[PROF-9821] Fix incorrect code provenance due to broken JSON monkey patch

lib/datadog/profiling/collectors/code_provenance.rb

@@ -98,19 +98,30 @@ def record_loaded_files(loaded_files)
         end
 
         # Represents metadata we have for a ruby gem
+        #
+        # Important note: This class gets encoded to JSON with the built-in JSON gem. But, we've found that in some
+        # buggy cases, some Ruby gems monkey patch the built-in JSON gem and forget to call #to_json, and instead
+        # encode this class instance-field-by-instance-field.
+        #
+        # Thus, this class was setup to match the JSON output. Take this into consideration if you are adding new
+        # fields. (Also, we have a spec for this)
         class Library
-          attr_reader :kind, :name, :version, :path
+          attr_reader :kind, :name, :version
 
           def initialize(kind:, name:, version:, path:)
             @kind = kind.freeze
             @name = name.dup.freeze
             @version = version.to_s.dup.freeze
-            @path = path.dup.freeze
+            @paths = [path.dup.freeze].freeze
             freeze
           end
 
           def to_json(arg = nil)
-            { kind: @kind, name: @name, version: @version, paths: [@path] }.to_json(arg)
+            { kind: @kind, name: @name, version: @version, paths: @paths }.to_json(arg)
+          end
+
+          def path
+            @paths.first
           end
         end
       end

spec/datadog/profiling/collectors/code_provenance_spec.rb

@@ -1,5 +1,6 @@
 require 'datadog/profiling/collectors/code_provenance'
 require 'json-schema'
+require 'yaml'
 
 RSpec.describe Datadog::Profiling::Collectors::CodeProvenance do
   subject(:code_provenance) { described_class.new }
@@ -187,5 +188,76 @@
     it 'renders the list of loaded libraries using the expected schema' do
       JSON::Validator.validate!(code_provenance_schema, code_provenance.generate_json)
     end
+
+    # In PROF-9821 we run into an issue where some versions of OJ + activesupport + monkey patching the JSON gem
+    # would result in our Library instance being encoded instance-field-by-instance-field instead of by calling #to_json.
+    #
+    # This would obviously result in broken code provenance files. To fix this, we've adjusted the class to make sure
+    # that if you serialize it field-by-field, you still get a correct result.
+    #
+    # Reproducing this exact issue in CI is really annoying -- because it would be one more set of appraisails we'd run
+    # just to reproduce it and test.
+    #
+    # So instead in this test we use YAML as an example of an encoder that doesn't use #to_json, and does it
+    # field-by-field. Thus if the Library class doesn't match exactly what we want in the output, this test will fail.
+    #
+    # In case you want to reproduce the exact JSON issue, here's a reproducer:
+    # ````ruby
+    # require 'bundler/inline'
+    #
+    # gemfile do
+    #   source 'https://rubygems.org'
+    #   gem 'activesupport', '= 5.0.7.2'
+    #   gem 'oj', '= 2.18.5'
+    # end
+    #
+    # require 'json'
+    #
+    # class Example
+    #   def initialize = @hello = 1
+    #   def to_json(arg = nil) = {world: 2}.to_json(arg)
+    # end
+    #
+    # example = Example.new
+    # puts JSON.fast_generate(example)
+    #
+    # require 'oj'
+    # require 'active_support/core_ext/object/json'
+    # Oj.mimic_JSON()
+    #
+    # puts JSON.fast_generate(example)
+    # ```
+    #
+    # Incorrect output:
+    # {"world":2}
+    # {"hello":1}
+    #
+    describe 'when JSON encoder is broken and skips #to_json' do
+      let(:library_class_without_to_json) do
+        Class.new(Datadog::Profiling::Collectors::CodeProvenance::Library) do
+          undef to_json
+        end
+      end
+
+      it 'is still able to correctly encode a library instance' do
+        instance = library_class_without_to_json.new(
+          name: 'datadog',
+          kind: 'library',
+          version: '1.2.3',
+          path: '/example/path/to/datadog/gem',
+        )
+
+        serialized_without_to_json = YAML.dump(instance)
+        # Remove class annotation, so it deserializes back as a hash and not an instance of our class
+        serialized_without_to_json.gsub!(/---.*/, '---')
+
+        expect(YAML.safe_load(serialized_without_to_json)).to eq(
+          'name' => 'datadog',
+          'kind' => 'library',
+          'version' => '1.2.3',
+          'paths' => ['/example/path/to/datadog/gem'],
+        )
+      end
+    end
   end
 end