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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 parente504d91 commita34c3ddCopy full SHA for a34c3dd

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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
`@@ -1050,7 +1050,7 @@ vac_truncate_clog(TransactionId frozenXID,`
`1050`	`1050`	`TransactionIdlastSaneFrozenXid,`
`1051`	`1051`	`MultiXactIdlastSaneMinMulti)`
`1052`	`1052`	`{`
`1053`		`-TransactionIdmyXID=GetCurrentTransactionId();`
	`1053`	`+TransactionIdnextXID=ReadNewTransactionId();`
`1054`	`1054`	`Relationrelation;`
`1055`	`1055`	`HeapScanDescscan;`
`1056`	`1056`	`HeapTupletuple;`
`@@ -1099,7 +1099,7 @@ vac_truncate_clog(TransactionId frozenXID,`
`1099`	`1099`	`MultiXactIdPrecedes(lastSaneMinMulti,dbform->datminmxid))`
`1100`	`1100`	`bogus= true;`
`1101`	`1101`
`1102`		`-if (TransactionIdPrecedes(myXID,dbform->datfrozenxid))`
	`1102`	`+if (TransactionIdPrecedes(nextXID,dbform->datfrozenxid))`
`1103`	`1103`	`frozenAlreadyWrapped= true;`
`1104`	`1104`	`elseif (TransactionIdPrecedes(dbform->datfrozenxid,frozenXID))`
`1105`	`1105`	`{`

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Commita34c3dd

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`‎src/backend/commands/vacuum.c`

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