puppet: Increase backlogged socket count based on uwsgi backlog.

Increasing the uwsgi listen backlog is intended to allow it to handle
higher connection rates during server restart, when many clients may
be trying to connect.  The kernel, in turn, needs to have a
proportionally increased somaxconn soas to not refuse the connection.

Set somaxconn to 2x the uwsgi backlog, but no lower than the
default (128).

puppet/zulip/manifests/app_frontend_base.pp

@@ -96,6 +96,7 @@ class zulip::app_frontend_base {
   $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)
+  $somaxconn = 2 * $uwsgi_listen_backlog_limit
   file { '/etc/zulip/uwsgi.ini':
     ensure  => file,
     require => Package[supervisor],
@@ -105,6 +106,21 @@ class zulip::app_frontend_base {
     content => template('zulip/uwsgi.ini.template.erb'),
     notify  => Service[$zulip::common::supervisor_service],
   }
+  file { '/etc/sysctl.d/40-uwsgi.conf':
+    ensure  => file,
+    owner   => 'root',
+    group   => 'root',
+    mode    => '0644',
+    content => template('zulip/sysctl.d/40-uwsgi.conf.erb'),
+  }
+  exec { 'sysctl_p_uwsgi':
+    command     => '/sbin/sysctl -p /etc/sysctl.d/40-uwsgi.conf',
+    subscribe   => File['/etc/sysctl.d/40-uwsgi.conf'],
+    refreshonly => true,
+    # We have to protect against running in Docker and other
+    # containerization which prevents adjusting these.
+    onlyif      => 'touch /proc/sys/net/core/somaxconn',
+  }
 
   file { '/home/zulip/tornado':
     ensure => directory,

puppet/zulip/templates/sysctl.d/40-uwsgi.conf.erb

@@ -0,0 +1,2 @@
+# Allow larger listen backlog
+net.core.somaxconn=<%= [128, @somaxconn].max %>