Replace def #{type}_func with define_singleton_method

When you run multiples pipeline and the config code get evaluated, on
every evaluation the class cached is clear, everytime you were calling a
`func` method you had the latest evaluated code. The `filter_func` and
the `output_func` need to be unique for every instance of the pipeline,
this PR replace the `def` with a `define_single_method` call ensuring
the uniqueness of the code on every instance.

This PR is based on #4254

Fixes #4298

logstash-core/lib/logstash/config/config_ast.rb

@@ -107,7 +107,10 @@ module LogStash; module Config; module AST
       ["filter", "output"].each do |type|
         # defines @filter_func and @output_func
 
-        definitions << "def #{type}_func(event)"
+        # This need to be defined as a singleton method
+        # so each instance of the pipeline has his own implementation
+        # of the output/filter function
+        definitions << "define_singleton_method :#{type}_func do |event|"
         definitions << "  targeted_outputs = []" if type == "output"
         definitions << "  events = [event]" if type == "filter"
         definitions << "  @logger.debug? && @logger.debug(\"#{type} received\", :event => event.to_hash)"
@@ -544,4 +547,4 @@ class Treetop::Runtime::SyntaxNode
         ""
       )
   end
-end
+end

logstash-core/spec/logstash/pipeline_spec.rb

@@ -47,7 +47,7 @@ class DummyOutput < LogStash::Outputs::Base
 
   def register
   end
-
+  
   def receive(event)
     @events << event
   end
@@ -405,4 +405,33 @@ describe LogStash::Pipeline do
       pipeline.shutdown
     end
   end
+
+  context "Multiple pipelines" do
+    before do
+      allow(LogStash::Plugin).to receive(:lookup).with("input", "generator").and_return(LogStash::Inputs::Generator)
+      allow(LogStash::Plugin).to receive(:lookup).with("codec", "plain").and_return(DummyCodec)
+      allow(LogStash::Plugin).to receive(:lookup).with("filter", "dummyfilter").and_return(DummyFilter)
+      allow(LogStash::Plugin).to receive(:lookup).with("output", "dummyoutput").and_return(DummyOutput)
+    end
+
+    let(:pipeline1) { LogStash::Pipeline.new("input { generator {} } filter { dummyfilter {} } output { dummyoutput {}}") }
+    let(:pipeline2) { LogStash::Pipeline.new("input { generator {} } filter { dummyfilter {} } output { dummyoutput {}}") }
+
+    it "should handle evaluating different config" do
+      # When the functions are compiled from the AST it will generate instance
+      # variables that are unique to the actual config, the intance are pointing
+      # to conditionals/plugins.
+      #
+      # Before the `defined_singleton_method`, the definition of the method was
+      # not unique per class, but the `instance variables` were unique per class.
+      #
+      # So the methods were trying to access instance variables that did not exist
+      # in the current instance and was returning an array containing nil values for
+      # the match.
+      expect(pipeline1.output_func(LogStash::Event.new)).not_to include(nil)
+      expect(pipeline1.filter_func(LogStash::Event.new)).not_to include(nil)
+      expect(pipeline2.output_func(LogStash::Event.new)).not_to include(nil)
+      expect(pipeline1.filter_func(LogStash::Event.new)).not_to include(nil)
+    end
+  end
 end