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Avoid consuming an XID during vac_truncate_clog().

vac_truncate_clog() uses its own transaction ID as the comparison point ina sanity check that no database's datfrozenxid has already wrapped around"into the future". That was probably fine when written, but in a lazyvacuum we won't have assigned an XID, so calling GetCurrentTransactionId()causes an XID to be assigned when otherwise one would not be. Most of thetime that's not a big problem ... but if we are hard up against thewraparound limit, consuming XIDs during antiwraparound vacuums is a verybad thing.Instead, use ReadNewTransactionId(), which not only avoids this problembut is in itself a better comparison point to test whether wraparoundhas already occurred.Report and patch by Alexander Korotkov. Back-patch to all versions.Report: <CAPpHfdspOkmiQsxh-UZw2chM6dRMwXAJGEmmbmqYR=yvM7-s6A@mail.gmail.com>

1 parent0c7cd45 commit996d273Copy full SHA for 996d273

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src/backend/commands
- vacuum.c

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`‎src/backend/commands/vacuum.c`

Lines changed: 2 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -1051,7 +1051,7 @@ vac_truncate_clog(TransactionId frozenXID,`
`1051`	`1051`	`TransactionIdlastSaneFrozenXid,`
`1052`	`1052`	`MultiXactIdlastSaneMinMulti)`
`1053`	`1053`	`{`
`1054`		`-TransactionIdmyXID=GetCurrentTransactionId();`
	`1054`	`+TransactionIdnextXID=ReadNewTransactionId();`
`1055`	`1055`	`Relationrelation;`
`1056`	`1056`	`HeapScanDescscan;`
`1057`	`1057`	`HeapTupletuple;`
`@@ -1100,7 +1100,7 @@ vac_truncate_clog(TransactionId frozenXID,`
`1100`	`1100`	`MultiXactIdPrecedes(lastSaneMinMulti,dbform->datminmxid))`
`1101`	`1101`	`bogus= true;`
`1102`	`1102`
`1103`		`-if (TransactionIdPrecedes(myXID,dbform->datfrozenxid))`
	`1103`	`+if (TransactionIdPrecedes(nextXID,dbform->datfrozenxid))`
`1104`	`1104`	`frozenAlreadyWrapped= true;`
`1105`	`1105`	`elseif (TransactionIdPrecedes(dbform->datfrozenxid,frozenXID))`
`1106`	`1106`	`{`

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Commit996d273

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`‎src/backend/commands/vacuum.c`

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