Implemented service reload for Prometheus

With this commit all the configuration files that can be reload on-the-fly
like prometheus.yml and alerts rules do not trigger a full service restart
like they did til now.
I left the possibility to choose if a change to a command-line flag should
or not restart prometheus (thus applying the change on puppet run) with
the already existing  flag (still true by default).

manifests/config.pp

@@ -10,6 +10,17 @@
 
   if $prometheus::init_style {
 
+    # the vast majority of files here are init-files
+    # so any change there should trigger a full service restart
+    if $::prometheus::restart_on_change {
+      File {
+        notify => [Class['::prometheus::run_service']]
+      }
+      $systemd_notify = [Exec['prometheus-systemd-reload'], Class['::prometheus::run_service']]
+    } else {
+      $systemd_notify = Exec['prometheus-systemd-reload']
+    }
+
     case $prometheus::init_style {
       'upstart' : {
         file { '/etc/init/prometheus.conf':
@@ -31,8 +42,10 @@
           mode    => '0644',
           owner   => 'root',
           group   => 'root',
+          notify  => $systemd_notify,
           content => template('prometheus/prometheus.systemd.erb'),
-        }~>
+        }
+
         exec { 'prometheus-systemd-reload':
           command     => 'systemctl daemon-reload',
           path        => [ '/usr/bin', '/bin', '/usr/sbin' ],
@@ -90,6 +103,7 @@
     owner   => $prometheus::user,
     group   => $prometheus::group,
     mode    => $prometheus::config_mode,
+    notify  => Class['::prometheus::service_reload'],
     content => template($config_template),
   }
 

manifests/init.pp

@@ -84,6 +84,8 @@
 #
 #  [*restart_on_change*]
 #  Should puppet restart prometheus on configuration change? (default true)
+#  Note: this applies only to command-line options changes. Configuration
+#  options are always *reloaded* without restarting.
 #
 #  [*init_style*]
 #  Service startup scripts style (e.g. rc, upstart or systemd)
@@ -158,10 +160,6 @@
 
   $config_hash_real = deep_merge($config_defaults, $config_hash)
   validate_hash($config_hash_real)
-  $notify_service = $restart_on_change ? {
-    true    => Class['::prometheus::run_service'],
-    default => undef,
-  }
 
   anchor {'prometheus_first': }
   ->
@@ -171,14 +169,13 @@
     rule_files      => $rule_files,
     scrape_configs  => $scrape_configs,
     purge           => $purge_config_dir,
-    notify          => $notify_service,
     config_template => $config_template,
   } ->
   class { '::prometheus::alerts':
     location => $config_dir,
     alerts   => $alerts,
-    notify   => $notify_service,
   } ->
   class { '::prometheus::run_service': } ->
+  class { '::prometheus::service_reload': } ->
   anchor {'prometheus_last': }
 }

manifests/run_service.pp

@@ -12,9 +12,9 @@
 
   if $prometheus::manage_service == true {
     service { 'prometheus':
-      ensure => $prometheus::service_ensure,
-      name   => $init_selector,
-      enable => $prometheus::service_enable,
+      ensure  => $prometheus::service_ensure,
+      name    => $init_selector,
+      enable  => $prometheus::service_enable,
     }
   }
 }

manifests/service_reload.pp

@@ -0,0 +1,23 @@
+# This class implements prometheus service reload
+# without restarting the whole service when a config
+# changes
+class prometheus::service_reload() {
+
+  if $prometheus::manage_service == true {
+    $init_selector = $prometheus::run_service::init_selector
+
+    $prometheus_reload = $prometheus::init_style ? {
+      'systemd' => "systemctl reload ${init_selector}",
+      'upstart' => "upstart reload ${init_selector}",
+      'sysv'    => "/etc/init.d/${init_selector} reload",
+      'sles'    => "/etc/init.d/${init_selector} reload",
+      'debian'  => "/etc/init.d/${init_selector} reload",
+      'launchd' => "launchctl stop ${init_selector} && launchctl start ${init_selector}",
+    }
+
+    exec { 'prometheus-reload':
+      command     => $prometheus_reload,
+      refreshonly => true,
+    }
+  }
+}