Instrument gems that are required after Datadog.configure

Gemfile

@@ -89,3 +89,6 @@ end
 #
 # TODO: Remove this once the issue is resolved: https://github.com/ffi/ffi/issues/1107
 gem 'ffi', '~> 1.16.3', require: false
+
+
+gem 'faraday'

Rakefile

@@ -497,9 +497,19 @@ namespace :spec do
     :suite,
     :trilogy
   ].each do |contrib|
+    # Test for each integration. These tests will pollute the environment (e.g. load gems, monkey-patch modules).
     desc '' # "Explicitly hiding from `rake -T`"
     RSpec::Core::RakeTask.new(contrib) do |t, args|
       t.pattern = "spec/datadog/tracing/contrib/#{contrib}/**/*_spec.rb"
+      t.exclude_pattern = "spec/datadog/tracing/contrib/#{contrib}/integration_spec.rb"
+      t.rspec_opts = args.to_a.join(' ')
+    end
+
+    # Tests that have to run with an environment where the instrumented gem has
+    # not been loaded yet nor modules have been modified.
+    desc '' # "Explicitly hiding from `rake -T`"
+    RSpec::Core::RakeTask.new("#{contrib}_clean") do |t, args|
+      t.pattern = "spec/datadog/tracing/contrib/#{contrib}/integration_spec.rb"
       t.rspec_opts = args.to_a.join(' ')
     end
   end

benchmarks/devweek.rb

@@ -0,0 +1,74 @@
+# rubocop:disable
+require 'ddtrace'
+
+module Datadog
+  module Tracing
+    module Contrib
+      module Kernel
+        def require(name)
+          just_loaded = super
+
+          @@dd_instance.require(name) if just_loaded
+
+          just_loaded
+        end
+
+        class Instance
+          def initialize
+            @on_require = {}
+          end
+
+          def require(name)
+            if @on_require.include?(name)
+              Datadog.logger.debug { "Gem '#{name}' loaded. Invoking callback." }
+
+              @on_require[name].call
+            end
+          rescue => e
+            Datadog.logger.debug do
+              "Failed to execute callback for gem '#{name}': #{e.class.name} #{e.message} at #{Array(e.backtrace).join("\n")}"
+            end
+          end
+
+          def on_require(gem, &block)
+            @on_require[gem] = block
+          end
+        end
+
+        @@dd_instance = Instance.new
+
+        def self.on_require(gem, &block)
+          @@dd_instance.on_require(gem, &block)
+        end
+
+        def self.patch!
+          ::Kernel.prepend(self)
+        end
+
+        DD_PATCH_ONLY_ONCE = Datadog::Core::Utils::OnlyOnce.new
+        private_constant :DD_PATCH_ONLY_ONCE
+      end
+    end
+  end
+end
+
+Datadog::Tracing::Contrib::Kernel.patch! # TODO: I think this stays here actually!
+
+ENV['DD_TRACE_DEBUG'] = 'true'
+
+# Setup
+require 'ddtrace'
+
+Datadog.configure do |c|
+  c.tracing.instrument :faraday
+end
+
+require 'faraday'
+
+# User application
+Faraday.get('http://example.com')
+
+# Tear down
+Datadog.shutdown! # Ensure traces have been flushed
+
+# rubocop:enable