Don't try to truncate multixact SLRU files in checkpoints done during xlog

recovery.  In the first place, it doesn't work because slru's
latest_page_number isn't set up yet (this is why we've been hearing reports
of strange "apparent wraparound" log messages during crash recovery, but
only from people who'd managed to advance their next-mxact counters some
considerable distance from 0).  In the second place, it seems a bit unwise
to be throwing away data during crash recovery anwyway.  This latter
consideration convinces me to just disable truncation during recovery,
rather than computing latest_page_number and pushing ahead.

src/backend/access/transam/multixact.c

@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.19 2006/07/13 16:49:13 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.20 2006/07/20 00:46:42 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1493,9 +1493,14 @@ CheckPointMultiXact(void)
 
 	/*
 	 * Truncate the SLRU files.  This could be done at any time, but
-	 * checkpoint seems a reasonable place for it.
+	 * checkpoint seems a reasonable place for it.  There is one exception:
+	 * if we are called during xlog recovery, then shared->latest_page_number
+	 * isn't valid (because StartupMultiXact hasn't been called yet) and
+	 * so SimpleLruTruncate would get confused.  It seems best not to risk
+	 * removing any data during recovery anyway, so don't truncate.
 	 */
-	TruncateMultiXact();
+	if (!InRecovery)
+		TruncateMultiXact();
 }
 
 /*