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

*** 22,31 ****
--- 22,35 ----
  /*
   * 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>
*** 976,985 ****
--- 980,991 ----
          } 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);
  
                  /*
*** 1003,1012 ****
--- 1009,1019 ----
                  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;
  
*** 1017,1029 ****
                                  return (ENOSYS);
                          }
                          *vpp = newvp;
                  }
  
!                 if (error == 0) {
                          vnevent_create(*vpp, ct);
!                 }
                  return (0);
          }
  
          if (error != ENOENT)
                  return (error);
--- 1024,1036 ----
                                  return (ENOSYS);
                          }
                          *vpp = newvp;
                  }
  
!                 if (trunc)
                          vnevent_create(*vpp, ct);
! 
                  return (0);
          }
  
          if (error != ENOENT)
                  return (error);