Fix handling of all-zero pages in SP-GiST vacuum.

SP-GiST initialized an all-zeros page at vacuum, but that was not
WAL-logged, which is not safe. You might get a torn page write, when it gets
flushed to disk, and end-up with a half-initialized index page. To fix,
leave it in the all-zeros state, and add it to the FSM. It will be
initialized when reused. Also don't set the page-deleted flag when recycling
an empty page. That was also not WAL-logged, and a torn write of that would
cause the page to have an invalid checksum.

Backpatch to 9.2, where SP-GiST indexes were added.

Author: hlinnaka

src/backend/access/spgist/spgvacuum.c

@@ -621,14 +621,10 @@ spgvacuumpage(spgBulkDeleteState *bds, BlockNumber blkno)
 	{
 		/*
 		 * We found an all-zero page, which could happen if the database
-		 * crashed just after extending the file.  Initialize and recycle it.
+		 * crashed just after extending the file.  Recycle it.
 		 */
-		SpGistInitBuffer(buffer, 0);
-		SpGistPageSetDeleted(page);
-		/* We don't bother to WAL-log this action; easy to redo */
-		MarkBufferDirty(buffer);
 	}
-	else if (SpGistPageIsDeleted(page))
+	else if (PageIsEmpty(page))
 	{
 		/* nothing to do */
 	}
@@ -654,30 +650,23 @@ spgvacuumpage(spgBulkDeleteState *bds, BlockNumber blkno)
 	/*
 	 * The root pages must never be deleted, nor marked as available in FSM,
 	 * because we don't want them ever returned by a search for a place to put
-	 * a new tuple.  Otherwise, check for empty/deletable page, and make sure
-	 * FSM knows about it.
+	 * a new tuple.  Otherwise, check for empty page, and make sure the FSM
+	 * knows about it.
 	 */
 	if (!SpGistBlockIsRoot(blkno))
 	{
-		/* If page is now empty, mark it deleted */
-		if (PageIsEmpty(page) && !SpGistPageIsDeleted(page))
-		{
-			SpGistPageSetDeleted(page);
-			/* We don't bother to WAL-log this action; easy to redo */
-			MarkBufferDirty(buffer);
-		}
-
-		if (SpGistPageIsDeleted(page))
+		if (PageIsEmpty(page))
 		{
 			RecordFreeIndexPage(index, blkno);
 			bds->stats->pages_deleted++;
 		}
 		else
+		{
+			SpGistSetLastUsedPage(index, buffer);
 			bds->lastFilledBlock = blkno;
+		}
 	}
 
-	SpGistSetLastUsedPage(index, buffer);
-
 	UnlockReleaseBuffer(buffer);
 }
 

src/include/access/spgist_private.h

@@ -48,14 +48,14 @@ typedef SpGistPageOpaqueData *SpGistPageOpaque;
 
 /* Flag bits in page special space */
 #define SPGIST_META			(1<<0)
-#define SPGIST_DELETED		(1<<1)
+#define SPGIST_DELETED		(1<<1)		/* never set, but keep for backwards
+										 * compatibility */
 #define SPGIST_LEAF			(1<<2)
 #define SPGIST_NULLS		(1<<3)
 
 #define SpGistPageGetOpaque(page) ((SpGistPageOpaque) PageGetSpecialPointer(page))
 #define SpGistPageIsMeta(page) (SpGistPageGetOpaque(page)->flags & SPGIST_META)
 #define SpGistPageIsDeleted(page) (SpGistPageGetOpaque(page)->flags & SPGIST_DELETED)
-#define SpGistPageSetDeleted(page) (SpGistPageGetOpaque(page)->flags |= SPGIST_DELETED)
 #define SpGistPageIsLeaf(page) (SpGistPageGetOpaque(page)->flags & SPGIST_LEAF)
 #define SpGistPageStoresNulls(page) (SpGistPageGetOpaque(page)->flags & SPGIST_NULLS)