FIX: avoid shelling to get hostname

Previously we were using `hostname` to get hostname, this is slow
and risks orphan processes.

This swaps it with Socket.gethostname which is both faster and more
reliable.

We also now cache it on the PrometheusExporter instance

CHANGELOG

@@ -1,6 +1,7 @@
-0.5.0 - pending
+0.5.0 - 14-02-2019
 
 - Breaking change: listen only to localhost by default to prevent unintended insecure configuration
+- FIX: Avoid calling `hostname` aggressively, instead cache it on the exporter instance
 
 0.4.17 - 13-01-2019
 

lib/prometheus_exporter.rb

@@ -20,6 +20,17 @@ def self.dump(obj)
     end
   end
 
+  def self.hostname
+    @hostname ||=
+      begin
+        require 'socket'
+        Socket.gethostname
+      rescue => e
+        STDERR.puts "Unable to lookup hostname #{e}"
+        "unknown-host"
+      end
+  end
+
   def self.detect_json_serializer(preferred)
     if preferred.nil?
       preferred = :oj if has_oj?

lib/prometheus_exporter/instrumentation/active_record.rb

@@ -51,17 +51,6 @@ def self.stop
     def initialize(metric_labels, config_labels)
       @metric_labels = metric_labels
       @config_labels = config_labels
-      @hostname = nil
-    end
-
-    def hostname
-      @hostname ||=
-        begin
-          `hostname`.strip
-        rescue => e
-          STDERR.puts "Unable to lookup hostname #{e}"
-          "unknown-host"
-        end
     end
 
     def collect
@@ -87,7 +76,7 @@ def collect_active_record_pool_stats(metrics)
         metric = {
           pid: pid,
           type: "active_record",
-          hostname: hostname,
+          hostname: ::PrometheusExporter.hostname,
           metric_labels: labels
         }
         metric.merge!(pool.stat)

lib/prometheus_exporter/instrumentation/process.rb

@@ -42,24 +42,13 @@ def self.stop
 
     def initialize(metric_labels)
       @metric_labels = metric_labels
-      @hostname = nil
-    end
-
-    def hostname
-      @hostname ||=
-        begin
-          `hostname`.strip
-        rescue => e
-          STDERR.puts "Unable to lookup hostname #{e}"
-          "unknown-host"
-        end
     end
 
     def collect
       metric = {}
       metric[:type] = "process"
       metric[:metric_labels] = @metric_labels
-      metric[:hostname] = hostname
+      metric[:hostname] = ::PrometheusExporter.hostname
       collect_gc_stats(metric)
       collect_v8_stats(metric)
       collect_process_stats(metric)

test/prometheus_exporter_test.rb

@@ -6,4 +6,8 @@ class PrometheusExporterTest < Minitest::Test
   def test_that_it_has_a_version_number
     refute_nil ::PrometheusExporter::VERSION
   end
+
+  def test_it_can_get_hostname
+    assert_equal `hostname`.strip, ::PrometheusExporter.hostname
+  end
 end