Fix some oversights in BRIN patch.

Remove HeapScanDescData.rs_initblock, which wasn't being used for anything
in the final version of the patch.

Fix IndexBuildHeapScan so that it supports syncscan again; the patch
broke synchronous scanning for index builds by forcing rs_startblk
to zero even when the caller did not care about that and had asked
for syncscan.

Add some commentary and usage defenses to heap_setscanlimits().

Fix heapam so that asking for rs_numblocks == 0 does what you would
reasonably expect.  As coded it amounted to requesting a whole-table
scan, because those "--x <= 0" tests on an unsigned variable would
behave surprisingly.

src/backend/access/heap/heapam.c

@@ -277,7 +277,6 @@ initscan(HeapScanDesc scan, ScanKey key, bool is_rescan)
 		scan->rs_startblock = 0;
 	}
 
-	scan->rs_initblock = 0;
 	scan->rs_numblocks = InvalidBlockNumber;
 	scan->rs_inited = false;
 	scan->rs_ctup.t_data = NULL;
@@ -302,11 +301,22 @@ initscan(HeapScanDesc scan, ScanKey key, bool is_rescan)
 		pgstat_count_heap_scan(scan->rs_rd);
 }
 
+/*
+ * heap_setscanlimits - restrict range of a heapscan
+ *
+ * startBlk is the page to start at
+ * numBlks is number of pages to scan (InvalidBlockNumber means "all")
+ */
 void
 heap_setscanlimits(HeapScanDesc scan, BlockNumber startBlk, BlockNumber numBlks)
 {
+	Assert(!scan->rs_inited);	/* else too late to change */
+	Assert(!scan->rs_syncscan); /* else rs_startblock is significant */
+
+	/* Check startBlk is valid (but allow case of zero blocks...) */
+	Assert(startBlk == 0 || startBlk < scan->rs_nblocks);
+
 	scan->rs_startblock = startBlk;
-	scan->rs_initblock = startBlk;
 	scan->rs_numblocks = numBlks;
 }
 
@@ -477,7 +487,7 @@ heapgettup(HeapScanDesc scan,
 			/*
 			 * return null immediately if relation is empty
 			 */
-			if (scan->rs_nblocks == 0)
+			if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
 			{
 				Assert(!BufferIsValid(scan->rs_cbuf));
 				tuple->t_data = NULL;
@@ -511,7 +521,7 @@ heapgettup(HeapScanDesc scan,
 			/*
 			 * return null immediately if relation is empty
 			 */
-			if (scan->rs_nblocks == 0)
+			if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
 			{
 				Assert(!BufferIsValid(scan->rs_cbuf));
 				tuple->t_data = NULL;
@@ -651,7 +661,7 @@ heapgettup(HeapScanDesc scan,
 		if (backward)
 		{
 			finished = (page == scan->rs_startblock) ||
-				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks <= 0 : false);
+				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);
 			if (page == 0)
 				page = scan->rs_nblocks;
 			page--;
@@ -662,7 +672,7 @@ heapgettup(HeapScanDesc scan,
 			if (page >= scan->rs_nblocks)
 				page = 0;
 			finished = (page == scan->rs_startblock) ||
-				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks <= 0 : false);
+				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);
 
 			/*
 			 * Report our new scan position for synchronization purposes. We
@@ -754,7 +764,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			/*
 			 * return null immediately if relation is empty
 			 */
-			if (scan->rs_nblocks == 0)
+			if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
 			{
 				Assert(!BufferIsValid(scan->rs_cbuf));
 				tuple->t_data = NULL;
@@ -785,7 +795,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			/*
 			 * return null immediately if relation is empty
 			 */
-			if (scan->rs_nblocks == 0)
+			if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
 			{
 				Assert(!BufferIsValid(scan->rs_cbuf));
 				tuple->t_data = NULL;
@@ -914,7 +924,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 		if (backward)
 		{
 			finished = (page == scan->rs_startblock) ||
-				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks <= 0 : false);
+				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);
 			if (page == 0)
 				page = scan->rs_nblocks;
 			page--;
@@ -925,7 +935,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			if (page >= scan->rs_nblocks)
 				page = 0;
 			finished = (page == scan->rs_startblock) ||
-				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks <= 0 : false);
+				(scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);
 
 			/*
 			 * Report our new scan position for synchronization purposes. We

src/backend/catalog/index.c

@@ -2168,7 +2168,8 @@ IndexBuildHeapScan(Relation heapRelation,
 /*
  * As above, except that instead of scanning the complete heap, only the given
  * number of blocks are scanned.  Scan to end-of-rel can be signalled by
- * passing InvalidBlockNumber as numblocks.
+ * passing InvalidBlockNumber as numblocks.  Note that restricting the range
+ * to scan cannot be done when requesting syncscan.
  */
 double
 IndexBuildHeapRangeScan(Relation heapRelation,
@@ -2251,7 +2252,14 @@ IndexBuildHeapRangeScan(Relation heapRelation,
 								allow_sync);	/* syncscan OK? */
 
 	/* set our scan endpoints */
+	if (!allow_sync)
 		heap_setscanlimits(scan, start_blockno, numblocks);
+	else
+	{
+		/* syncscan can only be requested on whole relation */
+		Assert(start_blockno == 0);
+		Assert(numblocks == InvalidBlockNumber);
+	}
 
 	reltuples = 0;
 

src/include/access/relscan.h

@@ -38,8 +38,8 @@ typedef struct HeapScanDescData
 	/* state set up at initscan time */
 	BlockNumber rs_nblocks;		/* total number of blocks in rel */
 	BlockNumber rs_startblock;	/* block # to start at */
-	BlockNumber rs_initblock;	/* block # to consider initial of rel */
+	BlockNumber rs_numblocks;	/* max number of blocks to scan */
-	BlockNumber rs_numblocks;	/* number of blocks to scan */
+	/* rs_numblocks is usually InvalidBlockNumber, meaning "scan whole rel" */
 	BufferAccessStrategy rs_strategy;	/* access strategy for reads */
 	bool		rs_syncscan;	/* report location to syncscan logic? */