FEATURE: Allow plugins to register demon processes (#11493)

This allows plugins to call `register_demon_process` with a Class inheriting from Demon::Base. The unicorn master process will take care of spawning, monitoring and restarting the process. This API should be used with extreme caution, but it is significantly cleaner than spawning processes/threads in an `after_initialize` block.

This commit also cleans up the demon spawning logging so that it uses the same format as unicorn worker logging. It also switches to the block form of `fork` to ensure that Demons exit after running, rather than returning execution to where the fork took place.

lib/demon/base.rb

@@ -141,15 +141,13 @@ class Demon::Base
   end
 
   def run
-    if @pid = fork
-      write_pid_file
-      return
+    @pid = fork do
+      Process.setproctitle("discourse #{self.class.prefix}")
+      monitor_parent
+      establish_app
+      after_fork
     end
-
-    Process.setproctitle("discourse #{self.class.prefix}")
-    monitor_parent
-    establish_app
-    after_fork
+    write_pid_file
   end
 
   def already_running?

lib/discourse_plugin_registry.rb

@@ -68,6 +68,7 @@ class DiscoursePluginRegistry
   define_register :vendored_pretty_text, Set
   define_register :vendored_core_pretty_text, Set
   define_register :seedfu_filter, Set
+  define_register :demon_processes, Set
 
   define_filtered_register :staff_user_custom_fields
   define_filtered_register :public_user_custom_fields

lib/plugin/instance.rb

@@ -861,6 +861,15 @@ class Plugin::Instance
       ), self)
   end
 
+  # Register a new demon process to be forked by the Unicorn master.
+  # The demon_class should inherit from Demon::Base.
+  # With great power comes great responsibility - this method should
+  # be used with extreme caution. See `config/unicorn.conf.rb`.
+  def register_demon_process(demon_class)
+    raise "Not a demon class" if !demon_class.ancestors.include?(Demon::Base)
+    DiscoursePluginRegistry.demon_processes << demon_class
+  end
+
   protected
 
   def self.js_path