PERF: avoid shelling to get hostname aggressively

Previously we had many places in the app that called `hostname` to get
hostname of a server. This commit replaces the pattern in 2 ways

1. We cache the result in `Discourse.os_hostname` so it is only ever called once

2. We prefer to use Socket.gethostname which avoids making a shell command

This improves performance as we are not spawning hostname processes throughout
the app lifetime

app/jobs/base.rb

@@ -40,7 +40,7 @@ module Jobs
         self.class.mutex.synchronize do
           @data = {}
 
-          @data["hostname"] = `hostname`.strip # Hostname
+          @data["hostname"] = Discourse.os_hostname
           @data["pid"] = Process.pid # Pid
           @data["database"] = db # DB name - multisite db name it ran on
           @data["job_id"] = jid # Job unique ID

app/models/optimized_image.rb

@@ -9,7 +9,7 @@ class OptimizedImage < ActiveRecord::Base
   URL_REGEX ||= /(\/optimized\/\dX[\/\.\w]*\/([a-zA-Z0-9]+)[\.\w]*)/
 
   def self.lock(upload_id, width, height)
-    @hostname ||= `hostname`.strip rescue "unknown"
+    @hostname ||= Discourse.os_hostname
     # note, the extra lock here ensures we only optimize one image per machine on webs
     # this can very easily lead to runaway CPU so slowing it down is beneficial and it is hijacked
     #

config/initializers/100-sidekiq.rb

@@ -69,7 +69,7 @@ if Sidekiq.server?
   Rails.application.config.after_initialize do
     scheduler_hostname = ENV["UNICORN_SCHEDULER_HOSTNAME"]
 
-    if !scheduler_hostname || scheduler_hostname.split(',').include?(`hostname`.strip)
+    if !scheduler_hostname || scheduler_hostname.split(',').include?(Discourse.os_hostname)
       MiniScheduler.start(workers: GlobalSetting.mini_scheduler_workers)
     end
   end

lib/demon/sidekiq.rb

@@ -39,7 +39,7 @@ class Demon::Sidekiq < ::Demon::Base
     [['critical', 8], ['default', 4], ['low', 2], ['ultra_low', 1]].each do |queue_name, weight|
       custom_queue_hostname = ENV["UNICORN_SIDEKIQ_#{queue_name.upcase}_QUEUE_HOSTNAME"]
 
-      if !custom_queue_hostname || custom_queue_hostname.split(',').include?(`hostname`.strip)
+      if !custom_queue_hostname || custom_queue_hostname.split(',').include?(Discourse.os_hostname)
         options << "-q"
         options << "#{queue_name},#{weight}"
       end

lib/discourse.rb

@@ -312,6 +312,24 @@ module Discourse
     end
   end
 
+  # hostname of the server, operating system level
+  # called os_hostname so we do no confuse it with current_hostname
+  def self.os_hostname
+    @os_hostname ||=
+      begin
+        require 'socket'
+        Socket.gethostname
+      rescue => e
+        warn_exception(e, message: 'Socket.gethostname is not working')
+        begin
+          `hostname`.strip
+        rescue => e
+          warn_exception(e, message: 'hostname command is not working')
+          'unknown_host'
+        end
+      end
+  end
+
   # Get the current base URL for the current site
   def self.current_hostname
     SiteSetting.force_hostname.presence || RailsMultisite::ConnectionManagement.current_hostname

lib/discourse_logstash_logger.rb

@@ -8,7 +8,7 @@ class DiscourseLogstashLogger
       uri: uri,
       sync: true,
       customize_event: ->(event) {
-        event['hostname'] = `hostname`.chomp
+        event['hostname'] = Discourse.os_hostname
         event['severity_name'] = event['severity']
         event['severity'] = Object.const_get("Logger::Severity::#{event['severity']}")
         event['type'] = type

lib/freedom_patches/schema_migration_details.rb

@@ -31,7 +31,7 @@ module FreedomPatches
       )
 SQL
 
-      hostname = `hostname` rescue ""
+      hostname = Discourse.os_hostname
       sql = ActiveRecord::Base.public_send(:sanitize_sql_array, [sql, {
         version: version || "",
         duration: (time.real * 1000).to_i,

lib/message_bus_diags.rb

@@ -5,7 +5,7 @@ class MessageBusDiags
   @host_info = {}
 
   def self.my_id
-    @my_id ||= "#{`hostname`}-#{Process.pid}"
+    @my_id ||= "#{Discourse.os_hostname}-#{Process.pid}"
   end
 
   def self.seen_host(name)

script/mwrap_sidekiq

@@ -128,7 +128,7 @@ begin
   [['critical', 8], ['default', 4], ['low', 2], ['ultra_low', 1]].each do |queue_name, weight|
     custom_queue_hostname = ENV["UNICORN_SIDEKIQ_#{queue_name.upcase}_QUEUE_HOSTNAME"]
 
-    if !custom_queue_hostname || custom_queue_hostname.split(',').include?(`hostname`.strip)
+    if !custom_queue_hostname || custom_queue_hostname.split(',').include?(Discourse.os_hostname)
       options << "-q"
       options << "#{queue_name},#{weight}"
     end