nbtree: Make BTMaxItemSize into object-like macro.

Make nbtree's "1/3 of a page limit" BTMaxItemSize function-like macro
(which accepts a "page" argument) into an object-like macro that can be
used from code that doesn't have convenient access to an nbtree page.

Preparation for an upcoming patch that adds skip scan to nbtree.
Parallel index scans that use skip scan will serialize datums (not just
SAOP array subscripts) when scheduling primitive scans.  BTMaxItemSize
will be used by btestimateparallelscan to determine how much DSM to
request.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wz=H_RG5weNGeUG_TkK87tRBnH9mGCQj6WpM4V4FNWKv2g@mail.gmail.com

contrib/amcheck/verify_nbtree.c

@@ -1597,8 +1597,7 @@ bt_target_page_check(BtreeCheckState *state)
 		 */
 		lowersizelimit = skey->heapkeyspace &&
 			(P_ISLEAF(topaque) || BTreeTupleGetHeapTID(itup) == NULL);
-		if (tupsize > (lowersizelimit ? BTMaxItemSize(state->target) :
-					   BTMaxItemSizeNoHeapTid(state->target)))
+		if (tupsize > (lowersizelimit ? BTMaxItemSize : BTMaxItemSizeNoHeapTid))
 		{
 			ItemPointer tid = BTreeTupleGetPointsToTID(itup);
 			char	   *itid,

src/backend/access/nbtree/nbtdedup.c

@@ -84,7 +84,7 @@ _bt_dedup_pass(Relation rel, Buffer buf, IndexTuple newitem, Size newitemsz,
 	state = (BTDedupState) palloc(sizeof(BTDedupStateData));
 	state->deduplicate = true;
 	state->nmaxitems = 0;
-	state->maxpostingsize = Min(BTMaxItemSize(page) / 2, INDEX_SIZE_MASK);
+	state->maxpostingsize = Min(BTMaxItemSize / 2, INDEX_SIZE_MASK);
 	/* Metadata about base tuple of current pending posting list */
 	state->base = NULL;
 	state->baseoff = InvalidOffsetNumber;
@@ -568,7 +568,7 @@ _bt_dedup_finish_pending(Page newpage, BTDedupState state)
 		/* Use original, unchanged base tuple */
 		tuplesz = IndexTupleSize(state->base);
 		Assert(tuplesz == MAXALIGN(IndexTupleSize(state->base)));
-		Assert(tuplesz <= BTMaxItemSize(newpage));
+		Assert(tuplesz <= BTMaxItemSize);
 		if (PageAddItem(newpage, (Item) state->base, tuplesz, tupoff,
 						false, false) == InvalidOffsetNumber)
 			elog(ERROR, "deduplication failed to add tuple to page");
@@ -588,7 +588,7 @@ _bt_dedup_finish_pending(Page newpage, BTDedupState state)
 		state->intervals[state->nintervals].nitems = state->nitems;
 
 		Assert(tuplesz == MAXALIGN(IndexTupleSize(final)));
-		Assert(tuplesz <= BTMaxItemSize(newpage));
+		Assert(tuplesz <= BTMaxItemSize);
 		if (PageAddItem(newpage, (Item) final, tuplesz, tupoff, false,
 						false) == InvalidOffsetNumber)
 			elog(ERROR, "deduplication failed to add tuple to page");

src/backend/access/nbtree/nbtinsert.c

@@ -827,7 +827,7 @@ _bt_findinsertloc(Relation rel,
 	opaque = BTPageGetOpaque(page);
 
 	/* Check 1/3 of a page restriction */
-	if (unlikely(insertstate->itemsz > BTMaxItemSize(page)))
+	if (unlikely(insertstate->itemsz > BTMaxItemSize))
 		_bt_check_third_page(rel, heapRel, itup_key->heapkeyspace, page,
 							 insertstate->itup);
 

src/backend/access/nbtree/nbtsort.c

@@ -829,7 +829,7 @@ _bt_buildadd(BTWriteState *wstate, BTPageState *state, IndexTuple itup,
 	 * make use of the reserved space.  This should never fail on internal
 	 * pages.
 	 */
-	if (unlikely(itupsz > BTMaxItemSize(npage)))
+	if (unlikely(itupsz > BTMaxItemSize))
 		_bt_check_third_page(wstate->index, wstate->heap, isleaf, npage,
 							 itup);
 
@@ -1305,7 +1305,7 @@ _bt_load(BTWriteState *wstate, BTSpool *btspool, BTSpool *btspool2)
 				 */
 				dstate->maxpostingsize = MAXALIGN_DOWN((BLCKSZ * 10 / 100)) -
 					sizeof(ItemIdData);
-				Assert(dstate->maxpostingsize <= BTMaxItemSize((Page) state->btps_buf) &&
+				Assert(dstate->maxpostingsize <= BTMaxItemSize &&
 					   dstate->maxpostingsize <= INDEX_SIZE_MASK);
 				dstate->htids = palloc(dstate->maxpostingsize);
 

src/backend/access/nbtree/nbtutils.c

@@ -3245,7 +3245,7 @@ _bt_check_third_page(Relation rel, Relation heap, bool needheaptidspace,
 	itemsz = MAXALIGN(IndexTupleSize(newtup));
 
 	/* Double check item size against limit */
-	if (itemsz <= BTMaxItemSize(page))
+	if (itemsz <= BTMaxItemSize)
 		return;
 
 	/*
@@ -3253,7 +3253,7 @@ _bt_check_third_page(Relation rel, Relation heap, bool needheaptidspace,
 	 * index uses version 2 or version 3, or that page is an internal page, in
 	 * which case a slightly higher limit applies.
 	 */
-	if (!needheaptidspace && itemsz <= BTMaxItemSizeNoHeapTid(page))
+	if (!needheaptidspace && itemsz <= BTMaxItemSizeNoHeapTid)
 		return;
 
 	/*
@@ -3270,8 +3270,7 @@ _bt_check_third_page(Relation rel, Relation heap, bool needheaptidspace,
 			 errmsg("index row size %zu exceeds btree version %u maximum %zu for index \"%s\"",
 					itemsz,
 					needheaptidspace ? BTREE_VERSION : BTREE_NOVAC_VERSION,
-					needheaptidspace ? BTMaxItemSize(page) :
-					BTMaxItemSizeNoHeapTid(page),
+					needheaptidspace ? BTMaxItemSize : BTMaxItemSizeNoHeapTid,
 					RelationGetRelationName(rel)),
 			 errdetail("Index row references tuple (%u,%u) in relation \"%s\".",
 					   ItemPointerGetBlockNumber(BTreeTupleGetHeapTID(newtup)),

src/backend/access/nbtree/nbtxlog.c

@@ -483,7 +483,7 @@ btree_xlog_dedup(XLogReaderState *record)
 		state->deduplicate = true;	/* unused */
 		state->nmaxitems = 0;	/* unused */
 		/* Conservatively use larger maxpostingsize than primary */
-		state->maxpostingsize = BTMaxItemSize(page);
+		state->maxpostingsize = BTMaxItemSize;
 		state->base = NULL;
 		state->baseoff = InvalidOffsetNumber;
 		state->basetupsize = 0;

src/include/access/nbtree.h

@@ -161,13 +161,13 @@ typedef struct BTMetaPageData
  * a heap index tuple to make space for a tiebreaker heap TID
  * attribute, which we account for here.
  */
-#define BTMaxItemSize(page) \
-	(MAXALIGN_DOWN((PageGetPageSize(page) - \
+#define BTMaxItemSize \
+	(MAXALIGN_DOWN((BLCKSZ - \
 					MAXALIGN(SizeOfPageHeaderData + 3*sizeof(ItemIdData)) - \
 					MAXALIGN(sizeof(BTPageOpaqueData))) / 3) - \
 					MAXALIGN(sizeof(ItemPointerData)))
-#define BTMaxItemSizeNoHeapTid(page) \
-	MAXALIGN_DOWN((PageGetPageSize(page) - \
+#define BTMaxItemSizeNoHeapTid \
+	MAXALIGN_DOWN((BLCKSZ - \
 				   MAXALIGN(SizeOfPageHeaderData + 3*sizeof(ItemIdData)) - \
 				   MAXALIGN(sizeof(BTPageOpaqueData))) / 3)