Move EventMachine initialization to Runner::EventLoop

app/models/runner.rb

@@ -41,7 +41,6 @@ class Runner < ApplicationRecord
   end
 
   def attach_to_execution(command, &block)
-    ensure_event_machine
     starting_time = Time.zone.now
     begin
       # As the EventMachine reactor is probably shared with other threads, we cannot use EventMachine.run with
@@ -66,22 +65,6 @@ class Runner < ApplicationRecord
 
   private
 
-  # If there are multiple threads trying to connect to the WebSocket of their execution at the same time,
-  # the Faye WebSocket connections will use the same reactor. We therefore only need to start an EventMachine
-  # if there isn't a running reactor yet.
-  # See this StackOverflow answer: https://stackoverflow.com/a/8247947
-  def ensure_event_machine
-    unless EventMachine.reactor_running? && EventMachine.reactor_thread.alive?
-      event_loop = Runner::EventLoop.new
-      Thread.new do
-        EventMachine.run { event_loop.stop }
-      ensure
-        ActiveRecord::Base.connection_pool.release_connection
-      end
-      event_loop.wait
-    end
-  end
-
   def request_id
     request_new_id if runner_id.blank?
   end

lib/runner/event_loop.rb

@@ -5,6 +5,7 @@
 class Runner::EventLoop
   def initialize
     @queue = Queue.new
+    ensure_event_machine
   end
 
   # wait waits until another thread calls stop on this EventLoop.
@@ -19,4 +20,22 @@ class Runner::EventLoop
   def stop
     @queue.push nil if @queue.empty?
   end
+
+  private
+
+  # If there are multiple threads trying to connect to the WebSocket of their execution at the same time,
+  # the Faye WebSocket connections will use the same reactor. We therefore only need to start an EventMachine
+  # if there isn't a running reactor yet.
+  # See this StackOverflow answer: https://stackoverflow.com/a/8247947
+  def ensure_event_machine
+    unless EventMachine.reactor_running? && EventMachine.reactor_thread.alive?
+      queue = Queue.new
+      Thread.new do
+        EventMachine.run { queue.push nil }
+      ensure
+        ActiveRecord::Base.connection_pool.release_connection
+      end
+      queue.pop
+    end
+  end
 end