Automatically patch Rack::Builder

* in tests, keep an unpatched ('untainted') version of Rack::Builder,
  restore before and after each test
* fix: ruby2.4: private method `define_method' called

Author: duonoid

adapters/rack/example/trace_demonstration.rb

@@ -15,10 +15,6 @@
 
 # setup fake rack application:
 builder = Rack::Builder.app do
-  # integration should be automatic in web frameworks (like rails),
-  # but for a plain Rack application, enable it in your config.ru, e.g.,
-  use OpenTelemetry::Adapters::Rack::Middlewares::TracerMiddleware
-
   app = ->(_env) { [200, { 'Content-Type' => 'text/plain' }, ['All responses are OK']] }
   run app
 end

adapters/rack/example/trace_demonstration2.rb

@@ -14,7 +14,6 @@
 app = ->(_env) { [200, { 'Content-Type' => 'text/plain' }, ['All responses are OK']] }
 builder.run app
 
-
 # demonstrate integration using config[:application] instead of 'builder.use':
 OpenTelemetry::SDK.configure do |c|
   c.use 'OpenTelemetry::Adapters::Rack', retain_middleware_names: true,

adapters/rack/lib/opentelemetry/adapters/rack/adapter.rb

@@ -14,7 +14,14 @@ class Adapter < OpenTelemetry::Instrumentation::Adapter
           require_dependencies
 
           retain_middleware_names if config[:retain_middleware_names]
-          config[:application]&.use Middlewares::TracerMiddleware
+
+          if (app = config[:application])
+            app.use Middlewares::TracerMiddleware
+          else
+            # monkey patch Rack::Builder:
+            require_relative 'patches/rack_builder'
+            ::Rack::Builder.prepend Rack::Patches::RackBuilder
+          end
         end
 
         present do

adapters/rack/lib/opentelemetry/adapters/rack/patches/rack_builder.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+# Copyright 2020 OpenTelemetry Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+require 'rack/builder'
+
+module OpenTelemetry
+  module Adapters
+    module Rack
+      module Patches
+        # Allows Rack::Builder to use middleware by default
+        module RackBuilder
+          def self.prepended(base)
+            old_initialize = base.instance_method(:initialize)
+
+            # NOTE: define_method is private in ruby-2.4:
+            base.send(:define_method, :initialize) do |*args, &block|
+              # pass original args to 'super':
+              old_initialize.bind(self).call(args, &block)
+
+              # add tracer middleware to default stack, once:
+              use Middlewares::TracerMiddleware if @use.empty?
+            end
+          end
+        end
+      end
+    end
+  end
+end
+
+require_relative '../middlewares/tracer_middleware'

adapters/rack/test/opentelemetry/adapters/rack/middlewares/tracer_middleware_test.rb

@@ -40,9 +40,8 @@
     # clear out cached config:
     described_class.send(:clear_cached_config)
 
-    # integrate tracer middleware:
+    # tracer middleware (automatically integrated):
     rack_builder.run app
-    rack_builder.use described_class
   end
 
   after do

adapters/rack/test/opentelemetry/adapters/rack/patches/rack_builder_test.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+# Copyright 2020 OpenTelemetry Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+require 'test_helper'
+
+require_relative '../../../../../lib/opentelemetry/adapters/rack'
+require_relative '../../../../../lib/opentelemetry/adapters/rack/patches/rack_builder'
+
+describe OpenTelemetry::Adapters::Rack::Patches::RackBuilder do
+  let(:adapter_module) { OpenTelemetry::Adapters::Rack }
+  let(:adapter_class) { adapter_module::Adapter }
+  let(:adapter) { adapter_class.instance }
+
+  let(:config) { {} }
+
+  before do
+    # allow for adapter re-installation:
+    adapter.instance_variable_set('@installed', false)
+  end
+
+  describe 'default installation' do
+    it 'adds middleware to default stack' do
+      _(::Rack::Builder.new.instance_variable_get(:@use).length).must_equal 0
+
+      adapter.install(config)
+
+      _(::Rack::Builder.new.instance_variable_get(:@use).length).must_equal 1
+    end
+  end
+end

adapters/rack/test/test_helper.rb

@@ -20,3 +20,28 @@
 end
 
 EXPORTER = exporter
+
+### "un-patch" Rack::Builder:
+#
+require 'rack/builder'
+UNTAINTED_RACK_BUILDER = ::Rack::Builder.dup
+
+module SafeRackBuilder
+  def after_setup
+    super
+    ::Rack.send(:remove_const, :Builder)
+    ::Rack.const_set(:Builder, UNTAINTED_RACK_BUILDER.dup)
+  end
+
+  def after_teardown
+    super
+    Rack.send(:remove_const, :Builder)
+    Rack.const_set(:Builder, UNTAINTED_RACK_BUILDER.dup)
+  end
+end
+
+module Minitest
+  class Test
+    include SafeRackBuilder
+  end
+end