From: Tom Lane Date: Tue, 24 May 2016 19:20:12 +0000 (-0400) Subject: Avoid consuming an XID during vac_truncate_clog(). X-Git-Tag: REL9_2_18~54 X-Git-Url: http://git.postgresql.org/gitweb/?a=commitdiff_plain;h=2e7f0c34aa6dd2f4c02f57d9dd5c37eeeb661198;p=postgresql.git 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: --- 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)) {