Make NOTIFY_PAYLOAD_MAX_LENGTH depend explicitly on BLCKSZ and

NAMEDATALEN, so this code doesn't go nuts with smaller than default
BLCKSZ or larger than default NAMEDATALEN.  The standard value is
still exactly 8000.

src/backend/commands/async.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.151 2010/02/16 22:34:43 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.152 2010/02/17 00:52:09 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -137,9 +137,12 @@
 /*
  * Maximum size of a NOTIFY payload, including terminating NULL.  This
  * must be kept small enough so that a notification message fits on one
- * SLRU page.
+ * SLRU page.  The magic fudge factor here is noncritical as long as it's
+ * more than AsyncQueueEntryEmptySize --- we make it significantly bigger
+ * than that, so changes in that data structure won't affect user-visible
+ * restrictions.
  */
-#define NOTIFY_PAYLOAD_MAX_LENGTH	8000
+#define NOTIFY_PAYLOAD_MAX_LENGTH	(BLCKSZ - NAMEDATALEN - 128)
 
 /*
  * Struct representing an entry in the global notify queue