Don't use the systemd plugin on JRuby

lib/puma/launcher.rb

@@ -59,7 +59,10 @@ def initialize(conf, launcher_args={})
 
       @environment = conf.environment
 
-      if ENV["NOTIFY_SOCKET"]
+      # Load the systemd integration if we detect systemd's NOTIFY_SOCKET.
+      # Skip this on JRuby though, because it is incompatible with the systemd
+      # integration due to https://github.com/jruby/jruby/issues/6504
+      if ENV["NOTIFY_SOCKET"] && !Puma.jruby?
         @config.plugins.create('systemd')
       end
 

test/test_plugin_systemd_jruby.rb

@@ -0,0 +1,27 @@
+require_relative "helper"
+require_relative "helpers/integration"
+
+class TestPluginSystemdJruby < TestIntegration
+
+  THREAD_LOG = TRUFFLE ? "{ 0/16 threads, 16 available, 0 backlog }" :
+    "{ 0/5 threads, 5 available, 0 backlog }"
+
+  def setup
+    skip "Skipped because Systemd support is linux-only" if windows? || osx?
+    skip_unless :unix
+    skip_unless_signal_exist? :TERM
+    skip_unless :jruby
+
+    super
+
+    ENV["NOTIFY_SOCKET"] = "/tmp/doesntmatter"
+  end
+
+  def test_systemd_plugin_not_loaded
+    cli_server "test/rackup/hello.ru"
+
+    assert_nil Puma::Plugins.instance_variable_get(:@plugins)["systemd"]
+
+    stop_server
+  end
+end