZw2chM6dRMwXAJGEmmbmqYR=yvM7-s6A@mail.gmail.com>
This commit is contained in:
Tom Lane
2016-05-24 15:20:12 -04:00
parent fca2eb3d43
commit 2e7f0c34aa
1 changed files with 2 additions and 2 deletions
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4
src/backend/commands/vacuum.c
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@ -802,7 +802,7 @@ vac_update_datfrozenxid(void)
static void
vac_truncate_clog(TransactionId frozenXID)
{
TransactionId myXID = GetCurrentTransactionId();
TransactionId nextXID = ReadNewTransactionId();
Relation relation;
HeapScanDesc scan;
HeapTuple tuple;
@ -834,7 +834,7 @@ vac_truncate_clog(TransactionId frozenXID)
Assert(TransactionIdIsNormal(dbform->datfrozenxid));
if (TransactionIdPrecedes(myXID, dbform->datfrozenxid))
if (TransactionIdPrecedes(nextXID, dbform->datfrozenxid))
frozenAlreadyWrapped = true;
else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
{