no mutex in shutdown

example-app/README.md

@@ -7,3 +7,14 @@ To run:
 bundle
 bundle exec clock
 ```
+
+To test invocation of existing signal handlers, put this code at the very top of exe/clock:
+
+```ruby
+%w[INT TERM].each do |signal|
+  Signal.trap(signal) do
+    puts "This is a well-behaving #{signal} handler from outside of ruby-clock"
+    exit
+  end
+end
+```

lib/ruby-clock.rb

@@ -24,22 +24,37 @@ def wait_seconds
     ENV['RUBY_CLOCK_SHUTDOWN_WAIT_SECONDS']&.to_i || 29
   end
 
-  def shutdown
-    puts "Shutting down ruby-clock. Waiting #{wait_seconds} seconds for jobs to finish..."
-    schedule.shutdown(wait: wait_seconds)
-    puts "...done 🐈️ 👋"
+  # shutdown is done async in a Thread because signal handlers should
+  # not have mutex locks created within them, which I believe can happen in
+  # the rufus shutdown process
+  def shutdown(old_handler=nil)
+    Thread.new do
+      sleep 0.1 # wait for trap block to exit
+      puts "Shutting down ruby-clock. Waiting #{wait_seconds} seconds for jobs to finish..."
+      schedule.shutdown(wait: wait_seconds)
+      puts "...done 🐈️ 👋"
+      if old_handler
+        puts "handing off shutdown to another signal handler..."
+        old_handler.call
+      else
+        exit
+      end
+    end
   end
 
   def listen_to_signals
     signals = %w[INT TERM]
     signals.each do |signal|
       old_handler = Signal.trap(signal) do
-        shutdown
         if old_handler.respond_to?(:call)
-          old_handler.call
+          shutdown(old_handler)
         else
-          exit
+          shutdown
         end
+
+        # keep this line here at the end, to serve as some degree of demonstration that
+        # the handler exited before shutdown begins
+        puts("received #{signal}") && STDOUT.flush
       end
     end
     puts "RUBY_CLOCK_SHUTDOWN_WAIT_SECONDS is set to #{wait_seconds}"