Silence deprecation warnings from concurrent-ruby

Concurrent-ruby is throwing warning when running under jdk7,
we have decided to silence theses errors unless you run logstash with
the `--debug` flag.

Fixes #3869

lib/logstash/patches.rb

@@ -3,3 +3,4 @@ require "logstash/patches/bugfix_jruby_2558"
 require "logstash/patches/cabin"
 require "logstash/patches/profile_require_calls"
 require "logstash/patches/stronger_openssl_defaults"
+require "logstash/patches/silence_concurrent_ruby_warning"

lib/logstash/patches/silence_concurrent_ruby_warning.rb

@@ -0,0 +1,54 @@
+# encoding: utf-8
+require "logstash/namespace"
+require "concurrent/concern/logging"
+require "concurrent/concern/deprecation"
+require "concurrent/version"
+require "cabin"
+
+# Concurrent-ruby is throwing warning when the code is run under jdk7, and they
+# will provide best effort support, logstash has to support JDK7 for a few months.
+#
+# By default all deprecation warnings of the concurrent ruby
+# library use the `WARN` level which is show everytime we boot logstash,
+# This monkeypatch change the log level of the deprecation warning to be `debug`
+# instead. This monkey patch might be a bit over kill but there is no
+# easy way to override the java version check.
+#
+# ref: https://github.com/ruby-concurrency/concurrent-ruby/blob/v0.9.1/lib/concurrent/configuration.rb#L284-L295
+#
+# This patch is only valid for 0.9.1
+if Concurrent::VERSION == "0.9.1"
+  module Concurrent
+    module Concern
+      module Deprecation
+        include Concern::Logging
+
+        def deprecated(message, strip = 2)
+          caller_line = caller(strip).first if strip > 0
+          klass       = if Module === self
+                          self
+                        else
+                          self.class
+                        end
+          message     = if strip > 0
+                          format("[DEPRECATED] %s\ncalled on: %s", message, caller_line)
+                        else
+                          format('[DEPRECATED] %s', message)
+                        end
+
+          # lets use our logger
+          logger = Cabin::Channel.get(LogStash)
+          logger.debug(message, :class => klass.to_s)
+        end
+
+        extend self
+      end
+    end
+  end
+else
+  # This is added a guard to check if we need to update this code or not.
+  # Keep in mind, the latest releases of concurrent-ruby brokes a few stuff.
+  #
+  # Even the latest master version changed how they handle deprecation.
+  raise "Logstash expects concurrent-ruby version 0.9.1 and version #{Concurrent::VERSION} is installed, please verify this patch: #{__FILE__}"
+end

logstash-core.gemspec

@@ -23,6 +23,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency "clamp", "~> 0.6.5" #(MIT license) for command line args/flags
   gem.add_runtime_dependency "filesize", "0.0.4" #(MIT license) for :bytes config validator
   gem.add_runtime_dependency "gems", "~> 0.8.3"  #(MIT license)
+  gem.add_runtime_dependency "concurrent-ruby", "0.9.1"
 
   # TODO(sissel): Treetop 1.5.x doesn't seem to work well, but I haven't
   # investigated what the cause might be. -Jordan