Ensure that only one EventMachine is running

lib/runner/event_loop.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+# EventLoop is an abstraction around Ruby's queue so that its usage is better
+# understandable in our context.
+class Runner::EventLoop
+  def initialize
+    @queue = Queue.new
+  end
+
+  # wait waits until another thread calls stop on this EventLoop.
+  # There may only be one active wait call per loop at a time, otherwise it is not
+  # deterministic which one will be unblocked if stop is called.
+  def wait
+    @queue.pop
+  end
+
+  # stop unblocks the currently active wait call. If there is none, the
+  # next call to wait will not be blocking.
+  def stop
+    @queue.push nil if @queue.empty?
+  end
+end