puppet: Make uwsgi listen backlog limit configurable.

This can be useful for busy servers to limit the risk of bursts of
traffic causing them to reject requests.

puppet/zulip/manifests/app_frontend_base.pp

@@ -89,6 +89,7 @@ class zulip::app_frontend_base {
     notify  => Service[$zulip::common::supervisor_service],
   }
 
+  $uwsgi_listen_backlog_limit = zulipconf('application_server', 'uwsgi_listen_backlog_limit', 128)
   $uwsgi_buffer_size = zulipconf('application_server', 'uwsgi_buffer_size', 8192)
   $uwsgi_processes = zulipconf('application_server', 'uwsgi_processes', $uwsgi_default_processes)
   file { '/etc/zulip/uwsgi.ini':

puppet/zulip/templates/uwsgi.ini.template.erb

@@ -8,6 +8,7 @@ chown-socket=zulip:zulip
 processes=<%= @uwsgi_processes %>
 harakiri=20
 buffer-size=<%= @uwsgi_buffer_size %>
+listen=<%= @uwsgi_listen_backlog_limit %>
 post-buffering=4096
 env= LANG=en_US.UTF-8
 uid=zulip