In rewriteheap.c (used by VACUUM FULL and CLUSTER), calculate the tuple

length stored in the line pointer the same way it's calculated in the normal
heap_insert() codepath. As noted by Jeff Davis, the length stored by
raw_heap_insert() included padding but the one stored by the normal codepath
did not. While the mismatch seems to be harmless, inconsistency isn't good,
and the normal codepath has received a lot more testing over the years.

Backpatch to 8.3 where the heap rewrite code was introduced.

src/backend/access/heap/rewriteheap.c

@@ -636,7 +636,7 @@ raw_heap_insert(RewriteState state, HeapTuple tup)
 	}
 
 	/* And now we can insert the tuple into the page */
-	newoff = PageAddItem(page, (Item) heaptup->t_data, len,
+	newoff = PageAddItem(page, (Item) heaptup->t_data, heaptup->t_len,
 						 InvalidOffsetNumber, false, true);
 	if (newoff == InvalidOffsetNumber)
 		elog(ERROR, "failed to add tuple");