shutdown node VS disconnect all hooks

[Gleb Smirnoff]

  Hi collegues,

  I'm again with the issue about node destruction process. If you
already forgot the problem, I'll remind you: in RELENG_4 node cuts down
all its hooks itself from shutdown method, while in HEAD it is done 
in ng_rmnode() _before_ calling shutdown method. This breaks
ng_tee behavior, and as I have noted, having hooks connected in
shutdown method may be useful sometimes.

  OK, If we destroy node in such a sequence

  1. nodes shutdown method
  2. ng_destroy_hook() on all hooks
  3. ng_flush_input_queue()

  in some cases we may have node be lost when its shutdown method calls
NG_NODE_UNREF. Ok, I have tried to put one more ref on node just before
calling its shutdown method. Seems like it works! However we may catch
panic in nodes which do not expect that ng_type_disconnect() may
be called after ng_type_shutdown(). Since RELENG_4 the following nodes
had been changed to don't expect this sequence:

  ng_socket
  ng_atmllc
  ng_bridge
  ng_eiface
  ng_etf
  ng_ether
  ng_frame_relay
  ng_gif
  ng_gif_demux
  ng_iface
  ng_l2tp
  ng_lmi
  ng_mppc
  ng_one2many
  ng_ppp
  ng_pppoe
  ng_pptpgre
  ng_rfc1490
  ng_source
  ng_sppp
  ng_tty
  ng_vjc
  ng_vlan

They all FREE priv in shutdown method, but use it in disconnect method.
The problem can be solved by inserting ng_cutlinks() into beginning of 
shutdown method for each of above nodes like it is done in RELENG_4.
Ok, we need to edit more than 20 nodes to get back ng_tees behavior. I'm
sure you don't like it :)

  So, what about adding one more netgraph method into struct ng_type, say
ng_preshutdown_t? This method is called first in ng_rmnode(), and then 
current shutdown sequence is followed. We will set it to NULL in all existing
nodes, and create a method in ng_tee, which calls ng_bypass(). Future
implementations may use this method to send "goodbye" messages down hooks when
shutting down.
  And this will be a POLA-friendly solution - we will not lose functionality
of RELENG_4 (which mpd relies on), and we will not break nodes which
rely on current shutdown sequence.

  What's your opinion? If it is positive - I'll send patches.

-- 
Totus tuus, Glebius.
GLEBIUS-RIPN GLEB-RIPE