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XidEpoch++ if wraparound during checkpoint.

If wal_level = hot_standby we update the checkpoint nextxid,though in the case where a wraparound occurred half-way througha checkpoint we would neglect updating the epoch also. Updatingthe nextxid is arguably the wrong thing to do, but changing thatmay introduce subtle bugs into hot standby startup, while updatingthe value doesn't cause any known bugs yet. Minimal fix now toHEAD and backbranches, wider fix later in HEAD.Bug reported in #6291 by Daniel Farina and slightly differently inCause analysis and recommended fixes from Tom Lane and Andres Freund.Applied patch is minimal version of Andres Freund's work.

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src/backend/access/transam
- xlog.c

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`‎src/backend/access/transam/xlog.c`

Lines changed: 8 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -7119,10 +7119,17 @@ CreateCheckPoint(int flags)`
`7119`	`7119`	`* If we are shutting down, or Startup process is completing crash`
`7120`	`7120`	`* recovery we don't need to write running xact data.`
`7121`	`7121`	`*`
`7122`		`- * Update checkPoint.nextXid since we have a later value`
	`7122`	`+ * Update checkPoint.nextXid since we may have a later value. If we`
	`7123`	`+ * do update the value, and we have wrapped, increment epoch also.`
`7123`	`7124`	`*/`
`7124`	`7125`	`if (!shutdown&&XLogStandbyInfoActive())`
	`7126`	`+{`
	`7127`	`+TransactionIdprevXid=checkPoint.nextXid;`
	`7128`	`+`
`7125`	`7129`	`LogStandbySnapshot(&checkPoint.nextXid);`
	`7130`	`+if (checkPoint.nextXid<prevXid)`
	`7131`	`+checkPoint.nextXidEpoch++;`
	`7132`	`+}`
`7126`	`7133`
`7127`	`7134`	`START_CRIT_SECTION();`
`7128`	`7135`

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Commitc111306

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`‎src/backend/access/transam/xlog.c`

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