auto convert :auto to :prepend/:chain in config

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## dev
 
+- **Feature: The agent configuration will now reflect whether module prepending or method chaining was used for instrumentation**
+
+  For `:'instrumentation.*'` configuration parameters that are set to :auto (the default), the agent will automatically determine whether to use module prepending or method chaining. The agent will now update its in-memory configuration to give each relevant parameter a value of either :prepend or :chain so that the result of the determination can be introspected. This is intended to help 3rd party libraries that wish to further enhance the agent's instrumentation capabilities by prepending or chaining additional logic. Environment variable, YAML file, and server-side configuration based values are not impacted. [PR#1930](https://github.com/newrelic/newrelic-ruby-agent/pull/1930)
+
 - **Feature: Deprecate memcached and memcache-client instrumentation**
 
   Instrumentation for the memcached and memcache-client libraries is deprecated and will be removed during the next major release.

lib/new_relic/agent/configuration/manager.rb

@@ -15,6 +15,8 @@ module NewRelic
   module Agent
     module Configuration
       class Manager
+        DEPENDENCY_DETECTION_VALUES = %i[prepend chain].freeze
+
         # Defining these explicitly saves object allocations that we incur
         # if we use Forwardable and def_delegators.
         def [](key)
@@ -357,7 +359,18 @@ def reset_to_defaults
           reset_cache
         end
 
+        # reset the configuration hash, but do not replace previously auto
+        # determined dependency detection values with nil or 'auto'
         def reset_cache
+          return new_cache unless defined?(@cache) && @cache
+
+          preserved = @cache.select { |_k, v| DEPENDENCY_DETECTION_VALUES.include?(v) }
+          new_cache
+          preserved.each { |k, v| @cache[k] = v unless @cache[k] && @cache[k] != 'auto' }
+          @cache
+        end
+
+        def new_cache
           @cache = Hash.new { |hash, key| hash[key] = self.fetch(key) }
         end
 

lib/new_relic/dependency_detection.rb

@@ -179,6 +179,15 @@ def fetch_config_value(key)
       return valid_value
     end
 
+    # update any :auto config value to be either :prepend or :chain after auto
+    # determination has selected one of those to use
+    def update_config_value(use_prepend)
+      if config_key && auto_configured?
+        NewRelic::Agent.config.instance_variable_get(:@cache)[config_key] = use_prepend ? :prepend : :chain
+      end
+      use_prepend
+    end
+
     def config_value
       return AUTO_CONFIG_VALUE unless config_key
 
@@ -202,7 +211,7 @@ def conflicts_with_prepend(&block)
     end
 
     def use_prepend?
-      prepend_configured? || (auto_configured? && !prepend_conflicts?)
+      update_config_value(prepend_configured? || (auto_configured? && !prepend_conflicts?))
     end
 
     def prepend_conflicts?

test/new_relic/agent/configuration/default_source_test.rb

@@ -10,6 +10,7 @@ class DefaultSourceTest < Minitest::Test
     def setup
       @default_source = DefaultSource.new
       @defaults = ::NewRelic::Agent::Configuration::DEFAULTS
+      NewRelic::Agent.config.send(:new_cache)
     end
 
     def test_default_values_have_a_public_setting

test/new_relic/agent/configuration/manager_test.rb

@@ -491,6 +491,26 @@ def test_apply_transformations_reraises_errors
       end
     end
 
+    def test_auto_determined_values_stay_cached
+      name = :knockbreck_manse
+
+      dd = DependencyDetection.defer do
+        named(name)
+        executes { use_prepend? }
+      end
+
+      key = :"instrumentation.#{name}"
+      with_config(key => 'auto') do
+        DependencyDetection.detect!
+
+        @manager.replace_or_add_config(ServerSource.new({}))
+
+        assert_equal :prepend, @manager.instance_variable_get(:@cache)[key]
+      end
+    end
+
+    private
+
     def assert_parsed_labels(expected)
       result = @manager.parsed_labels
 

test/new_relic/dependency_detection_test.rb

@@ -380,4 +380,40 @@ def test_log_and_instrument_uses_supportability_name_when_provided
     assert_log_contains(log, /#{supportability_name}/)
     assert_log_contains(log, /#{method}/)
   end
+
+  # when an instrumentation value of :auto (the default) is present, the agent
+  # automatically determines whether to use :prepend or :chain. after the
+  # determination is made, the config value should be updated to be either
+  # :prepend or :chain so that the determined value can be introspected.
+  def test_prepend_or_chain_based_values_have_auto_converted_into_one_of_those
+    name = :bandu_gumbo
+
+    dd = DependencyDetection.defer do
+      named(name)
+      executes { use_prepend? }
+    end
+
+    with_config(:"instrumentation.#{name}" => 'auto') do
+      DependencyDetection.detect!
+
+      assert_equal :prepend, dd.config_value
+    end
+  end
+
+  # confirm that :auto becomes :chain when :chain is automatically determined
+  def test_auto_is_replaced_by_chain_when_chain_is_used
+    name = :blank_and_jones
+
+    dd = DependencyDetection.defer do
+      named(name)
+      conflicts_with_prepend { true }
+      executes { use_prepend? }
+    end
+
+    with_config(:"instrumentation.#{name}" => 'auto') do
+      DependencyDetection.detect!
+
+      assert_equal :chain, dd.config_value
+    end
+  end
 end