[RFC/PATCH] check_client_connection: document local-only requirement

[Eric Wong <normalperson@yhbt.net>]

In my testing, only dropped clients over Unix domain sockets or
loopback TCP were detected with this option.  Since many
nginx+unicorn combinations run on the same host, this is not a
problem.

Furthermore, tcp_nodelay:true appears to work over loopback,
so remove the requirement for tcp_nodelay:false.
---
  Eric Wong <normalperson@yhbt.net> wrote:
  > Tom Burns <tom.burns@jadedpixel.com> wrote:
  > > +    if set[:check_client_connection]
  > > +      set[:listeners].each do |address|
  > > +        if set[:listener_opts][address][:tcp_nopush] == true
  > > +          raise ArgumentError,
  > > +            "check_client_connection is incompatible with tcp_nopush:true"
  > > +        end
  > 
  > Btw, were you using:
  > 
  > 1) TCP over loopback (bound to 0.0.0.0, client comes from 127.0.0.1)
  > 2) TCP over a LAN (separate client/server hosts)
  > 3) Unix domain socket
  > 
  > I wonder if we can drop the below hunk for checking :tcp_nodelay,
  > and document that check_client_connection requires the client to
  > either be from a Unix domain socket or TCP loopback to work.
  > 
  > > +        if set[:listener_opts][address][:tcp_nodelay] == true
  > > +          raise ArgumentError,
  > > +            "check_client_connection is incompatible with tcp_nodelay:true"
  > > +        end
  > 
  > I couldn't get 2) to work with either value of tcp_nodelay.  My small
  > LAN at home only has ~0.100ms latency.
  > 
  > Happily, with TCP over loopback (on Linux 3.6), either value of
  > tcp_nodelay works, so the tcp_nodelay check seems unnecessary after
  > all.

 lib/unicorn/configurator.rb | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/lib/unicorn/configurator.rb b/lib/unicorn/configurator.rb
index 9752cdd..7651093 100644
--- a/lib/unicorn/configurator.rb
+++ b/lib/unicorn/configurator.rb
@@ -103,10 +103,6 @@ class Unicorn::Configurator
           raise ArgumentError,
             "check_client_connection is incompatible with tcp_nopush:true"
         end
-        if set[:listener_opts][address][:tcp_nodelay] == true
-          raise ArgumentError,
-            "check_client_connection is incompatible with tcp_nodelay:true"
-        end
       end
     end
     set.each do |key, value|
@@ -473,8 +469,11 @@ class Unicorn::Configurator
   # This will prevent calling the application for clients who have
   # disconnected while their connection was queued.
   #
-  # This option cannot be used in conjunction with tcp_nodelay or
-  # tcp_nopush.
+  # This only affects clients connecting over Unix domain sockets
+  # and TCP via loopback (127.*.*.*).  It is unlikely to detect
+  # disconnects if the client is on a remote host (even on a fast LAN).
+  #
+  # This option cannot be used in conjunction with :tcp_nopush.
   def check_client_connection(bool)
     set_bool(:check_client_connection, bool)
   end
-- 
1.8.0.3.gdd57fab.dirty
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