Add XXX comment

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@530 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/cache_expire.c

@@ -2,6 +2,13 @@
  * $Id$
  *
  * Expiry of cached objects and execution of prefetcher
+ *
+ * XXX: Objects can linger on deathrow as long as a slow client
+ * XXX: tickles data away from it.  With many slow clients this could
+ * XXX: possibly make deathrow very long and make the hangman waste
+ * XXX: time.  The solution is to have another queue for such "pending
+ * XXX: cases" and have HSH_Deref() move them to deathrow when they
+ * XXX: are ready.
  */
 
 #include <unistd.h>