Fix bug in GIN posting tree root creation.

The root page is filled with as many items as fit, and the rest are inserted
using normal insertions. However, I fumbled the variable names, and the code
actually memcpy'd all the items on the page, overflowing the buffer. While
at it, rename the variable to make the distinction more clear.

Reported by Teodor Sigaev. This bug was introduced by my recent
refactorings, so no backpatching required.

Author: hlinnaka

src/backend/access/gin/gindatapage.c

@@ -623,10 +623,10 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
 	BlockNumber blkno;
 	Buffer		buffer;
 	Page		page;
-	int			itemsCount;
+	int			nrootitems;
 
 	/* Calculate how many TIDs will fit on first page. */
-	itemsCount = Min(nitems, GinMaxLeafDataItems);
+	nrootitems = Min(nitems, GinMaxLeafDataItems);
 
 	/*
 	 * Create the root page.
@@ -638,8 +638,8 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
 	START_CRIT_SECTION();
 
 	GinInitBuffer(buffer, GIN_DATA | GIN_LEAF);
-	memcpy(GinDataPageGetData(page), items, sizeof(ItemPointerData) * nitems);
-	GinPageGetOpaque(page)->maxoff = nitems;
+	memcpy(GinDataPageGetData(page), items, sizeof(ItemPointerData) * nrootitems);
+	GinPageGetOpaque(page)->maxoff = nrootitems;
 
 	MarkBufferDirty(buffer);
 
@@ -651,7 +651,7 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
 
 		data.node = index->rd_node;
 		data.blkno = blkno;
-		data.nitem = nitems;
+		data.nitem = nrootitems;
 
 		rdata[0].buffer = InvalidBuffer;
 		rdata[0].data = (char *) &data;
@@ -660,7 +660,7 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
 
 		rdata[1].buffer = InvalidBuffer;
 		rdata[1].data = (char *) items;
-		rdata[1].len = sizeof(ItemPointerData) * itemsCount;
+		rdata[1].len = sizeof(ItemPointerData) * nrootitems;
 		rdata[1].next = NULL;
 
 		recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_CREATE_PTREE, rdata);
@@ -678,16 +678,16 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
 	/*
 	 * Add any remaining TIDs to the newly-created posting tree.
 	 */
-	if (itemsCount < nitems)
+	if (nitems > nrootitems)
 	{
 		GinPostingTreeScan *gdi;
 
 		gdi = ginPrepareScanPostingTree(index, blkno, FALSE);
 		gdi->btree.isBuild = (buildStats != NULL);
 
 		ginInsertItemPointers(gdi,
-							  items + itemsCount,
-							  nitems - itemsCount,
+							  items + nrootitems,
+							  nitems - nrootitems,
 							  buildStats);
 
 		pfree(gdi);