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Protect against torn pages when deleting GIN list pages.

To-be-deleted list pages contain no useful information, as they are beingdeleted, but we must still protect the writes from being torn by a crashafter a partial write. To do that, re-initialize the pages on WAL replay.Jeff Janes caught this with a test program to test partial writes.Backpatch to all supported versions.

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src/backend/access/gin
- ginxlog.c

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+13

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`‎src/backend/access/gin/ginxlog.c`

Lines changed: 13 additions & 13 deletions

Original file line number	Diff line number	Diff line change
`@@ -884,25 +884,25 @@ ginRedoDeleteListPages(XLogRecPtr lsn, XLogRecord *record)`
`884`	`884`	`* cannot get past a reader that is on, or due to visit, any page we are`
`885`	`885`	`* going to delete. New incoming readers will block behind our metapage`
`886`	`886`	`* lock and then see a fully updated page list.`
	`887`	`+ *`
	`888`	`+ * No full-page images are taken of the deleted pages. Instead, they are`
	`889`	`+ * re-initialized as empty, deleted pages. Their right-links don't need to`
	`890`	`+ * be preserved, because no new readers can see the pages, as explained`
	`891`	`+ * above.`
`887`	`892`	`*/`
`888`	`893`	`for (i=0;i<data->ndeleted;i++)`
`889`	`894`	`{`
`890`		`-Bufferbuffer=XLogReadBuffer(data->node,data->toDelete[i], false);`
	`895`	`+Bufferbuffer;`
	`896`	`+Pagepage;`
`891`	`897`
`892`		`-if (BufferIsValid(buffer))`
`893`		`-{`
`894`		`-Pagepage=BufferGetPage(buffer);`
`895`		`-`
`896`		`-if (lsn>PageGetLSN(page))`
`897`		`-{`
`898`		`-GinPageGetOpaque(page)->flags=GIN_DELETED;`
	`898`	`+buffer=XLogReadBuffer(data->node,data->toDelete[i], true);`
	`899`	`+page=BufferGetPage(buffer);`
	`900`	`+GinInitBuffer(buffer,GIN_DELETED);`
`899`	`901`
`900`		`-PageSetLSN(page,lsn);`
`901`		`-MarkBufferDirty(buffer);`
`902`		`-}`
	`902`	`+PageSetLSN(page,lsn);`
	`903`	`+MarkBufferDirty(buffer);`
`903`	`904`
`904`		`-UnlockReleaseBuffer(buffer);`
`905`		`-}`
	`905`	`+UnlockReleaseBuffer(buffer);`
`906`	`906`	`}`
`907`	`907`	`UnlockReleaseBuffer(metabuffer);`
`908`	`908`	`}`

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Commit4f7bb4b

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`‎src/backend/access/gin/ginxlog.c`

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