3484 enhance and document tail follow support
Reviewed by: Joshua M. Clulow <jmc@joyent.com>

@@ -29,10 +29,14 @@
 /*
  *      Copyright 1983,1984,1985,1986,1987,1988,1989 AT&T.
  *      All Rights Reserved
  */
 
+/*
+ * Copyright (c) 2013, Joyent, Inc. All rights reserved.
+ */
+
 #include <sys/param.h>
 #include <sys/types.h>
 #include <sys/systm.h>
 #include <sys/cred.h>
 #include <sys/time.h>

@@ -6651,20 +6655,24 @@
         if (error) {
                 VN_RELE(vp);
         } else {
                 vnode_t *tvp;
                 rnode4_t *trp;
-                /*
-                 * existing file got truncated, notify.
-                 */
                 tvp = vp;
                 if (vp->v_type == VREG) {
                         trp = VTOR4(vp);
                         if (IS_SHADOW(vp, trp))
                                 tvp = RTOV4(trp);
                 }
+
+                if (must_trunc) {
+                        /*
+                         * existing file got truncated, notify.
+                         */
                 vnevent_create(tvp, ct);
+                }
+
                 *vpp = vp;
         }
         return (error);
 
 create_otw: