From fe1731fca27008836f2208f4be8c22acf8ec1f67 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Tue, 24 May 2016 15:20:12 -0400 Subject: [PATCH] Avoid consuming an XID during vac_truncate_clog(). 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: --- src/backend/commands/vacuum.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c index c5170efea50..15c36eca94f 100644 --- a/src/backend/commands/vacuum.c +++ b/src/backend/commands/vacuum.c @@ -784,7 +784,7 @@ vac_update_datfrozenxid(void) static void vac_truncate_clog(TransactionId frozenXID) { - TransactionId myXID = GetCurrentTransactionId(); + TransactionId nextXID = ReadNewTransactionId(); Relation relation; HeapScanDesc scan; HeapTuple tuple; @@ -816,7 +816,7 @@ 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)) {