add Java 9+ support to ChildProcess dependency

this can safely be removed when the childprocess gem supports Java9+
enkessler/childprocess#141

qa/integration/patch/childprocess-modern-java.rb

@@ -0,0 +1,23 @@
+# encoding: utf-8
+
+# Implementation of ChildProcess::JRuby::Process#pid depends heavily on
+# what Java SDK is being used; here, we look it up once at load, then
+# override that method with an implementation that works on modern Javas
+# if necessary.
+#
+# This patch can be removed when the upstream childprocess gem supports Java 9+
+# https://github.com/enkessler/childprocess/pull/141
+normalised_java_version_major = java.lang.System.get_property("java.version")
+                                    .slice(/^(1\.)?([0-9]+)/, 2)
+                                    .to_i
+
+if normalised_java_version_major >= 9
+  $stderr.puts("patching childprocess for Java9+ support...")
+  ChildProcess::JRuby::Process.class_exec do
+    def pid
+      @process.pid
+    rescue java.lang.UnsupportedOperationException => e
+      raise NotImplementedError, "pid is not supported on this platform: #{e.message}"
+    end
+  end
+end

qa/integration/services/logstash_service.rb

@@ -1,6 +1,7 @@
 require_relative "monitoring_api"
 
 require "childprocess"
+require_relative "../patch/childprocess-modern-java"
 require "bundler"
 require "socket"
 require "tempfile"