settings: add support for observing settings after post-process hooks

logstash-core/lib/logstash/settings.rb

@@ -197,6 +197,13 @@ def post_process
           callback.call(self)
         end
       end
+
+      # because we cannot rely on deprecation logger being wired up when the setters
+      # are initially used, we re-emit setter-related deprecations after all post-processing
+      # hooks have been activated (and therefore after logging has been configured)
+      @settings.each_value do |setting|
+        setting.observe_post_process if setting.respond_to?(:observe_post_process)
+      end
     end
 
     def on_post_process(&block)
@@ -839,9 +846,7 @@ def initialize(canonical_proxy, alias_name)
       end
 
       def set(value)
-        deprecation_logger.deprecated(I18n.t("logstash.settings.deprecation.set",
-                                             :deprecated_alias => name,
-                                             :canonical_name => canonical_proxy.name))
+        do_log_setter_deprecation
         super
       end
 
@@ -856,6 +861,18 @@ def validate_value
         # bypass deprecation warning
         wrapped.validate_value if set?
       end
+
+      def observe_post_process
+        do_log_setter_deprecation if set?
+      end
+
+      private
+
+      def do_log_setter_deprecation
+        deprecation_logger.deprecated(I18n.t("logstash.settings.deprecation.set",
+                                             :deprecated_alias => name,
+                                             :canonical_name => canonical_proxy.name))
+      end
     end
 
     ##

logstash-core/spec/logstash/settings/setting_with_deprecated_alias_spec.rb

@@ -52,6 +52,13 @@
     end
 
     include_examples '#validate_value success'
+
+    context "#observe_post_process" do
+      it 'does not emit a deprecation warning' do
+        expect(LogStash::Settings.deprecation_logger).to_not receive(:deprecated).with(a_string_including(deprecated_setting_name))
+        settings.get_setting(deprecated_setting_name).observe_post_process
+      end
+    end
   end
 
   context "when only the deprecated alias is set" do
@@ -71,6 +78,13 @@
 
     include_examples '#validate_value success'
 
+    context "#observe_post_process" do
+      it 're-emits the deprecation warning' do
+        expect(LogStash::Settings.deprecation_logger).to receive(:deprecated).with(a_string_including(deprecated_setting_name))
+        settings.get_setting(deprecated_setting_name).observe_post_process
+      end
+    end
+
     it 'validates deprecated alias' do
       expect { settings.get_setting(canonical_setting_name).deprecated_alias.validate_value }.to_not raise_error
     end
@@ -107,6 +121,13 @@
     end
 
     include_examples '#validate_value success'
+
+    context "#observe_post_process" do
+      it 'does not emit a deprecation warning' do
+        expect(LogStash::Settings.deprecation_logger).to_not receive(:deprecated).with(a_string_including(deprecated_setting_name))
+        settings.get_setting(deprecated_setting_name).observe_post_process
+      end
+    end
   end
 
   context "when both the canonical setting and deprecated alias are set" do

logstash-core/spec/logstash/settings_spec.rb

@@ -144,6 +144,18 @@
     it "should preserve original settings" do
       expect(settings.get("foo")).to eq("bar")
     end
+
+    context 'when a registered setting responds to `observe_post_process`' do
+      let(:observe_post_process_setting) do
+        LogStash::Setting::Boolean.new("this.that", true).tap { |s| allow(s).to receive(:observe_post_process) }
+      end
+      subject(:settings) do
+        described_class.new.tap { |s| s.register(observe_post_process_setting) }
+      end
+      it 'it sends `observe_post_process`' do
+        expect(observe_post_process_setting).to have_received(:observe_post_process)
+      end
+    end
   end
 
   context "transient settings" do