Convert Assert checking for empty page into a regular test and elog.

The consequences of overwriting a non-empty page are bad enough that
we should not omit this test in production builds.

src/backend/access/heap/hio.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/hio.c,v 1.59 2005/11/22 18:17:06 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/hio.c,v 1.60 2006/01/06 00:15:50 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -281,10 +281,17 @@ RelationGetBufferForTuple(Relation relation, Size len,
 		UnlockRelationForExtension(relation, ExclusiveLock);
 
 	/*
-	 * We need to initialize the empty new page.
+	 * We need to initialize the empty new page.  Double-check that it really
+	 * is empty (this should never happen, but if it does we don't want to
+	 * risk wiping out valid data).
 	 */
 	pageHeader = (Page) BufferGetPage(buffer);
-	Assert(PageIsNew((PageHeader) pageHeader));
+
+	if (!PageIsNew((PageHeader) pageHeader))
+		elog(ERROR, "page %u of relation \"%s\" should be empty but is not",
+			 BufferGetBlockNumber(buffer),
+			 RelationGetRelationName(relation));
+
 	PageInit(pageHeader, BufferGetPageSize(buffer), 0);
 
 	if (len > PageGetFreeSpace(pageHeader))