Make rs_cindex and rs_ntuples unsigned

HeapScanDescData.rs_cindex and rs_ntuples can't be less than 0. All scan
types using the heap scan descriptor expect these values to be >= 0.
Make that expectation clear by making rs_cindex and rs_ntuples unsigned.

Also remove the test in heapam_scan_bitmap_next_tuple() that checks if
rs_cindex < 0. This was never true, but now that rs_cindex is unsigned,
it makes even less sense.

While we are at it, initialize both rs_cindex and rs_ntuples to 0 in
initscan().

Author: Melanie Plageman
Reviewed-by: Dilip Kumar
Discussion: https://postgr.es/m/CAAKRu_ZxF8cDCM_BFi_L-t%3DRjdCZYP1usd1Gd45mjHfZxm0nZw%40mail.gmail.com

src/backend/access/heap/heapam.c

@@ -378,6 +378,8 @@ initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
 	ItemPointerSetInvalid(&scan->rs_ctup.t_self);
 	scan->rs_cbuf = InvalidBuffer;
 	scan->rs_cblock = InvalidBlockNumber;
+	scan->rs_ntuples = 0;
+	scan->rs_cindex = 0;
 
 	/*
 	 * Initialize to ForwardScanDirection because it is most common and
@@ -943,8 +945,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 {
 	HeapTuple	tuple = &(scan->rs_ctup);
 	Page		page;
-	int			lineindex;
-	int			linesleft;
+	uint32		lineindex;
+	uint32		linesleft;
 
 	if (likely(scan->rs_inited))
 	{
@@ -989,6 +991,7 @@ continue_page:
 			ItemId		lpp;
 			OffsetNumber lineoff;
 
+			Assert(lineindex <= scan->rs_ntuples);
 			lineoff = scan->rs_vistuples[lineindex];
 			lpp = PageGetItemId(page, lineoff);
 			Assert(ItemIdIsNormal(lpp));

src/backend/access/heap/heapam_handler.c

@@ -2303,7 +2303,7 @@ heapam_scan_bitmap_next_tuple(TableScanDesc scan,
 	/*
 	 * Out of range?  If so, nothing more to look at on this page
 	 */
-	if (hscan->rs_cindex < 0 || hscan->rs_cindex >= hscan->rs_ntuples)
+	if (hscan->rs_cindex >= hscan->rs_ntuples)
 		return false;
 
 	targoffset = hscan->rs_vistuples[hscan->rs_cindex];

src/include/access/heapam.h

@@ -103,8 +103,8 @@ typedef struct HeapScanDescData
 	int			rs_empty_tuples_pending;
 
 	/* these fields only used in page-at-a-time mode and for bitmap scans */
-	int			rs_cindex;		/* current tuple's index in vistuples */
-	int			rs_ntuples;		/* number of visible tuples on page */
+	uint32		rs_cindex;		/* current tuple's index in vistuples */
+	uint32		rs_ntuples;		/* number of visible tuples on page */
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }			HeapScanDescData;
 typedef struct HeapScanDescData *HeapScanDesc;