Add thread_local_tracer_state configuration option that switches betw…

…een thread-local and fiber-local storage

lib/new_relic/agent/configuration/default_source.rb

@@ -554,6 +554,13 @@ def self.enforce_fallback(allowed_values: nil, fallback: nil)
           :allowed_from_server => false,
           :description => 'When set to `true`, forces a synchronous connection to the New Relic [collector](/docs/using-new-relic/welcome-new-relic/get-started/glossary/#collector) during application startup. For very short-lived processes, this helps ensure the New Relic agent has time to report.'
         },
+        :thread_local_tracer_state => {
+          :default => false,
+          :public => true,
+          :type => Boolean,
+          :allowed_from_server => false,
+          :description => 'If `true`, tracer state storage is thread-local, otherwise, fiber-local'
+        },
         :timeout => {
           :default => 2 * 60, # 2 minutes
           :public => true,

lib/new_relic/thread_local_storage.rb

@@ -5,11 +5,19 @@
 module NewRelic
   module ThreadLocalStorage
     def self.get(thread, key)
-      thread[key]
+      if Agent.config[:thread_local_tracer_state]
+        thread.thread_variable_get(key)
+      else
+        thread[key]
+      end
     end
 
     def self.set(thread, key, value)
-      thread[key] = value
+      if Agent.config[:thread_local_tracer_state]
+        thread.thread_variable_set(key, value)
+      else
+        thread[key] = value
+      end
     end
 
     def self.[](key)

newrelic.yml

@@ -704,6 +704,9 @@ common: &default_settings
   # the New Relic agent has time to report.
   # sync_startup: false
 
+  # If true, tracer state storage is thread-local, otherwise, fiber-local
+  # thread_local_tracer_state: false
+
   # If true, enables use of the thread profiler.
   # thread_profiler.enabled: false
 

test/multiverse/suites/rake/instrumentation_test.rb

@@ -57,7 +57,7 @@ def test_invoke_with_tracing_with_exception
     NewRelic::Agent::Instrumentation::Rake.stub :should_trace?, true, [instance.name] do
       error = RuntimeError.new('expected')
       # produce the exception we want to have the method rescue
-      NewRelic::Agent.stub :config, -> { raise error } do
+      NewRelic::Agent.stub :instance, -> { raise error } do
         logger = MiniTest::Mock.new
         NewRelic::Agent.stub :logger, logger do
           logger.expect :error, nil, [/^Exception/, error]

test/new_relic/agent/tracer_test.rb

@@ -17,6 +17,16 @@ def test_tracer_aliases
         refute_nil state
       end
 
+      def test_tracer_state_in_fiber_with_thread_local_tracer_state
+        with_config(:thread_local_tracer_state => true) do
+          state = Tracer.state
+          fiber = Fiber.new do
+            assert_equal Tracer.state, state
+          end
+          fiber.resume
+        end
+      end
+
       def test_current_transaction_with_transaction
         in_transaction do |txn|
           assert_equal txn, Tracer.current_transaction

test/new_relic/thread_local_storage_test.rb

@@ -29,6 +29,8 @@ def test_shortcut_ops
   def test_new_thread
     NewRelic::ThreadLocalStorage[:new_thread] = :parent
     thread = Thread.new do
+      Thread.current.abort_on_exception = true
+
       assert_nil NewRelic::ThreadLocalStorage[:new_thread]
       NewRelic::ThreadLocalStorage[:new_thread] = :child
       sleep
@@ -39,4 +41,30 @@ def test_new_thread
     assert_equal(:child, NewRelic::ThreadLocalStorage.get(thread, :new_thread))
     thread.exit
   end
+
+  def test_new_fiber_without_thread_local_tracer_state_flag
+    with_config(:thread_local_tracer_state => false) do
+      NewRelic::ThreadLocalStorage[:new_fiber_no_flag] = 1
+      fiber = Fiber.new do
+        assert_nil NewRelic::ThreadLocalStorage[:new_fiber_no_flag]
+        NewRelic::ThreadLocalStorage[:new_fiber_no_flag] = 2
+      end
+      fiber.resume
+
+      assert_equal(1, NewRelic::ThreadLocalStorage[:new_fiber_no_flag])
+    end
+  end
+
+  def test_new_fiber_with_thread_local_tracer_state_flag
+    with_config(:thread_local_tracer_state => true) do
+      NewRelic::ThreadLocalStorage[:new_fiber_with_flag] = 1
+      fiber = Fiber.new do
+        assert_equal(1, NewRelic::ThreadLocalStorage[:new_fiber_with_flag])
+        NewRelic::ThreadLocalStorage[:new_fiber_with_flag] = 2
+      end
+      fiber.resume
+
+      assert_equal(2, NewRelic::ThreadLocalStorage[:new_fiber_with_flag])
+    end
+  end
 end