Fix bugs in contrib/pg_visibility.

collect_corrupt_items() failed to initialize tuple.t_self.  While
HeapTupleSatisfiesVacuum() doesn't actually use that value, it does
Assert that it's valid, so that the code would dump core if ip_posid
chanced to be zero.  (That's somewhat unlikely, which probably explains
how this got missed.  In any case it wouldn't matter for field use.)

Also, collect_corrupt_items was returning the wrong TIDs, that is the
contents of t_ctid rather than the tuple's own location.  This would
be the same thing in simple cases, but it could be wrong if, for
example, a past update attempt had been rolled back, leaving a live
tuple whose t_ctid doesn't point at itself.

Also, in pg_visibility(), guard against trying to read a page past
the end of the rel.  The VM code handles inquiries beyond the end
of the map by silently returning zeroes, and it seems like we should
do the same thing here.

I ran into the assertion failure while using pg_visibility to check
pg_upgrade's behavior, and then noted the other problems while
reading the code.

Report: <29043.1475288648@sss.pgh.pa.us>

Author: tglsfdc

contrib/pg_visibility/pg_visibility.c

@@ -3,6 +3,8 @@
  * pg_visibility.c
  *	  display visibility map information and page-level visibility bits
  *
+ * Copyright (c) 2016, PostgreSQL Global Development Group
+ *
  *	  contrib/pg_visibility/pg_visibility.c
  *-------------------------------------------------------------------------
  */
@@ -54,6 +56,10 @@ static bool tuple_all_visible(HeapTuple tup, TransactionId OldestXmin,
 
 /*
  * Visibility map information for a single block of a relation.
+ *
+ * Note: the VM code will silently return zeroes for pages past the end
+ * of the map, so we allow probes up to MaxBlockNumber regardless of the
+ * actual relation size.
  */
 Datum
 pg_visibility_map(PG_FUNCTION_ARGS)
@@ -122,13 +128,22 @@ pg_visibility(PG_FUNCTION_ARGS)
 	values[0] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0);
 	values[1] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0);
 
-	buffer = ReadBuffer(rel, blkno);
-	LockBuffer(buffer, BUFFER_LOCK_SHARE);
+	/* Here we have to explicitly check rel size ... */
+	if (blkno < RelationGetNumberOfBlocks(rel))
+	{
+		buffer = ReadBuffer(rel, blkno);
+		LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
-	page = BufferGetPage(buffer);
-	values[2] = BoolGetDatum(PageIsAllVisible(page));
+		page = BufferGetPage(buffer);
+		values[2] = BoolGetDatum(PageIsAllVisible(page));
 
-	UnlockReleaseBuffer(buffer);
+		UnlockReleaseBuffer(buffer);
+	}
+	else
+	{
+		/* As with the vismap, silently return 0 for pages past EOF */
+		values[2] = BoolGetDatum(false);
+	}
 
 	relation_close(rel, AccessShareLock);
 
@@ -611,14 +626,13 @@ collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
 			/* Dead line pointers are neither all-visible nor frozen. */
 			if (ItemIdIsDead(itemid))
 			{
-				ItemPointerData tid;
-
-				ItemPointerSet(&tid, blkno, offnum);
-				record_corrupt_item(items, &tid);
+				ItemPointerSet(&(tuple.t_self), blkno, offnum);
+				record_corrupt_item(items, &tuple.t_self);
 				continue;
 			}
 
 			/* Initialize a HeapTupleData structure for checks below. */
+			ItemPointerSet(&(tuple.t_self), blkno, offnum);
 			tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
 			tuple.t_len = ItemIdGetLength(itemid);
 			tuple.t_tableOid = relid;
@@ -649,12 +663,12 @@ collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
 				RecomputedOldestXmin = GetOldestXmin(NULL, true);
 
 				if (!TransactionIdPrecedes(OldestXmin, RecomputedOldestXmin))
-					record_corrupt_item(items, &tuple.t_data->t_ctid);
+					record_corrupt_item(items, &tuple.t_self);
 				else
 				{
 					OldestXmin = RecomputedOldestXmin;
 					if (!tuple_all_visible(&tuple, OldestXmin, buffer))
-						record_corrupt_item(items, &tuple.t_data->t_ctid);
+						record_corrupt_item(items, &tuple.t_self);
 				}
 			}
 
@@ -665,7 +679,7 @@ collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
 			if (check_frozen)
 			{
 				if (heap_tuple_needs_eventual_freeze(tuple.t_data))
-					record_corrupt_item(items, &tuple.t_data->t_ctid);
+					record_corrupt_item(items, &tuple.t_self);
 			}
 		}