From 2e7f0c34aa6dd2f4c02f57d9dd5c37eeeb661198 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
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: <CAPpHfdspOkmiQsxh-UZw2chM6dRMwXAJGEmmbmqYR=yvM7-s6A@mail.gmail.com>
---
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 a5235148d49..c019b7230bf 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -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))
{
--
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