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Correct StartupSUBTRANS for page wraparound

StartupSUBTRANS() incorrectly handled cases near the max pageid in the subtransdata structure, which in some cases could lead to errors in startup for HotStandby.This patch wraps the pageids correctly, avoiding any such errors.Identified by exhaustive crash testing by Jeff Janes.Jeff Janes

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src/backend/access/transam
- subtrans.c

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`‎src/backend/access/transam/subtrans.c`

Lines changed: 5 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -44,7 +44,8 @@`
`44`	`44`	`* 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE, and segment numbering at`
`45`	`45`	`* 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no`
`46`	`46`	`* explicit notice of that fact in this module, except when comparing segment`
`47`		`- * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes).`
	`47`	`+ * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes) and zeroing`
	`48`	`+ * them in StartupSUBTRANS.`
`48`	`49`	`*/`
`49`	`50`
`50`	`51`	`/* We need four bytes per xact */`
`@@ -254,6 +255,9 @@ StartupSUBTRANS(TransactionId oldestActiveXID)`
`254`	`255`	`{`
`255`	`256`	`(void)ZeroSUBTRANSPage(startPage);`
`256`	`257`	`startPage++;`
	`258`	`+/* must account for wraparound */`
	`259`	`+if (startPage>TransactionIdToPage(MaxTransactionId))`
	`260`	`+startPage=0;`
`257`	`261`	`}`
`258`	`262`	`(void)ZeroSUBTRANSPage(startPage);`
`259`	`263`

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Commit481725c

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`‎src/backend/access/transam/subtrans.c`

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