Add various assertions to heap pruning code.

These assertions document (and verify) our high level assumptions about
how pruning can and cannot affect existing items from target heap pages.
For example, one of the new assertions verifies that pruning does not
set a heap-only tuple to LP_DEAD.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wz=vhvBx1GjF+oueHh8YQcHoQYrMi0F0zFMHEr8yc4sCoA@mail.gmail.com

src/backend/access/heap/pruneheap.c

@@ -844,39 +844,115 @@ heap_page_prune_execute(Buffer buffer,
 {
 	Page		page = (Page) BufferGetPage(buffer);
 	OffsetNumber *offnum;
-	int			i;
+	HeapTupleHeader htup PG_USED_FOR_ASSERTS_ONLY;
 
 	/* Shouldn't be called unless there's something to do */
 	Assert(nredirected > 0 || ndead > 0 || nunused > 0);
 
 	/* Update all redirected line pointers */
 	offnum = redirected;
-	for (i = 0; i < nredirected; i++)
+	for (int i = 0; i < nredirected; i++)
 	{
 		OffsetNumber fromoff = *offnum++;
 		OffsetNumber tooff = *offnum++;
 		ItemId		fromlp = PageGetItemId(page, fromoff);
+		ItemId		tolp PG_USED_FOR_ASSERTS_ONLY;
+
+#ifdef USE_ASSERT_CHECKING
+
+		/*
+		 * Any existing item that we set as an LP_REDIRECT (any 'from' item)
+		 * must be the first item from a HOT chain.  If the item has tuple
+		 * storage then it can't be a heap-only tuple.  Otherwise we are just
+		 * maintaining an existing LP_REDIRECT from an existing HOT chain that
+		 * has been pruned at least once before now.
+		 */
+		if (!ItemIdIsRedirected(fromlp))
+		{
+			Assert(ItemIdHasStorage(fromlp) && ItemIdIsNormal(fromlp));
+
+			htup = (HeapTupleHeader) PageGetItem(page, fromlp);
+			Assert(!HeapTupleHeaderIsHeapOnly(htup));
+		}
+		else
+		{
+			/* We shouldn't need to redundantly set the redirect */
+			Assert(ItemIdGetRedirect(fromlp) != tooff);
+		}
+
+		/*
+		 * The item that we're about to set as an LP_REDIRECT (the 'from'
+		 * item) will point to an existing item (the 'to' item) that is
+		 * already a heap-only tuple.  There can be at most one LP_REDIRECT
+		 * item per HOT chain.
+		 *
+		 * We need to keep around an LP_REDIRECT item (after original
+		 * non-heap-only root tuple gets pruned away) so that it's always
+		 * possible for VACUUM to easily figure out what TID to delete from
+		 * indexes when an entire HOT chain becomes dead.  A heap-only tuple
+		 * can never become LP_DEAD; an LP_REDIRECT item or a regular heap
+		 * tuple can.
+		 */
+		tolp = PageGetItemId(page, tooff);
+		Assert(ItemIdHasStorage(tolp) && ItemIdIsNormal(tolp));
+		htup = (HeapTupleHeader) PageGetItem(page, tolp);
+		Assert(HeapTupleHeaderIsHeapOnly(htup));
+#endif
 
 		ItemIdSetRedirect(fromlp, tooff);
 	}
 
 	/* Update all now-dead line pointers */
 	offnum = nowdead;
-	for (i = 0; i < ndead; i++)
+	for (int i = 0; i < ndead; i++)
 	{
 		OffsetNumber off = *offnum++;
 		ItemId		lp = PageGetItemId(page, off);
 
+#ifdef USE_ASSERT_CHECKING
+
+		/*
+		 * An LP_DEAD line pointer must be left behind when the original item
+		 * (which is dead to everybody) could still be referenced by a TID in
+		 * an index.  This should never be necessary with any individual
+		 * heap-only tuple item, though. (It's not clear how much of a problem
+		 * that would be, but there is no reason to allow it.)
+		 */
+		if (ItemIdHasStorage(lp))
+		{
+			Assert(ItemIdIsNormal(lp));
+			htup = (HeapTupleHeader) PageGetItem(page, lp);
+			Assert(!HeapTupleHeaderIsHeapOnly(htup));
+		}
+		else
+		{
+			/* Whole HOT chain becomes dead */
+			Assert(ItemIdIsRedirected(lp));
+		}
+#endif
+
 		ItemIdSetDead(lp);
 	}
 
 	/* Update all now-unused line pointers */
 	offnum = nowunused;
-	for (i = 0; i < nunused; i++)
+	for (int i = 0; i < nunused; i++)
 	{
 		OffsetNumber off = *offnum++;
 		ItemId		lp = PageGetItemId(page, off);
 
+#ifdef USE_ASSERT_CHECKING
+
+		/*
+		 * Only heap-only tuples can become LP_UNUSED during pruning.  They
+		 * don't need to be left in place as LP_DEAD items until VACUUM gets
+		 * around to doing index vacuuming.
+		 */
+		Assert(ItemIdHasStorage(lp) && ItemIdIsNormal(lp));
+		htup = (HeapTupleHeader) PageGetItem(page, lp);
+		Assert(HeapTupleHeaderIsHeapOnly(htup));
+#endif
+
 		ItemIdSetUnused(lp);
 	}