Prevent SSH connection restarts

Set a high idle timeout on the sshkit connection pool. This will
reduce the incidence of re-connection storms when a deployment has been
idle for a while (e.g. when waiting for a docker build).

The default timeout was 30 seconds, so we'll enable keepalives at a
30s interval to match. This is to help prevent connections from being
killed during long idle periods.

lib/mrsk/commander.rb

@@ -143,6 +143,7 @@ class Mrsk::Commander
   private
     # Lazy setup of SSHKit
     def configure_sshkit_with(config)
+      SSHKit::Backend::Netssh.pool.idle_timeout = config.sshkit_pool_idle_timeout
       SSHKit::Backend::Netssh.configure do |sshkit|
         sshkit.max_concurrent_starts = config.sshkit_max_concurrent_starts if config.sshkit_max_concurrent_starts
         sshkit.ssh_options = config.ssh_options

lib/mrsk/configuration.rb

@@ -153,7 +153,7 @@ class Mrsk::Configuration
   end
 
   def ssh_options
-    { user: ssh_user, proxy: ssh_proxy, auth_methods: [ "publickey" ] }.compact
+    { user: ssh_user, proxy: ssh_proxy, auth_methods: [ "publickey" ], keepalive: true, keepalive_interval: 30 }.compact
   end
 
 
@@ -161,6 +161,15 @@ class Mrsk::Configuration
     raw_config.sshkit["max_concurrent_starts"] if raw_config.sshkit.present?
   end
 
+  def sshkit_pool_idle_timeout
+    if raw_config.sshkit.present?
+      raw_config.sshkit["pool_idle_timeout"] || 900
+    else
+      900
+    end
+  end
+
+
   def healthcheck
     { "path" => "/up", "port" => 3000, "max_attempts" => 7 }.merge(raw_config.healthcheck || {})
   end