Cosmetic cleanup of ginInsertValue().

Make it clearer that the passed stack mustn't be empty, and that we
are not supposed to fall off the end of the stack in the main loop.
Tighten the loop that extracts the root block number, too.

Markus Wanner and Tom Lane

src/backend/access/gin/ginbtree.c

@@ -275,20 +275,22 @@ ginFindParents(GinBtree btree, GinBtreeStack *stack,
 void
 ginInsertValue(GinBtree btree, GinBtreeStack *stack, GinStatsData *buildStats)
 {
-	GinBtreeStack *parent = stack;
-	BlockNumber rootBlkno = InvalidBuffer;
+	GinBtreeStack *parent;
+	BlockNumber rootBlkno;
 	Page		page,
 				rpage,
 				lpage;
 
-	/* remember root BlockNumber */
-	while (parent)
-	{
-		rootBlkno = parent->blkno;
+	/* extract root BlockNumber from stack */
+	Assert(stack != NULL);
+	parent = stack;
+	while (parent->parent)
 		parent = parent->parent;
-	}
+	rootBlkno = parent->blkno;
+	Assert(BlockNumberIsValid(rootBlkno));
 
-	while (stack)
+	/* this loop crawls up the stack until the insertion is complete */
+	for (;;)
 	{
 		XLogRecData *rdata;
 		BlockNumber savedRightLink;
@@ -457,7 +459,7 @@ ginInsertValue(GinBtree btree, GinBtreeStack *stack, GinStatsData *buildStats)
 				 */
 				ginFindParents(btree, stack, rootBlkno);
 				parent = stack->parent;
-				page = BufferGetPage(parent->buffer);
+				Assert(parent != NULL);
 				break;
 			}