Add a transient_settings concept to the settings handling

When we load a yml file in logstash if a key isn't not found in the
settings, we move that key and the corresponding value to a
`transient_settings` hash. This give the time to the plugin to register
new settings. When we call #validate_all we merge the
`transient_settings` hash and do the validation, if a key is not found
at that stage an excepton will be throw.

Fixes #6109

logstash-core/lib/logstash/settings.rb

@@ -9,6 +9,11 @@ module LogStash
 
     def initialize
       @settings = {}
+      # Theses settings were loaded from the yaml file
+      # but we didn't find any settings to validate them,
+      # lets keep them around until we do `validate_all` at that
+      # time universal plugins could have added new settings.
+      @transient_settings = {}
     end
 
     def register(setting)
@@ -52,8 +57,14 @@ module LogStash
     end
     alias_method :get, :get_value
 
-    def set_value(setting_name, value)
+    def set_value(setting_name, value, graceful = false)
       get_setting(setting_name).set(value)
+    rescue ArgumentError => e
+      if graceful
+        @transient_settings[setting_name] = value
+      else
+        raise e
+      end
     end
     alias_method :set, :set_value
 
@@ -65,8 +76,8 @@ module LogStash
       hash
     end
 
-    def merge(hash)
-      hash.each {|key, value| set_value(key, value) }
+    def merge(hash, graceful = false)
+      hash.each {|key, value| set_value(key, value, graceful) }
       self
     end
 
@@ -96,10 +107,13 @@ module LogStash
 
     def from_yaml(yaml_path)
       settings = read_yaml(::File.join(yaml_path, "logstash.yml"))
-      self.merge(flatten_hash(settings))
+      self.merge(flatten_hash(settings), true)
     end
 
     def validate_all
+      # lets merge the transient_settings again to see if new setting were added.
+      self.merge(@transient_settings)
+
       @settings.each do |name, setting|
         setting.validate_value
       end

logstash-core/spec/logstash/settings_spec.rb

@@ -1,6 +1,7 @@
 # encoding: utf-8
 require "spec_helper"
 require "logstash/settings"
+require "fileutils"
 
 describe LogStash::Settings do
   let(:numeric_setting_name) { "number" }
@@ -86,4 +87,42 @@ describe LogStash::Settings do
       end
     end
   end
+
+  context "transient settings" do
+    subject do
+      settings = described_class.new
+      settings.register(LogStash::Setting::String.new("exist", "bonsoir"))
+      settings
+    end
+
+    let(:values) { { "do.not.exist.on.boot" => true, "exist" => "bonjour" } }
+    let(:yaml_path) do
+      p = Stud::Temporary.pathname
+      FileUtils.mkdir_p(p)
+
+      ::File.open(::File.join(p, "logstash.yml"), "w+") do |f|
+        f.write(YAML.dump(values))
+      end
+      p
+    end
+
+    it "allow to read yml file that contains unknown settings" do
+      expect { subject.from_yaml(yaml_path) }.not_to raise_error
+    end
+
+    context "when running #validate_all" do
+      it "merge and validate all the registered setting" do
+        subject.from_yaml(yaml_path)
+        subject.register(LogStash::Setting::Boolean.new("do.not.exist.on.boot", false))
+
+        expect { subject.validate_all }.not_to raise_error
+        expect(subject.get("do.not.exist.on.boot")).to be_truthy
+      end
+
+      it "raise an error when the settings doesn't exist" do
+        subject.from_yaml(yaml_path)
+        expect { subject.validate_all }.to raise_error(ArgumentError)
+      end
+    end
+  end
 end