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

@@ -22,10 +22,14 @@
 /*
  * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
  */
 
+/*
+ * Copyright (c) 2013, Joyent, Inc. All rights reserved.
+ */
+
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/t_lock.h>
 #include <sys/systm.h>
 #include <sys/sysmacros.h>

@@ -976,10 +980,12 @@
         } else {
                 error = tdirlookup(parent, nm, &oldtp, cred);
         }
 
         if (error == 0) {       /* name found */
+                boolean_t trunc = B_FALSE;
+
                 ASSERT(oldtp);
 
                 rw_enter(&oldtp->tn_rwlock, RW_WRITER);
 
                 /*

@@ -1003,10 +1009,11 @@
                 if ((*vpp)->v_type == VREG && (vap->va_mask & AT_SIZE) &&
                     vap->va_size == 0) {
                         rw_enter(&oldtp->tn_contents, RW_WRITER);
                         (void) tmpnode_trunc(tm, oldtp, 0);
                         rw_exit(&oldtp->tn_contents);
+                        trunc = B_TRUE;
                 }
                 rw_exit(&oldtp->tn_rwlock);
                 if (IS_DEVVP(*vpp)) {
                         struct vnode *newvp;
 

@@ -1017,13 +1024,13 @@
                                 return (ENOSYS);
                         }
                         *vpp = newvp;
                 }
 
-                if (error == 0) {
+                if (trunc)
                         vnevent_create(*vpp, ct);
-                }
+
                 return (0);
         }
 
         if (error != ENOENT)
                 return (error);