nbtree: Pass down MAXALIGN()'d itemsz for new item.

Refactor nbtinsert.c so that the final itemsz of each new non-pivot
tuple (the MAXALIGN()'d size) is determined once.  Most of the functions
used by leaf page inserts used the insertstate.itemsz value already.
This commit makes everything use insertstate.itemsz as standard
practice.  The goal is to decouple tuple size from "effective" tuple
size.  Making this distinction isn't truly necessary right now, but that
might change in the future.

Also explain why we consistently apply MAXALIGN() to get an effective
index tuple size.  This was rather unclear, in part because it isn't
actually strictly necessary right now.

src/backend/access/nbtree/nbtinsert.c

@@ -44,6 +44,7 @@ static void _bt_insertonpg(Relation rel, BTScanInsert itup_key,
 						   Buffer cbuf,
 						   BTStack stack,
 						   IndexTuple itup,
+						   Size itemsz,
 						   OffsetNumber newitemoff,
 						   int postingoff,
 						   bool split_only_page);
@@ -118,10 +119,18 @@ _bt_doinsert(Relation rel, IndexTuple itup,
 
 	/*
 	 * Fill in the BTInsertState working area, to track the current page and
-	 * position within the page to insert on
+	 * position within the page to insert on.
+	 *
+	 * Note that itemsz is passed down to lower level code that deals with
+	 * inserting the item.  It must be MAXALIGN()'d.  This ensures that space
+	 * accounting code consistently considers the alignment overhead that we
+	 * expect PageAddItem() will add later.  (Actually, index_form_tuple() is
+	 * already conservative about alignment, but we don't rely on that from
+	 * this distance.  Besides, preserving the "true" tuple size in index
+	 * tuple headers for the benefit of nbtsplitloc.c might happen someday.
+	 * Note that heapam does not MAXALIGN() each heap tuple's lp_len field.)
 	 */
 	insertstate.itup = itup;
-	/* PageAddItem will MAXALIGN(), but be consistent */
 	insertstate.itemsz = MAXALIGN(IndexTupleSize(itup));
 	insertstate.itup_key = itup_key;
 	insertstate.bounds_valid = false;
@@ -299,7 +308,8 @@ top:
 		newitemoff = _bt_findinsertloc(rel, &insertstate, checkingunique,
 									   stack, heapRel);
 		_bt_insertonpg(rel, itup_key, insertstate.buf, InvalidBuffer, stack,
-					   itup, newitemoff, insertstate.postingoff, false);
+					   itup, insertstate.itemsz, newitemoff,
+					   insertstate.postingoff, false);
 	}
 	else
 	{
@@ -1058,13 +1068,13 @@ _bt_insertonpg(Relation rel,
 			   Buffer cbuf,
 			   BTStack stack,
 			   IndexTuple itup,
+			   Size itemsz,
 			   OffsetNumber newitemoff,
 			   int postingoff,
 			   bool split_only_page)
 {
 	Page		page;
 	BTPageOpaque lpageop;
-	Size		itemsz;
 	IndexTuple	oposting = NULL;
 	IndexTuple	origitup = NULL;
 	IndexTuple	nposting = NULL;
@@ -1082,6 +1092,7 @@ _bt_insertonpg(Relation rel,
 		   BTreeTupleGetNAtts(itup, rel) <=
 		   IndexRelationGetNumberOfKeyAttributes(rel));
 	Assert(!BTreeTupleIsPosting(itup));
+	Assert(MAXALIGN(IndexTupleSize(itup)) == itemsz);
 
 	/*
 	 * Every internal page should have exactly one negative infinity item at
@@ -1096,10 +1107,6 @@ _bt_insertonpg(Relation rel,
 		elog(ERROR, "cannot insert to incompletely split page %u",
 			 BufferGetBlockNumber(buf));
 
-	itemsz = IndexTupleSize(itup);
-	itemsz = MAXALIGN(itemsz);	/* be safe, PageAddItem will do this but we
-								 * need to be consistent */
-
 	/*
 	 * Do we need to split an existing posting list item?
 	 */
@@ -2103,8 +2110,8 @@ _bt_insert_parent(Relation rel,
 
 		/* Recursively insert into the parent */
 		_bt_insertonpg(rel, NULL, pbuf, buf, stack->bts_parent,
-					   new_item, stack->bts_offset + 1, 0,
-					   is_only);
+					   new_item, MAXALIGN(IndexTupleSize(new_item)),
+					   stack->bts_offset + 1, 0, is_only);
 
 		/* be tidy */
 		pfree(new_item);